Linux Virtualization - Jun 2023 - [PATCH 1/1] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression

When switching from kthreads to vhost_tasks two bugs were added:
1. The vhost worker tasks's now show up as processes so scripts doing
ps or ps a would not incorrectly detect the vhost task as another
process.  2. kthreads disabled freeze by setting PF_NOFREEZE, but
vhost tasks's didn't disable or add support for them.

To fix both bugs, this switches the vhost task to be thread in the
process that does the VHOST_SET_OWNER ioctl, and has vhost_worker call
get_signal to support SIGKILL/SIGSTOP and freeze signals. Note that
SIGKILL/STOP support is required because CLONE_THREAD requires
CLONE_SIGHAND which requires those 2 signals to be supported.

This is a modified version of the patch written by Mike Christie
<michael.christie at oracle.com> which was a modified version of patch
originally written by Linus.

Much of what depended upon PF_IO_WORKER now depends on PF_USER_WORKER.
Including ignoring signals, setting up the register state, and having
get_signal return instead of calling do_group_exit.

Tidied up the vhost_task abstraction so that the definition of
vhost_task only needs to be visible inside of vhost_task.c.  Making
it easier to review the code and tell what needs to be done where.
As part of this the main loop has been moved from vhost_worker into
vhost_task_fn.  vhost_worker now returns true if work was done.

The main loop has been updated to call get_signal which handles
SIGSTOP, freezing, and collects the message that tells the thread to
exit as part of process exit.  This collection clears
__fatal_signal_pending.  This collection is not guaranteed to
clear signal_pending() so clear that explicitly so the schedule()
sleeps.

For now the vhost thread continues to exist and run work until the
last file descriptor is closed and the release function is called as
part of freeing struct file.  To avoid hangs in the coredump
rendezvous and when killing threads in a multi-threaded exec.  The
coredump code and de_thread have been modified to ignore vhost threads.

Remvoing the special case for exec appears to require teaching
vhost_dev_flush how to directly complete transactions in case
the vhost thread is no longer running.

Removing the special case for coredump rendezvous requires either the
above fix needed for exec or moving the coredump rendezvous into
get_signal.

Fixes: 6e890c5d5021 ("vhost: use vhost_tasks for worker threads")
Signed-off-by: Eric W. Biederman <ebiederm at xmission.com>
Co-developed-by: Mike Christie <michael.christie at oracle.com>
Signed-off-by: Mike Christie <michael.christie at oracle.com>
---
 arch/x86/include/asm/fpu/sched.h |  2 +-
 arch/x86/kernel/fpu/context.h    |  2 +-
 arch/x86/kernel/fpu/core.c       |  2 +-
 drivers/vhost/vhost.c            | 22 ++------
 fs/coredump.c                    |  4 +-
 include/linux/sched/task.h       |  1 -
 include/linux/sched/vhost_task.h | 15 ++----
 kernel/exit.c                    |  5 +-
 kernel/fork.c                    | 13 ++---
 kernel/signal.c                  |  8 +--
 kernel/vhost_task.c              | 92 +++++++++++++++++++++-----------
 11 files changed, 89 insertions(+), 77 deletions(-)

diff --git a/arch/x86/include/asm/fpu/sched.h b/arch/x86/include/asm/fpu/sched.h
index c2d6cd78ed0c..78fcde7b1f07 100644
--- a/arch/x86/include/asm/fpu/sched.h
+++ b/arch/x86/include/asm/fpu/sched.h
@@ -39,7 +39,7 @@ extern void fpu_flush_thread(void);
 static inline void switch_fpu_prepare(struct fpu *old_fpu, int cpu)
 {
 	if (cpu_feature_enabled(X86_FEATURE_FPU) &&
-	    !(current->flags & (PF_KTHREAD | PF_IO_WORKER))) {
+	    !(current->flags & (PF_KTHREAD | PF_USER_WORKER))) {
 		save_fpregs_to_fpstate(old_fpu);
 		/*
 		 * The save operation preserved register state, so the
diff --git a/arch/x86/kernel/fpu/context.h b/arch/x86/kernel/fpu/context.h
index 9fcfa5c4dad7..af5cbdd9bd29 100644
--- a/arch/x86/kernel/fpu/context.h
+++ b/arch/x86/kernel/fpu/context.h
@@ -57,7 +57,7 @@ static inline void fpregs_restore_userregs(void)
 	struct fpu *fpu = &current->thread.fpu;
 	int cpu = smp_processor_id();
 
-	if (WARN_ON_ONCE(current->flags & (PF_KTHREAD | PF_IO_WORKER)))
+	if (WARN_ON_ONCE(current->flags & (PF_KTHREAD | PF_USER_WORKER)))
 		return;
 
 	if (!fpregs_state_valid(fpu, cpu)) {
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index caf33486dc5e..1015af1ae562 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -426,7 +426,7 @@ void kernel_fpu_begin_mask(unsigned int kfpu_mask)
 
 	this_cpu_write(in_kernel_fpu, true);
 
-	if (!(current->flags & (PF_KTHREAD | PF_IO_WORKER)) &&
+	if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER)) &&
 	    !test_thread_flag(TIF_NEED_FPU_LOAD)) {
 		set_thread_flag(TIF_NEED_FPU_LOAD);
 		save_fpregs_to_fpstate(&current->thread.fpu);
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index a92af08e7864..074273020849 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -256,7 +256,7 @@ void vhost_work_queue(struct vhost_dev *dev, struct
vhost_work *work)
 		 * test_and_set_bit() implies a memory barrier.
 		 */
 		llist_add(&work->node, &dev->worker->work_list);
-		wake_up_process(dev->worker->vtsk->task);
+		vhost_task_wake(dev->worker->vtsk);
 	}
 }
 EXPORT_SYMBOL_GPL(vhost_work_queue);
@@ -333,31 +333,19 @@ static void vhost_vq_reset(struct vhost_dev *dev,
 	__vhost_vq_meta_reset(vq);
 }
 
