Virtualization - Mar 2021 - [PATCH 2/2] vhost-vdpa: set v->config_ctx to NULL if eventfd_ctx_fdget() fails

While writing a test for a Rust library [1] to handle vhost-vdpa devices,
I experienced the 'use-after-free' issue fixed in patch 1, then I
discovered the potential issue when eventfd_ctx_fdget() fails fixed in
patch 2.

Do you think it might be useful to write a vdpa test suite, perhaps using
this Rust library [2] ?
Could we put it in the kernel tree in tool/testing?

Thanks,
Stefano

[1] https://github.com/stefano-garzarella/vhost/tree/vdpa
[2] https://github.com/rust-vmm/vhost

Stefano Garzarella (2):
  vhost-vdpa: fix use-after-free of v->config_ctx
  vhost-vdpa: set v->config_ctx to NULL if eventfd_ctx_fdget() fails

 drivers/vhost/vdpa.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

-- 
2.29.2

When the 'v->config_ctx' eventfd_ctx reference is released we
didn't
set it to NULL. So if the same character device (e.g. /dev/vhost-vdpa-0)
is re-opened, the 'v->config_ctx' is invalid and calling again
vhost_vdpa_config_put() causes use-after-free issues like the
following refcount_t underflow:

    refcount_t: underflow; use-after-free.
    WARNING: CPU: 2 PID: 872 at lib/refcount.c:28
refcount_warn_saturate+0xae/0xf0
    RIP: 0010:refcount_warn_saturate+0xae/0xf0
    Call Trace:
     eventfd_ctx_put+0x5b/0x70
     vhost_vdpa_release+0xcd/0x150 [vhost_vdpa]
     __fput+0x8e/0x240
     ____fput+0xe/0x10
     task_work_run+0x66/0xa0
     exit_to_user_mode_prepare+0x118/0x120
     syscall_exit_to_user_mode+0x21/0x50
     ? __x64_sys_close+0x12/0x40
     do_syscall_64+0x45/0x50
     entry_SYSCALL_64_after_hwframe+0x44/0xae

Fixes: 776f395004d8 ("vhost_vdpa: Support config interrupt in vdpa")
Cc: lingshan.zhu at intel.com
Cc: stable at vger.kernel.org
Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>
---
 drivers/vhost/vdpa.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
index ef688c8c0e0e..00796e4ecfdf 100644
--- a/drivers/vhost/vdpa.c
+++ b/drivers/vhost/vdpa.c
@@ -308,8 +308,10 @@ static long vhost_vdpa_get_vring_num(struct vhost_vdpa *v,
u16 __user *argp)
 
 static void vhost_vdpa_config_put(struct vhost_vdpa *v)
 {
-	if (v->config_ctx)
+	if (v->config_ctx) {
 		eventfd_ctx_put(v->config_ctx);
+		v->config_ctx = NULL;
+	}
 }
 
 static long vhost_vdpa_set_config_call(struct vhost_vdpa *v, u32 __user *argp)
-- 
2.29.2

In vhost_vdpa_set_config_call() if eventfd_ctx_fdget() fails the
'v->config_ctx' contains an error instead of a valid pointer.

Since we consider 'v->config_ctx' valid if it is not NULL, we should
set it to NULL in this case to avoid to use an invalid pointer in
other functions such as vhost_vdpa_config_put().

Fixes: 776f395004d8 ("vhost_vdpa: Support config interrupt in vdpa")
Cc: lingshan.zhu at intel.com
Cc: stable at vger.kernel.org
Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>
---
 drivers/vhost/vdpa.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
index 00796e4ecfdf..f9ecdce5468a 100644
--- a/drivers/vhost/vdpa.c
+++ b/drivers/vhost/vdpa.c
@@ -331,8 +331,12 @@ static long vhost_vdpa_set_config_call(struct vhost_vdpa
*v, u32 __user *argp)
 	if (!IS_ERR_OR_NULL(ctx))
 		eventfd_ctx_put(ctx);
 
-	if (IS_ERR(v->config_ctx))
-		return PTR_ERR(v->config_ctx);
+	if (IS_ERR(v->config_ctx)) {
+		long ret = PTR_ERR(v->config_ctx);
+
+		v->config_ctx = NULL;
+		return ret;
+	}
 
 	v->vdpa->config->set_config_cb(v->vdpa, &cb);
 
-- 
2.29.2

On Thu, Mar 11, 2021 at 02:52:55PM +0100, Stefano Garzarella
wrote:
> While writing a test for a Rust library [1] to handle vhost-vdpa devices,
> I experienced the 'use-after-free' issue fixed in patch 1, then I
> discovered the potential issue when eventfd_ctx_fdget() fails fixed in
> patch 2.
> 
> Do you think it might be useful to write a vdpa test suite, perhaps using
> this Rust library [2] ?
> Could we put it in the kernel tree in tool/testing?
I can add tools/vdpa, no problem ...
> Thanks,
> Stefano
> 
> [1] https://github.com/stefano-garzarella/vhost/tree/vdpa
> [2] https://github.com/rust-vmm/vhost
> 
> Stefano Garzarella (2):
>   vhost-vdpa: fix use-after-free of v->config_ctx
>   vhost-vdpa: set v->config_ctx to NULL if eventfd_ctx_fdget() fails
> 
>  drivers/vhost/vdpa.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> -- 
> 2.29.2