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peterz at infradead.org

2020-Aug-05 14:12 UTC

[PATCH] x86/paravirt: Add missing noinstr to arch_local*() helpers

On Wed, Aug 05, 2020 at 03:59:40PM +0200, Marco Elver
wrote:> On Wed, Aug 05, 2020 at 03:42PM +0200, peterz at infradead.org wrote:
> > Shouldn't we __always_inline those? They're going to be really
small.
> 
> I can send a v2, and you can choose. For reference, though:
> 
> 	ffffffff86271ee0 <arch_local_save_flags>:
> 	ffffffff86271ee0:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
> 	ffffffff86271ee5:       48 83 3d 43 87 e4 01    cmpq  
$0x0,0x1e48743(%rip)        # ffffffff880ba630 <pv_ops+0x120>
> 	ffffffff86271eec:       00
> 	ffffffff86271eed:       74 0d                   je     ffffffff86271efc
<arch_local_save_flags+0x1c>
> 	ffffffff86271eef:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
> 	ffffffff86271ef4:       ff 14 25 30 a6 0b 88    callq  *0xffffffff880ba630
> 	ffffffff86271efb:       c3                      retq
> 	ffffffff86271efc:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
> 	ffffffff86271f01:       0f 0b                   ud2
> 	ffffffff86271a90 <arch_local_irq_restore>:
> 	ffffffff86271a90:       53                      push   %rbx
> 	ffffffff86271a91:       48 89 fb                mov    %rdi,%rbx
> 	ffffffff86271a94:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
> 	ffffffff86271a99:       48 83 3d 97 8b e4 01    cmpq  
$0x0,0x1e48b97(%rip)        # ffffffff880ba638 <pv_ops+0x128>
> 	ffffffff86271aa0:       00
> 	ffffffff86271aa1:       74 11                   je     ffffffff86271ab4
<arch_local_irq_restore+0x24>
> 	ffffffff86271aa3:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
> 	ffffffff86271aa8:       48 89 df                mov    %rbx,%rdi
> 	ffffffff86271aab:       ff 14 25 38 a6 0b 88    callq  *0xffffffff880ba638
> 	ffffffff86271ab2:       5b                      pop    %rbx
> 	ffffffff86271ab3:       c3                      retq
> 	ffffffff86271ab4:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
> 	ffffffff86271ab9:       0f 0b                   ud2

Blergh, that's abysmall. In part I suspect because you have
CONFIG_PARAVIRT_DEBUG, let me try and untangle that PV macro maze.

Jürgen Groß

2020-Aug-05 14:17 UTC

head link

[PATCH] x86/paravirt: Add missing noinstr to arch_local*() helpers

On 05.08.20 16:12, peterz at infradead.org wrote:> On Wed, Aug 05, 2020 at 03:59:40PM +0200, Marco Elver wrote:
>> On Wed, Aug 05, 2020 at 03:42PM +0200, peterz at infradead.org wrote:
> 
>>> Shouldn't we __always_inline those? They're going to be
really small.
>>
>> I can send a v2, and you can choose. For reference, though:
>>
>> 	ffffffff86271ee0 <arch_local_save_flags>:
>> 	ffffffff86271ee0:       0f 1f 44 00 00          nopl  
0x0(%rax,%rax,1)
>> 	ffffffff86271ee5:       48 83 3d 43 87 e4 01    cmpq  
$0x0,0x1e48743(%rip)        # ffffffff880ba630 <pv_ops+0x120>
>> 	ffffffff86271eec:       00
>> 	ffffffff86271eed:       74 0d                   je    
ffffffff86271efc <arch_local_save_flags+0x1c>
>> 	ffffffff86271eef:       0f 1f 44 00 00          nopl  
0x0(%rax,%rax,1)
>> 	ffffffff86271ef4:       ff 14 25 30 a6 0b 88    callq 
*0xffffffff880ba630
>> 	ffffffff86271efb:       c3                      retq
>> 	ffffffff86271efc:       0f 1f 44 00 00          nopl  
0x0(%rax,%rax,1)
>> 	ffffffff86271f01:       0f 0b                   ud2
> 
>> 	ffffffff86271a90 <arch_local_irq_restore>:
>> 	ffffffff86271a90:       53                      push   %rbx
>> 	ffffffff86271a91:       48 89 fb                mov    %rdi,%rbx
>> 	ffffffff86271a94:       0f 1f 44 00 00          nopl  
0x0(%rax,%rax,1)
>> 	ffffffff86271a99:       48 83 3d 97 8b e4 01    cmpq  
$0x0,0x1e48b97(%rip)        # ffffffff880ba638 <pv_ops+0x128>
>> 	ffffffff86271aa0:       00
>> 	ffffffff86271aa1:       74 11                   je    
ffffffff86271ab4 <arch_local_irq_restore+0x24>
>> 	ffffffff86271aa3:       0f 1f 44 00 00          nopl  
0x0(%rax,%rax,1)
>> 	ffffffff86271aa8:       48 89 df                mov    %rbx,%rdi
>> 	ffffffff86271aab:       ff 14 25 38 a6 0b 88    callq 
*0xffffffff880ba638
>> 	ffffffff86271ab2:       5b                      pop    %rbx
>> 	ffffffff86271ab3:       c3                      retq
>> 	ffffffff86271ab4:       0f 1f 44 00 00          nopl  
0x0(%rax,%rax,1)
>> 	ffffffff86271ab9:       0f 0b                   ud2
> 
> 
> Blergh, that's abysmall. In part I suspect because you have
> CONFIG_PARAVIRT_DEBUG, let me try and untangle that PV macro maze.
> 
Probably. I have found the following in my kernel:

fffffff81540a5f <arch_local_save_flags>:
ffffffff81540a5f:   ff 14 25 40 a4 23 82    callq  *0xffffffff8223a440
ffffffff81540a66:   c3                      retq


Juergen

peterz at infradead.org

2020-Aug-05 14:17 UTC

head link

[PATCH] x86/paravirt: Add missing noinstr to arch_local*() helpers

On Wed, Aug 05, 2020 at 04:12:37PM +0200, peterz at infradead.org
wrote:> On Wed, Aug 05, 2020 at 03:59:40PM +0200, Marco Elver wrote:
> > On Wed, Aug 05, 2020 at 03:42PM +0200, peterz at infradead.org wrote:
> 
> > > Shouldn't we __always_inline those? They're going to be
really small.
> > 
> > I can send a v2, and you can choose. For reference, though:
> > 
> > 	ffffffff86271ee0 <arch_local_save_flags>:
> > 	ffffffff86271ee0:       0f 1f 44 00 00          nopl  
0x0(%rax,%rax,1)
> > 	ffffffff86271ee5:       48 83 3d 43 87 e4 01    cmpq  
$0x0,0x1e48743(%rip)        # ffffffff880ba630 <pv_ops+0x120>
> > 	ffffffff86271eec:       00
> > 	ffffffff86271eed:       74 0d                   je    
ffffffff86271efc <arch_local_save_flags+0x1c>
> > 	ffffffff86271eef:       0f 1f 44 00 00          nopl  
0x0(%rax,%rax,1)
> > 	ffffffff86271ef4:       ff 14 25 30 a6 0b 88    callq 
*0xffffffff880ba630
> > 	ffffffff86271efb:       c3                      retq
> > 	ffffffff86271efc:       0f 1f 44 00 00          nopl  
0x0(%rax,%rax,1)
> > 	ffffffff86271f01:       0f 0b                   ud2
> 
> > 	ffffffff86271a90 <arch_local_irq_restore>:
> > 	ffffffff86271a90:       53                      push   %rbx
> > 	ffffffff86271a91:       48 89 fb                mov    %rdi,%rbx
> > 	ffffffff86271a94:       0f 1f 44 00 00          nopl  
0x0(%rax,%rax,1)
> > 	ffffffff86271a99:       48 83 3d 97 8b e4 01    cmpq  
$0x0,0x1e48b97(%rip)        # ffffffff880ba638 <pv_ops+0x128>
> > 	ffffffff86271aa0:       00
> > 	ffffffff86271aa1:       74 11                   je    
ffffffff86271ab4 <arch_local_irq_restore+0x24>
> > 	ffffffff86271aa3:       0f 1f 44 00 00          nopl  
0x0(%rax,%rax,1)
> > 	ffffffff86271aa8:       48 89 df                mov    %rbx,%rdi
> > 	ffffffff86271aab:       ff 14 25 38 a6 0b 88    callq 
*0xffffffff880ba638
> > 	ffffffff86271ab2:       5b                      pop    %rbx
> > 	ffffffff86271ab3:       c3                      retq
> > 	ffffffff86271ab4:       0f 1f 44 00 00          nopl  
0x0(%rax,%rax,1)
> > 	ffffffff86271ab9:       0f 0b                   ud2
> 
> 
> Blergh, that's abysmall. In part I suspect because you have
> CONFIG_PARAVIRT_DEBUG, let me try and untangle that PV macro maze.
Yeah, look here:

0000 0000000000462149 <arch_local_save_flags>:
0000   462149:  ff 14 25 00 00 00 00    callq  *0x0
0003                    46214c: R_X86_64_32S    pv_ops+0x120
0007   462150:  c3                      retq


That's exactly what I was expecting.

peterz at infradead.org

2020-Aug-06 11:32 UTC

head link

[PATCH] x86/paravirt: Add missing noinstr to arch_local*() helpers

On Thu, Aug 06, 2020 at 09:47:23AM +0200, Marco Elver
wrote:> Testing my hypothesis that raw then nested non-raw
> local_irq_save/restore() breaks IRQ state tracking -- see the reproducer
> below. This is at least 1 case I can think of that we're bound to hit.
Aaargh!
> diff --git a/init/main.c b/init/main.c
> index 15bd0efff3df..0873319dcff4 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1041,6 +1041,22 @@ asmlinkage __visible void __init start_kernel(void)
>  	sfi_init_late();
>  	kcsan_init();
>  
> +	/* DEBUG CODE */
> +	lockdep_assert_irqs_enabled(); /* Pass. */
> +	{
> +		unsigned long flags1;
> +		raw_local_irq_save(flags1);
This disables IRQs but doesn't trace..
> +		{
> +			unsigned long flags2;
> +			lockdep_assert_irqs_enabled(); /* Pass - expectedly blind. */
Indeed, we didn't trace the above disable, so software state is still
on.
> +			local_irq_save(flags2);
So here we save IRQ state, and unconditionally disable IRQs and trace
them disabled.
> +			lockdep_assert_irqs_disabled(); /* Pass. */
> +			local_irq_restore(flags2);
But here, we restore IRQ state to 'disabled' and explicitly trace it
disabled *again* (which is a bit daft, but whatever).
> +		}
> +		raw_local_irq_restore(flags1);
This then restores the IRQ state to enable, but no tracing.
> +	}
> +	lockdep_assert_irqs_enabled(); /* FAIL! */
And we're out of sync... :/

/me goes ponder things...

How's something like this then?

---
 include/linux/sched.h |  3 ---
 kernel/kcsan/core.c   | 62 ++++++++++++++++++++++++++++++++++++---------------
 2 files changed, 44 insertions(+), 21 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 06ec60462af0..2f5aef57e687 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1193,9 +1193,6 @@ struct task_struct {
 
 #ifdef CONFIG_KCSAN
 	struct kcsan_ctx		kcsan_ctx;
-#ifdef CONFIG_TRACE_IRQFLAGS
-	struct irqtrace_events		kcsan_save_irqtrace;
-#endif
 #endif
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 9147ff6a12e5..9c4436bf0561 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -291,17 +291,50 @@ static inline unsigned int get_delay(void)
 				0);
 }
 
-void kcsan_save_irqtrace(struct task_struct *task)
+/*
+ * KCSAN hooks are everywhere, which means they're NMI like for interrupt
+ * tracing. In order to present a 'normal' as possible context to the
code
+ * called by KCSAN when reporting errors we need to update the irq-tracing
+ * state.
+ *
+ * Save and restore the IRQ state trace touched by KCSAN, since KCSAN's
+ * runtime is entered for every memory access, and potentially useful
+ * information is lost if dirtied by KCSAN.
+ */
+
+struct kcsan_irq_state {
+	unsigned long		flags;
+#ifdef CONFIG_TRACE_IRQFLAGS
+	int			hardirqs;
+	struct irqtrace_events	irqtrace;
+#endif
+};
+
+void kcsan_save_irqtrace(struct kcsan_irq_state *irq_state)
 {
 #ifdef CONFIG_TRACE_IRQFLAGS
-	task->kcsan_save_irqtrace = task->irqtrace;
+	irq_state->irqtrace = task->irqtrace;
+	irq_state->hardirq = lockdep_hardirqs_enabled();
 #endif
+	if (!kcsan_interrupt_watcher) {
+		raw_local_irq_save(irq_state->flags);
+		lockdep_hardirqs_off(CALLER_ADDR0);
+	}
 }
 
-void kcsan_restore_irqtrace(struct task_struct *task)
+void kcsan_restore_irqtrace(struct kcsan_irq_state *irq_state)
 {
+	if (!kcsan_interrupt_watcher) {
+#ifdef CONFIG_TRACE_IRQFLAGS
+		if (irq_state->hardirqs) {
+			lockdep_hardirqs_on_prepare(CALLER_ADDR0);
+			lockdep_hardirqs_on(CALLER_ADDR0);
+		}
+#endif
+		raw_local_irq_restore(irq_state->flags);
+	}
 #ifdef CONFIG_TRACE_IRQFLAGS
-	task->irqtrace = task->kcsan_save_irqtrace;
+	task->irqtrace = irq_state->irqtrace;
 #endif
 }
 