-static int vhost_worker(void *data)
+static bool vhost_worker(void *data)
 {
 	struct vhost_worker *worker = data;
 	struct vhost_work *work, *work_next;
 	struct llist_node *node;
 
-	for (;;) {
-		/* mb paired w/ kthread_stop */
-		set_current_state(TASK_INTERRUPTIBLE);
-
-		if (vhost_task_should_stop(worker->vtsk)) {
-			__set_current_state(TASK_RUNNING);
-			break;
-		}
-
-		node = llist_del_all(&worker->work_list);
-		if (!node)
-			schedule();
-
+	node = llist_del_all(&worker->work_list);
+	if (node) {
 		node = llist_reverse_order(node);
 		/* make sure flag is seen after deletion */
 		smp_wmb();
 		llist_for_each_entry_safe(work, work_next, node, node) {
 			clear_bit(VHOST_WORK_QUEUED, &work->flags);
-			__set_current_state(TASK_RUNNING);
 			kcov_remote_start_common(worker->kcov_handle);
 			work->fn(work);
 			kcov_remote_stop();
@@ -365,7 +353,7 @@ static int vhost_worker(void *data)
 		}
 	}
 
-	return 0;
+	return !!node;
 }
 
 static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
diff --git a/fs/coredump.c b/fs/coredump.c
index ece7badf701b..88740c51b942 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -371,7 +371,9 @@ static int zap_process(struct task_struct *start, int
exit_code)
 		if (t != current && !(t->flags & PF_POSTCOREDUMP)) {
 			sigaddset(&t->pending.signal, SIGKILL);
 			signal_wake_up(t, 1);
-			nr++;
+			/* The vhost_worker does not particpate in coredumps */
+			if ((t->flags & (PF_USER_WORKER | PF_IO_WORKER)) != PF_USER_WORKER)
+				nr++;
 		}
 	}
 
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 537cbf9a2ade..e0f5ac90a228 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -29,7 +29,6 @@ struct kernel_clone_args {
 	u32 io_thread:1;
 	u32 user_worker:1;
 	u32 no_files:1;
-	u32 ignore_signals:1;
 	unsigned long stack;
 	unsigned long stack_size;
 	unsigned long tls;
diff --git a/include/linux/sched/vhost_task.h b/include/linux/sched/vhost_task.h
index 6123c10b99cf..837a23624a66 100644
--- a/include/linux/sched/vhost_task.h
+++ b/include/linux/sched/vhost_task.h
@@ -2,22 +2,13 @@
 #ifndef _LINUX_VHOST_TASK_H
 #define _LINUX_VHOST_TASK_H
 
-#include <linux/completion.h>
 
-struct task_struct;
+struct vhost_task;
 
-struct vhost_task {
-	int (*fn)(void *data);
-	void *data;
-	struct completion exited;
-	unsigned long flags;
-	struct task_struct *task;
-};
-
-struct vhost_task *vhost_task_create(int (*fn)(void *), void *arg,
+struct vhost_task *vhost_task_create(bool (*fn)(void *), void *arg,
 				     const char *name);
 void vhost_task_start(struct vhost_task *vtsk);
 void vhost_task_stop(struct vhost_task *vtsk);
-bool vhost_task_should_stop(struct vhost_task *vtsk);
+void vhost_task_wake(struct vhost_task *vtsk);
 
 #endif
diff --git a/kernel/exit.c b/kernel/exit.c
index 34b90e2e7cf7..edb50b4c9972 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -411,7 +411,10 @@ static void coredump_task_exit(struct task_struct *tsk)
 	tsk->flags |= PF_POSTCOREDUMP;
 	core_state = tsk->signal->core_state;
 	spin_unlock_irq(&tsk->sighand->siglock);
-	if (core_state) {
+
+	/* The vhost_worker does not particpate in coredumps */
+	if (core_state &&
+	    ((tsk->flags & (PF_IO_WORKER | PF_USER_WORKER)) != PF_USER_WORKER))
{
 		struct core_thread self;
 
 		self.task = current;
diff --git a/kernel/fork.c b/kernel/fork.c
index ed4e01daccaa..81cba91f30bb 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2336,16 +2336,16 @@ __latent_entropy struct task_struct *copy_process(
 	p->flags &= ~PF_KTHREAD;
 	if (args->kthread)
 		p->flags |= PF_KTHREAD;
-	if (args->user_worker)
-		p->flags |= PF_USER_WORKER;
-	if (args->io_thread) {
+	if (args->user_worker) {
 		/*
-		 * Mark us an IO worker, and block any signal that isn't
+		 * Mark us a user worker, and block any signal that isn't
 		 * fatal or STOP
 		 */
-		p->flags |= PF_IO_WORKER;
+		p->flags |= PF_USER_WORKER;
 		siginitsetinv(&p->blocked, sigmask(SIGKILL)|sigmask(SIGSTOP));
 	}
+	if (args->io_thread)
+		p->flags |= PF_IO_WORKER;
 
 	if (args->name)
 		strscpy_pad(p->comm, args->name, sizeof(p->comm));
@@ -2517,9 +2517,6 @@ __latent_entropy struct task_struct *copy_process(
 	if (retval)
 		goto bad_fork_cleanup_io;
 
-	if (args->ignore_signals)
-		ignore_signals(p);
-
 	stackleak_task_init(p);
 
 	if (pid != &init_struct_pid) {
diff --git a/kernel/signal.c b/kernel/signal.c
index 8f6330f0e9ca..2547fa73bde5 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1368,7 +1368,9 @@ int zap_other_threads(struct task_struct *p)
 
 	while_each_thread(p, t) {
 		task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
-		count++;
+		/* Don't require de_thread to wait for the vhost_worker */
+		if ((t->flags & (PF_IO_WORKER | PF_USER_WORKER)) != PF_USER_WORKER)
+			count++;
 
 		/* Don't bother with already dead threads */
 		if (t->exit_state)
@@ -2861,11 +2863,11 @@ bool get_signal(struct ksignal *ksig)
 		}
 
 		/*
-		 * PF_IO_WORKER threads will catch and exit on fatal signals
+		 * PF_USER_WORKER threads will catch and exit on fatal signals
 		 * themselves. They have cleanup that must be performed, so
 		 * we cannot call do_exit() on their behalf.
 		 */
-		if (current->flags & PF_IO_WORKER)
+		if (current->flags & PF_USER_WORKER)
 			goto out;
 
 		/*
diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c
index b7cbd66f889e..f80d5c51ae67 100644
--- a/kernel/vhost_task.c
+++ b/kernel/vhost_task.c
@@ -12,58 +12,88 @@ enum vhost_task_flags {
 	VHOST_TASK_FLAGS_STOP,
 };
 
+struct vhost_task {
+	bool (*fn)(void *data);
+	void *data;
+	struct completion exited;
+	unsigned long flags;
+	struct task_struct *task;
+};
+
 static int vhost_task_fn(void *data)
 {
 	struct vhost_task *vtsk = data;
-	int ret;
+	bool dead = false;
+
+	for (;;) {
+		bool did_work;
+
+		/* mb paired w/ vhost_task_stop */
+		if (test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags))
+			break;
+
+		if (!dead && signal_pending(current)) {
+			struct ksignal ksig;
+			/*
+			 * Calling get_signal will block in SIGSTOP,
+			 * or clear fatal_signal_pending, but remember
+			 * what was set.
+			 *
+			 * This thread won't actually exit until all
+			 * of the file descriptors are closed, and
+			 * the release function is called.
+			 */
+			dead = get_signal(&ksig);
+			if (dead)
+				clear_thread_flag(TIF_SIGPENDING);
+		}
+
+		did_work = vtsk->fn(vtsk->data);
+		if (!did_work) {
+			set_current_state(TASK_INTERRUPTIBLE);
+			schedule();
+		}
+	}
 
-	ret = vtsk->fn(vtsk->data);
 	complete(&vtsk->exited);
-	do_exit(ret);
+	do_exit(0);
+}
+
+/**
+ * vhost_task_wake - wakeup the vhost_task
+ * @vtsk: vhost_task to wake
+ *
+ * wake up the vhost_task worker thread
+ */
+void vhost_task_wake(struct vhost_task *vtsk)
+{
+	wake_up_process(vtsk->task);
 }
+EXPORT_SYMBOL_GPL(vhost_task_wake);
 
 /**
  * vhost_task_stop - stop a vhost_task
  * @vtsk: vhost_task to stop
  *
- * Callers must call vhost_task_should_stop and return from their worker
- * function when it returns true;
+ * vhost_task_fn ensures the worker thread exits after
+ * VHOST_TASK_FLAGS_SOP becomes true.
  */
 void vhost_task_stop(struct vhost_task *vtsk)
 {
-	pid_t pid = vtsk->task->pid;
-
 	set_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags);
-	wake_up_process(vtsk->task);
+	vhost_task_wake(vtsk);
 	/*
 	 * Make sure vhost_task_fn is no longer accessing the vhost_task before
-	 * freeing it below. If userspace crashed or exited without closing,
-	 * then the vhost_task->task could already be marked dead so
-	 * kernel_wait will return early.
+	 * freeing it below.
 	 */
 	wait_for_completion(&vtsk->exited);
-	/*
-	 * If we are just closing/removing a device and the parent process is
-	 * not exiting then reap the task.
-	 */
-	kernel_wait4(pid, NULL, __WCLONE, NULL);
 	kfree(vtsk);
 }
 EXPORT_SYMBOL_GPL(vhost_task_stop);
 
 /**
- * vhost_task_should_stop - should the vhost task return from the work function
- * @vtsk: vhost_task to stop
- */
-bool vhost_task_should_stop(struct vhost_task *vtsk)
-{
-	return test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags);
-}
-EXPORT_SYMBOL_GPL(vhost_task_should_stop);
-
-/**
- * vhost_task_create - create a copy of a process to be used by the kernel
- * @fn: thread stack
+ * vhost_task_create - create a copy of a task to be used by the kernel
+ * @fn: vhost worker function
  * @arg: data to be passed to fn
  * @name: the thread's name
  *
@@ -71,17 +101,17 @@ EXPORT_SYMBOL_GPL(vhost_task_should_stop);
  * failure. The returned task is inactive, and the caller must fire it up
  * through vhost_task_start().
  */
-struct vhost_task *vhost_task_create(int (*fn)(void *), void *arg,
+struct vhost_task *vhost_task_create(bool (*fn)(void *), void *arg,
 				     const char *name)
 {
 	struct kernel_clone_args args = {
-		.flags		= CLONE_FS | CLONE_UNTRACED | CLONE_VM,
+		.flags		= CLONE_FS | CLONE_UNTRACED | CLONE_VM |
+				  CLONE_THREAD | CLONE_SIGHAND,
 		.exit_signal	= 0,
 		.fn		= vhost_task_fn,
 		.name		= name,
 		.user_worker	= 1,
 		.no_files	= 1,
-		.ignore_signals	= 1,
 	};
 	struct vhost_task *vtsk;
 	struct task_struct *tsk;
-- 
2.25.1