@@ -350,11 +383,13 @@ static noinline void kcsan_found_watchpoint(const volatile
void *ptr,
 	flags = user_access_save();
 
 	if (consumed) {
-		kcsan_save_irqtrace(current);
+		struct kcsan_irq_state irqstate;
+
+		kcsan_save_irqtrace(&irqstate);
 		kcsan_report(ptr, size, type, KCSAN_VALUE_CHANGE_MAYBE,
 			     KCSAN_REPORT_CONSUMED_WATCHPOINT,
 			     watchpoint - watchpoints);
-		kcsan_restore_irqtrace(current);
+		kcsan_restore_irqtrace(&irqstate);
 	} else {
 		/*
 		 * The other thread may not print any diagnostics, as it has
@@ -387,7 +422,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t
size, int type)
 	unsigned long access_mask;
 	enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE;
 	unsigned long ua_flags = user_access_save();
-	unsigned long irq_flags = 0;
+	struct kcsan_irq_state irqstate;
 
 	/*
 	 * Always reset kcsan_skip counter in slow-path to avoid underflow; see
@@ -412,14 +447,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t
size, int type)
 		goto out;
 	}
 
-	/*
-	 * Save and restore the IRQ state trace touched by KCSAN, since KCSAN's
-	 * runtime is entered for every memory access, and potentially useful
-	 * information is lost if dirtied by KCSAN.
-	 */
-	kcsan_save_irqtrace(current);
-	if (!kcsan_interrupt_watcher)
-		local_irq_save(irq_flags);
+	kcsan_save_irqtrace(&irqstate);
 
 	watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
 	if (watchpoint == NULL) {
@@ -559,9 +587,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t
size, int type)
 	remove_watchpoint(watchpoint);
 	kcsan_counter_dec(KCSAN_COUNTER_USED_WATCHPOINTS);
 out_unlock:
-	if (!kcsan_interrupt_watcher)
-		local_irq_restore(irq_flags);
-	kcsan_restore_irqtrace(current);
+	kcsan_restore_irqtrace(&irqstate);
 out:
 	user_access_restore(ua_flags);
 }

Jürgen Groß

2020-Aug-07 09:24 UTC

head link

[PATCH] x86/paravirt: Add missing noinstr to arch_local*() helpers

On 07.08.20 11:01, Marco Elver wrote:> On Thu, 6 Aug 2020 at 18:06, Marco Elver <elver at google.com> wrote:
>> On Thu, 6 Aug 2020 at 15:17, Marco Elver <elver at google.com>
wrote:
>>> On Thu, Aug 06, 2020 at 01:32PM +0200, peterz at infradead.org
wrote:
>>>> On Thu, Aug 06, 2020 at 09:47:23AM +0200, Marco Elver wrote:
>>>>> Testing my hypothesis that raw then nested non-raw
>>>>> local_irq_save/restore() breaks IRQ state tracking -- see
the reproducer
>>>>> below. This is at least 1 case I can think of that
we're bound to hit.
>>> ...
>>>>
>>>> /me goes ponder things...
>>>>
>>>> How's something like this then?
>>>>
>>>> ---
>>>>   include/linux/sched.h |  3 ---
>>>>   kernel/kcsan/core.c   | 62
++++++++++++++++++++++++++++++++++++---------------
>>>>   2 files changed, 44 insertions(+), 21 deletions(-)
>>>
>>> Thank you! That approach seems to pass syzbot (also with
>>> CONFIG_PARAVIRT) and kcsan-test tests.
>>>
>>> I had to modify it some, so that report.c's use of the restore
logic
>>> works and not mess up the IRQ trace printed on KCSAN reports (with
>>> CONFIG_KCSAN_VERBOSE).
>>>
>>> I still need to fully convince myself all is well now and we
don't end
>>> up with more fixes. :-) If it passes further testing, I'll send
it as a
>>> real patch (I want to add you as Co-developed-by, but would need
your
>>> Signed-off-by for the code you pasted, I think.)
> 
> I let it run on syzbot through the night, and it's fine without
> PARAVIRT (see below). I have sent the patch (need your Signed-off-by
> as it's based on your code, thank you!):
> https://lkml.kernel.org/r/20200807090031.3506555-1-elver at google.com
> 
>> With CONFIG_PARAVIRT=y (without the notrace->noinstr patch), I still
>> get lockdep DEBUG_LOCKS_WARN_ON(!lockdep_hardirqs_enabled()), although
>> it takes longer for syzbot to hit them. But I think that's expected
>> because we can still get the recursion that I pointed out, and will
>> need that patch.
> 
> Never mind, I get these warnings even if I don't turn on KCSAN
> (CONFIG_KCSAN=n). Something else is going on with PARAVIRT=y that
> throws off IRQ state tracking. :-/
What are the settings of CONFIG_PARAVIRT_XXL and
CONFIG_PARAVIRT_SPINLOCKS in this case?