On Thu, Jun 01, 2023 at 01:32:32PM -0500, Mike Christie
wrote:
> When switching from kthreads to vhost_tasks two bugs were added:
> 1. The vhost worker tasks's now show up as processes so scripts doing
> ps or ps a would not incorrectly detect the vhost task as another
> process.  2. kthreads disabled freeze by setting PF_NOFREEZE, but
> vhost tasks's didn't disable or add support for them.
> 
> To fix both bugs, this switches the vhost task to be thread in the
> process that does the VHOST_SET_OWNER ioctl, and has vhost_worker call
> get_signal to support SIGKILL/SIGSTOP and freeze signals. Note that
> SIGKILL/STOP support is required because CLONE_THREAD requires
> CLONE_SIGHAND which requires those 2 signals to be supported.
> 
> This is a modified version of the patch written by Mike Christie
> <michael.christie at oracle.com> which was a modified version of
patch
> originally written by Linus.
> 
> Much of what depended upon PF_IO_WORKER now depends on PF_USER_WORKER.
> Including ignoring signals, setting up the register state, and having
> get_signal return instead of calling do_group_exit.
> 
> Tidied up the vhost_task abstraction so that the definition of
> vhost_task only needs to be visible inside of vhost_task.c.  Making
> it easier to review the code and tell what needs to be done where.
> As part of this the main loop has been moved from vhost_worker into
> vhost_task_fn.  vhost_worker now returns true if work was done.
> 
> The main loop has been updated to call get_signal which handles
> SIGSTOP, freezing, and collects the message that tells the thread to
> exit as part of process exit.  This collection clears
> __fatal_signal_pending.  This collection is not guaranteed to
> clear signal_pending() so clear that explicitly so the schedule()
> sleeps.
> 
> For now the vhost thread continues to exist and run work until the
> last file descriptor is closed and the release function is called as
> part of freeing struct file.  To avoid hangs in the coredump
> rendezvous and when killing threads in a multi-threaded exec.  The
> coredump code and de_thread have been modified to ignore vhost threads.
> 
> Remvoing the special case for exec appears to require teaching
> vhost_dev_flush how to directly complete transactions in case
> the vhost thread is no longer running.
> 
> Removing the special case for coredump rendezvous requires either the
> above fix needed for exec or moving the coredump rendezvous into
> get_signal.
> 
> Fixes: 6e890c5d5021 ("vhost: use vhost_tasks for worker threads")
> Signed-off-by: Eric W. Biederman <ebiederm at xmission.com>
> Co-developed-by: Mike Christie <michael.christie at oracle.com>
> Signed-off-by: Mike Christie <michael.christie at oracle.com>

Acked-by: Michael S. Tsirkin <mst at redhat.com>

> ---
>  arch/x86/include/asm/fpu/sched.h |  2 +-
>  arch/x86/kernel/fpu/context.h    |  2 +-
>  arch/x86/kernel/fpu/core.c       |  2 +-
>  drivers/vhost/vhost.c            | 22 ++------
>  fs/coredump.c                    |  4 +-
>  include/linux/sched/task.h       |  1 -
>  include/linux/sched/vhost_task.h | 15 ++----
>  kernel/exit.c                    |  5 +-
>  kernel/fork.c                    | 13 ++---
>  kernel/signal.c                  |  8 +--
>  kernel/vhost_task.c              | 92 +++++++++++++++++++++-----------
>  11 files changed, 89 insertions(+), 77 deletions(-)
> 
> diff --git a/arch/x86/include/asm/fpu/sched.h
b/arch/x86/include/asm/fpu/sched.h
> index c2d6cd78ed0c..78fcde7b1f07 100644
> --- a/arch/x86/include/asm/fpu/sched.h
> +++ b/arch/x86/include/asm/fpu/sched.h
> @@ -39,7 +39,7 @@ extern void fpu_flush_thread(void);
>  static inline void switch_fpu_prepare(struct fpu *old_fpu, int cpu)
>  {
>  	if (cpu_feature_enabled(X86_FEATURE_FPU) &&
> -	    !(current->flags & (PF_KTHREAD | PF_IO_WORKER))) {
> +	    !(current->flags & (PF_KTHREAD | PF_USER_WORKER))) {
>  		save_fpregs_to_fpstate(old_fpu);
>  		/*
>  		 * The save operation preserved register state, so the
> diff --git a/arch/x86/kernel/fpu/context.h b/arch/x86/kernel/fpu/context.h
> index 9fcfa5c4dad7..af5cbdd9bd29 100644
> --- a/arch/x86/kernel/fpu/context.h
> +++ b/arch/x86/kernel/fpu/context.h
> @@ -57,7 +57,7 @@ static inline void fpregs_restore_userregs(void)
>  	struct fpu *fpu = &current->thread.fpu;
>  	int cpu = smp_processor_id();
>  
> -	if (WARN_ON_ONCE(current->flags & (PF_KTHREAD | PF_IO_WORKER)))
> +	if (WARN_ON_ONCE(current->flags & (PF_KTHREAD | PF_USER_WORKER)))
>  		return;
>  
>  	if (!fpregs_state_valid(fpu, cpu)) {
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index caf33486dc5e..1015af1ae562 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -426,7 +426,7 @@ void kernel_fpu_begin_mask(unsigned int kfpu_mask)
>  
>  	this_cpu_write(in_kernel_fpu, true);
>  
> -	if (!(current->flags & (PF_KTHREAD | PF_IO_WORKER)) &&
> +	if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER)) &&
>  	    !test_thread_flag(TIF_NEED_FPU_LOAD)) {
>  		set_thread_flag(TIF_NEED_FPU_LOAD);
>  		save_fpregs_to_fpstate(&current->thread.fpu);
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index a92af08e7864..074273020849 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -256,7 +256,7 @@ void vhost_work_queue(struct vhost_dev *dev, struct
vhost_work *work)
>  		 * test_and_set_bit() implies a memory barrier.
>  		 */
>  		llist_add(&work->node, &dev->worker->work_list);
> -		wake_up_process(dev->worker->vtsk->task);
> +		vhost_task_wake(dev->worker->vtsk);
>  	}
>  }
>  EXPORT_SYMBOL_GPL(vhost_work_queue);
> @@ -333,31 +333,19 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>  	__vhost_vq_meta_reset(vq);
>  }
>  
> -static int vhost_worker(void *data)
> +static bool vhost_worker(void *data)
>  {
>  	struct vhost_worker *worker = data;
>  	struct vhost_work *work, *work_next;
>  	struct llist_node *node;
>  
> -	for (;;) {
> -		/* mb paired w/ kthread_stop */
> -		set_current_state(TASK_INTERRUPTIBLE);
> -
> -		if (vhost_task_should_stop(worker->vtsk)) {
> -			__set_current_state(TASK_RUNNING);
> -			break;
> -		}
> -
> -		node = llist_del_all(&worker->work_list);
> -		if (!node)
> -			schedule();
> -
> +	node = llist_del_all(&worker->work_list);
> +	if (node) {
>  		node = llist_reverse_order(node);
>  		/* make sure flag is seen after deletion */
>  		smp_wmb();
>  		llist_for_each_entry_safe(work, work_next, node, node) {
>  			clear_bit(VHOST_WORK_QUEUED, &work->flags);
> -			__set_current_state(TASK_RUNNING);
>  			kcov_remote_start_common(worker->kcov_handle);
>  			work->fn(work);
>  			kcov_remote_stop();
> @@ -365,7 +353,7 @@ static int vhost_worker(void *data)
>  		}
>  	}
>  
> -	return 0;
> +	return !!node;
>  }
>  
>  static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
> diff --git a/fs/coredump.c b/fs/coredump.c
> index ece7badf701b..88740c51b942 100644
> --- a/fs/coredump.c
> +++ b/fs/coredump.c
> @@ -371,7 +371,9 @@ static int zap_process(struct task_struct *start, int
exit_code)
>  		if (t != current && !(t->flags & PF_POSTCOREDUMP)) {
>  			sigaddset(&t->pending.signal, SIGKILL);
>  			signal_wake_up(t, 1);
> -			nr++;
> +			/* The vhost_worker does not particpate in coredumps */
> +			if ((t->flags & (PF_USER_WORKER | PF_IO_WORKER)) !=
PF_USER_WORKER)
> +				nr++;
>  		}
>  	}
>  
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index 537cbf9a2ade..e0f5ac90a228 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -29,7 +29,6 @@ struct kernel_clone_args {
>  	u32 io_thread:1;
>  	u32 user_worker:1;
>  	u32 no_files:1;
> -	u32 ignore_signals:1;
>  	unsigned long stack;
>  	unsigned long stack_size;
>  	unsigned long tls;
> diff --git a/include/linux/sched/vhost_task.h
b/include/linux/sched/vhost_task.h
> index 6123c10b99cf..837a23624a66 100644
> --- a/include/linux/sched/vhost_task.h
> +++ b/include/linux/sched/vhost_task.h
> @@ -2,22 +2,13 @@
>  #ifndef _LINUX_VHOST_TASK_H
>  #define _LINUX_VHOST_TASK_H
>  
> -#include <linux/completion.h>
>  
> -struct task_struct;
> +struct vhost_task;
>  
> -struct vhost_task {
> -	int (*fn)(void *data);
> -	void *data;
> -	struct completion exited;
> -	unsigned long flags;
> -	struct task_struct *task;
> -};
> -
> -struct vhost_task *vhost_task_create(int (*fn)(void *), void *arg,
> +struct vhost_task *vhost_task_create(bool (*fn)(void *), void *arg,
>  				     const char *name);
>  void vhost_task_start(struct vhost_task *vtsk);
>  void vhost_task_stop(struct vhost_task *vtsk);
> -bool vhost_task_should_stop(struct vhost_task *vtsk);
> +void vhost_task_wake(struct vhost_task *vtsk);
>  
>  #endif
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 34b90e2e7cf7..edb50b4c9972 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -411,7 +411,10 @@ static void coredump_task_exit(struct task_struct
*tsk)
>  	tsk->flags |= PF_POSTCOREDUMP;
>  	core_state = tsk->signal->core_state;
>  	spin_unlock_irq(&tsk->sighand->siglock);
> -	if (core_state) {
> +
> +	/* The vhost_worker does not particpate in coredumps */
> +	if (core_state &&
> +	    ((tsk->flags & (PF_IO_WORKER | PF_USER_WORKER)) !=
PF_USER_WORKER)) {
>  		struct core_thread self;
>  
>  		self.task = current;
> diff --git a/kernel/fork.c b/kernel/fork.c
> index ed4e01daccaa..81cba91f30bb 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2336,16 +2336,16 @@ __latent_entropy struct task_struct *copy_process(
>  	p->flags &= ~PF_KTHREAD;
>  	if (args->kthread)
>  		p->flags |= PF_KTHREAD;
> -	if (args->user_worker)
> -		p->flags |= PF_USER_WORKER;
> -	if (args->io_thread) {
> +	if (args->user_worker) {
>  		/*
> -		 * Mark us an IO worker, and block any signal that isn't
> +		 * Mark us a user worker, and block any signal that isn't
>  		 * fatal or STOP
>  		 */
> -		p->flags |= PF_IO_WORKER;
> +		p->flags |= PF_USER_WORKER;
>  		siginitsetinv(&p->blocked, sigmask(SIGKILL)|sigmask(SIGSTOP));
>  	}
> +	if (args->io_thread)
> +		p->flags |= PF_IO_WORKER;
>  
>  	if (args->name)
>  		strscpy_pad(p->comm, args->name, sizeof(p->comm));
> @@ -2517,9 +2517,6 @@ __latent_entropy struct task_struct *copy_process(
>  	if (retval)
>  		goto bad_fork_cleanup_io;
>  
> -	if (args->ignore_signals)
> -		ignore_signals(p);
> -
>  	stackleak_task_init(p);
>  
>  	if (pid != &init_struct_pid) {
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 8f6330f0e9ca..2547fa73bde5 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1368,7 +1368,9 @@ int zap_other_threads(struct task_struct *p)
>  
>  	while_each_thread(p, t) {
>  		task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
> -		count++;
> +		/* Don't require de_thread to wait for the vhost_worker */
> +		if ((t->flags & (PF_IO_WORKER | PF_USER_WORKER)) !=
PF_USER_WORKER)
> +			count++;
>  
>  		/* Don't bother with already dead threads */
>  		if (t->exit_state)
> @@ -2861,11 +2863,11 @@ bool get_signal(struct ksignal *ksig)
>  		}
>  
>  		/*
> -		 * PF_IO_WORKER threads will catch and exit on fatal signals
> +		 * PF_USER_WORKER threads will catch and exit on fatal signals
>  		 * themselves. They have cleanup that must be performed, so
>  		 * we cannot call do_exit() on their behalf.
>  		 */
> -		if (current->flags & PF_IO_WORKER)
> +		if (current->flags & PF_USER_WORKER)
>  			goto out;
>  
>  		/*
> diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c
> index b7cbd66f889e..f80d5c51ae67 100644
> --- a/kernel/vhost_task.c
> +++ b/kernel/vhost_task.c
> @@ -12,58 +12,88 @@ enum vhost_task_flags {
>  	VHOST_TASK_FLAGS_STOP,
>  };
>  
> +struct vhost_task {
> +	bool (*fn)(void *data);
> +	void *data;
> +	struct completion exited;
> +	unsigned long flags;
> +	struct task_struct *task;
> +};
> +
>  static int vhost_task_fn(void *data)
>  {
>  	struct vhost_task *vtsk = data;
> -	int ret;
> +	bool dead = false;
> +
> +	for (;;) {
> +		bool did_work;
> +
> +		/* mb paired w/ vhost_task_stop */
> +		if (test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags))
> +			break;
> +
> +		if (!dead && signal_pending(current)) {
> +			struct ksignal ksig;
> +			/*
> +			 * Calling get_signal will block in SIGSTOP,
> +			 * or clear fatal_signal_pending, but remember
> +			 * what was set.
> +			 *
> +			 * This thread won't actually exit until all
> +			 * of the file descriptors are closed, and
> +			 * the release function is called.
> +			 */
> +			dead = get_signal(&ksig);
> +			if (dead)
> +				clear_thread_flag(TIF_SIGPENDING);
> +		}
> +
> +		did_work = vtsk->fn(vtsk->data);
> +		if (!did_work) {
> +			set_current_state(TASK_INTERRUPTIBLE);
> +			schedule();
> +		}
> +	}
>  
> -	ret = vtsk->fn(vtsk->data);
>  	complete(&vtsk->exited);
> -	do_exit(ret);
> +	do_exit(0);
> +}
> +
> +/**
> + * vhost_task_wake - wakeup the vhost_task
> + * @vtsk: vhost_task to wake
> + *
> + * wake up the vhost_task worker thread
> + */
> +void vhost_task_wake(struct vhost_task *vtsk)
> +{
> +	wake_up_process(vtsk->task);
>  }
> +EXPORT_SYMBOL_GPL(vhost_task_wake);
>  
>  /**
>   * vhost_task_stop - stop a vhost_task
>   * @vtsk: vhost_task to stop
>   *
> - * Callers must call vhost_task_should_stop and return from their worker
> - * function when it returns true;
> + * vhost_task_fn ensures the worker thread exits after
> + * VHOST_TASK_FLAGS_SOP becomes true.
>   */
>  void vhost_task_stop(struct vhost_task *vtsk)
>  {
> -	pid_t pid = vtsk->task->pid;
> -
>  	set_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags);
> -	wake_up_process(vtsk->task);
> +	vhost_task_wake(vtsk);
>  	/*
>  	 * Make sure vhost_task_fn is no longer accessing the vhost_task before
> -	 * freeing it below. If userspace crashed or exited without closing,
> -	 * then the vhost_task->task could already be marked dead so
> -	 * kernel_wait will return early.
> +	 * freeing it below.
>  	 */
>  	wait_for_completion(&vtsk->exited);
> -	/*
> -	 * If we are just closing/removing a device and the parent process is
> -	 * not exiting then reap the task.
> -	 */
> -	kernel_wait4(pid, NULL, __WCLONE, NULL);
>  	kfree(vtsk);
>  }
>  EXPORT_SYMBOL_GPL(vhost_task_stop);
>  
>  /**
> - * vhost_task_should_stop - should the vhost task return from the work
function
> - * @vtsk: vhost_task to stop
> - */
> -bool vhost_task_should_stop(struct vhost_task *vtsk)
> -{
> -	return test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags);
> -}
> -EXPORT_SYMBOL_GPL(vhost_task_should_stop);
> -
> -/**
> - * vhost_task_create - create a copy of a process to be used by the kernel
> - * @fn: thread stack
> + * vhost_task_create - create a copy of a task to be used by the kernel
> + * @fn: vhost worker function
>   * @arg: data to be passed to fn
>   * @name: the thread's name
>   *
> @@ -71,17 +101,17 @@ EXPORT_SYMBOL_GPL(vhost_task_should_stop);
>   * failure. The returned task is inactive, and the caller must fire it up
>   * through vhost_task_start().
>   */
> -struct vhost_task *vhost_task_create(int (*fn)(void *), void *arg,
> +struct vhost_task *vhost_task_create(bool (*fn)(void *), void *arg,
>  				     const char *name)
>  {
>  	struct kernel_clone_args args = {
> -		.flags		= CLONE_FS | CLONE_UNTRACED | CLONE_VM,
> +		.flags		= CLONE_FS | CLONE_UNTRACED | CLONE_VM |
> +				  CLONE_THREAD | CLONE_SIGHAND,
>  		.exit_signal	= 0,
>  		.fn		= vhost_task_fn,
>  		.name		= name,
>  		.user_worker	= 1,
>  		.no_files	= 1,
> -		.ignore_signals	= 1,
>  	};
>  	struct vhost_task *vtsk;
>  	struct task_struct *tsk;
> -- 
> 2.25.1

Mike Christie <michael.christie at oracle.com> writes:
> When switching from kthreads to vhost_tasks two bugs were added:
> 1. The vhost worker tasks's now show up as processes so scripts doing
> ps or ps a would not incorrectly detect the vhost task as another
> process.  2. kthreads disabled freeze by setting PF_NOFREEZE, but
> vhost tasks's didn't disable or add support for them.
>
> To fix both bugs, this switches the vhost task to be thread in the
> process that does the VHOST_SET_OWNER ioctl, and has vhost_worker call
> get_signal to support SIGKILL/SIGSTOP and freeze signals. Note that
> SIGKILL/STOP support is required because CLONE_THREAD requires
> CLONE_SIGHAND which requires those 2 signals to be supported.
>
> This is a modified version of the patch written by Mike Christie
> <michael.christie at oracle.com> which was a modified version of
patch
> originally written by Linus.
>
> Much of what depended upon PF_IO_WORKER now depends on PF_USER_WORKER.
> Including ignoring signals, setting up the register state, and having
> get_signal return instead of calling do_group_exit.
>
> Tidied up the vhost_task abstraction so that the definition of
> vhost_task only needs to be visible inside of vhost_task.c.  Making
> it easier to review the code and tell what needs to be done where.
> As part of this the main loop has been moved from vhost_worker into
> vhost_task_fn.  vhost_worker now returns true if work was done.
Please see below for a race in that tidying up.
> The main loop has been updated to call get_signal which handles
> SIGSTOP, freezing, and collects the message that tells the thread to
> exit as part of process exit.  This collection clears
> __fatal_signal_pending.  This collection is not guaranteed to
> clear signal_pending() so clear that explicitly so the schedule()
> sleeps.
>
> For now the vhost thread continues to exist and run work until the
> last file descriptor is closed and the release function is called as
> part of freeing struct file.  To avoid hangs in the coredump
> rendezvous and when killing threads in a multi-threaded exec.  The
> coredump code and de_thread have been modified to ignore vhost threads.
>
> Remvoing the special case for exec appears to require teaching
> vhost_dev_flush how to directly complete transactions in case
> the vhost thread is no longer running.
>
> Removing the special case for coredump rendezvous requires either the
> above fix needed for exec or moving the coredump rendezvous into
> get_signal.


In just fixing the hang after exec I am afraid I may have introduced
something worse.

Two different sighand_struct's (and their associated locks) pointing
at the same signal_struct.  (Should be fixable?)

I am worried about what happens with that vhost task after an exec.
It retains it's existing cred (and technically the old mm) but shares
signal_struct so it might be possible to use permission checks against
the old vhost task cred to bypass permission checks on the new tasks
cred.  In particular for exec's that gain privilege.

It doesn't look like that is an issue for signals and suid exec as
kill_ok_by_cred seems to deliberately allow the same thing.


We may be ok but the way that vhost task remains after exec it smells
like the setup for a local privilege escalation.


Oleg do you have any insights?

Does anyone see why using the vhost task to modify the process should
not result in privilege escalation?

> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index a92af08e7864..074273020849 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -256,7 +256,7 @@ void vhost_work_queue(struct vhost_dev *dev, struct
vhost_work *work)
>  		 * test_and_set_bit() implies a memory barrier.
>  		 */
>  		llist_add(&work->node, &dev->worker->work_list);
> -		wake_up_process(dev->worker->vtsk->task);
> +		vhost_task_wake(dev->worker->vtsk);
>  	}
>  }
>  EXPORT_SYMBOL_GPL(vhost_work_queue);
> @@ -333,31 +333,19 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>  	__vhost_vq_meta_reset(vq);
>  }
>  
> -static int vhost_worker(void *data)
> +static bool vhost_worker(void *data)
>  {
>  	struct vhost_worker *worker = data;
>  	struct vhost_work *work, *work_next;
>  	struct llist_node *node;
>  
> -	for (;;) {
> -		/* mb paired w/ kthread_stop */
> -		set_current_state(TASK_INTERRUPTIBLE);
> -
> -		if (vhost_task_should_stop(worker->vtsk)) {
> -			__set_current_state(TASK_RUNNING);
> -			break;
> -		}
> -
> -		node = llist_del_all(&worker->work_list);
> -		if (!node)
> -			schedule();
> -
> +	node = llist_del_all(&worker->work_list);
> +	if (node) {
>  		node = llist_reverse_order(node);
>  		/* make sure flag is seen after deletion */
>  		smp_wmb();
>  		llist_for_each_entry_safe(work, work_next, node, node) {
>  			clear_bit(VHOST_WORK_QUEUED, &work->flags);
> -			__set_current_state(TASK_RUNNING);
>  			kcov_remote_start_common(worker->kcov_handle);
>  			work->fn(work);
>  			kcov_remote_stop();
> @@ -365,7 +353,7 @@ static int vhost_worker(void *data)
>  		}
>  	}
>  
> -	return 0;
> +	return !!node;
>  }
>  
>  static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
> diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c
> index b7cbd66f889e..f80d5c51ae67 100644
> --- a/kernel/vhost_task.c
> +++ b/kernel/vhost_task.c
> @@ -12,58 +12,88 @@ enum vhost_task_flags {
>  	VHOST_TASK_FLAGS_STOP,
>  };
>  
> +struct vhost_task {
> +	bool (*fn)(void *data);
> +	void *data;
> +	struct completion exited;
> +	unsigned long flags;
> +	struct task_struct *task;
> +};
> +
>  static int vhost_task_fn(void *data)
>  {
>  	struct vhost_task *vtsk = data;
> -	int ret;
> +	bool dead = false;
> +
> +	for (;;) {
> +		bool did_work;
> +
> +		/* mb paired w/ vhost_task_stop */
> +		if (test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags))
> +			break;
> +
> +		if (!dead && signal_pending(current)) {
> +			struct ksignal ksig;
> +			/*
> +			 * Calling get_signal will block in SIGSTOP,
> +			 * or clear fatal_signal_pending, but remember
> +			 * what was set.
> +			 *
> +			 * This thread won't actually exit until all
> +			 * of the file descriptors are closed, and
> +			 * the release function is called.
> +			 */
> +			dead = get_signal(&ksig);
> +			if (dead)
> +				clear_thread_flag(TIF_SIGPENDING);
> +		}
> +
> +		did_work = vtsk->fn(vtsk->data);
> +		if (!did_work) {
> +			set_current_state(TASK_INTERRUPTIBLE);
I am about to head off on vacation for a week or so, but I want to
add some comments before I go.


First moving set_current_state(TASK_INTERRUPTIBLE) here in the loop
won't work.  It introduces a race between vhost_work_queue and this wake
up.

Better would be to move the "if (!dead && signal_pending(current))
..."
check up above "test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags)"

Eric

Hi Mike,

sorry, but somehow I can't understand this patch...

I'll try to read it with a fresh head on Weekend, but for example,

On 06/01, Mike Christie wrote:
>
>  static int vhost_task_fn(void *data)
>  {
>  	struct vhost_task *vtsk = data;
> -	int ret;
> +	bool dead = false;
> +
> +	for (;;) {
> +		bool did_work;
> +
> +		/* mb paired w/ vhost_task_stop */
> +		if (test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags))
> +			break;
> +
> +		if (!dead && signal_pending(current)) {
> +			struct ksignal ksig;
> +			/*
> +			 * Calling get_signal will block in SIGSTOP,
> +			 * or clear fatal_signal_pending, but remember
> +			 * what was set.
> +			 *
> +			 * This thread won't actually exit until all
> +			 * of the file descriptors are closed, and
> +			 * the release function is called.
> +			 */
> +			dead = get_signal(&ksig);
> +			if (dead)
> +				clear_thread_flag(TIF_SIGPENDING);
this can't be right or I am totally confused.

Another signal_wake_up() can come right after clear(SIGPENDING).


Again, I'll try to re-read this patch, but let me ask anyway...

Do we have a plan B? I mean... iirc you have mentioned that you can
change these code paths to do something like

	if (killed)
		tell_the_drivers_that_all_callbacks_will_fail();


so that vhost_worker() can exit after get_signal() returns SIGKILL.

Probably I misunderstood you, but it would be nice to avoid the changes
in coredump/etc code just to add a temporary (iiuc!) fix.

Oleg.

On 06/01, Mike Christie wrote:
>
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1368,7 +1368,9 @@ int zap_other_threads(struct task_struct *p)
>  
>  	while_each_thread(p, t) {
>  		task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
> -		count++;
> +		/* Don't require de_thread to wait for the vhost_worker */
> +		if ((t->flags & (PF_IO_WORKER | PF_USER_WORKER)) !=
PF_USER_WORKER)
> +			count++;
Well if you do this, then you should also change __exit_signal() to
not decrement sig->notify_count. Otherwise de_thread() can succeed
before the "normal" sub-threads exit.

But this can't be right anyway... If nothing else, suppose we have
a process with 3 threads:

	M	- the main thread, group leader
	T	- sub-thread
	V	- vhost worker

T does exec and calls de_thread().

M exits. T takes the leadership and does release_task()

V is still running but V->group_leader points to already freed M.

Or unshare_sighand() after that... If nothing else this means that
lock_task_sighand(T) and lock_task_sighand(V) will take different
locks.

Oleg.