Juergen

Jürgen Groß

2020-Aug-07 10:35 UTC

head link

[PATCH] x86/paravirt: Add missing noinstr to arch_local*() helpers

On 07.08.20 11:50, Marco Elver wrote:> On Fri, Aug 07, 2020 at 11:24AM +0200, J?rgen Gro? wrote:
>> On 07.08.20 11:01, Marco Elver wrote:
>>> On Thu, 6 Aug 2020 at 18:06, Marco Elver <elver at
google.com> wrote:
>>>> On Thu, 6 Aug 2020 at 15:17, Marco Elver <elver at
google.com> wrote:
>>>>> On Thu, Aug 06, 2020 at 01:32PM +0200, peterz at
infradead.org wrote:
>>>>>> On Thu, Aug 06, 2020 at 09:47:23AM +0200, Marco Elver
wrote:
>>>>>>> Testing my hypothesis that raw then nested non-raw
>>>>>>> local_irq_save/restore() breaks IRQ state tracking
-- see the reproducer
>>>>>>> below. This is at least 1 case I can think of that
we're bound to hit.
>>>>> ...
>>>>>>
>>>>>> /me goes ponder things...
>>>>>>
>>>>>> How's something like this then?
>>>>>>
>>>>>> ---
>>>>>>    include/linux/sched.h |  3 ---
>>>>>>    kernel/kcsan/core.c   | 62
++++++++++++++++++++++++++++++++++++---------------
>>>>>>    2 files changed, 44 insertions(+), 21 deletions(-)
>>>>>
>>>>> Thank you! That approach seems to pass syzbot (also with
>>>>> CONFIG_PARAVIRT) and kcsan-test tests.
>>>>>
>>>>> I had to modify it some, so that report.c's use of the
restore logic
>>>>> works and not mess up the IRQ trace printed on KCSAN
reports (with
>>>>> CONFIG_KCSAN_VERBOSE).
>>>>>
>>>>> I still need to fully convince myself all is well now and
we don't end
>>>>> up with more fixes. :-) If it passes further testing,
I'll send it as a
>>>>> real patch (I want to add you as Co-developed-by, but would
need your
>>>>> Signed-off-by for the code you pasted, I think.)
>>>
>>> I let it run on syzbot through the night, and it's fine without
>>> PARAVIRT (see below). I have sent the patch (need your
Signed-off-by
>>> as it's based on your code, thank you!):
>>> https://lkml.kernel.org/r/20200807090031.3506555-1-elver at
google.com
>>>
>>>> With CONFIG_PARAVIRT=y (without the notrace->noinstr patch),
I still
>>>> get lockdep DEBUG_LOCKS_WARN_ON(!lockdep_hardirqs_enabled()),
although
>>>> it takes longer for syzbot to hit them. But I think that's
expected
>>>> because we can still get the recursion that I pointed out, and
will
>>>> need that patch.
>>>
>>> Never mind, I get these warnings even if I don't turn on KCSAN
>>> (CONFIG_KCSAN=n). Something else is going on with PARAVIRT=y that
>>> throws off IRQ state tracking. :-/
>>
>> What are the settings of CONFIG_PARAVIRT_XXL and
>> CONFIG_PARAVIRT_SPINLOCKS in this case?
> 
> I attached a config.
> 
> 	$> grep PARAVIRT .config
> 	CONFIG_PARAVIRT=y
> 	CONFIG_PARAVIRT_XXL=y
> 	# CONFIG_PARAVIRT_DEBUG is not set
> 	CONFIG_PARAVIRT_SPINLOCKS=y
> 	# CONFIG_PARAVIRT_TIME_ACCOUNTING is not set
> 	CONFIG_PARAVIRT_CLOCK=y
Anything special I need to do to reproduce the problem? Or would you be
willing to do some more rounds with different config settings?

I think CONFIG_PARAVIRT_XXL shouldn't matter, but I'm not completely
sure about that. CONFIG_PARAVIRT_SPINLOCKS would be my primary suspect.


Juergen

Jürgen Groß

2020-Aug-07 12:04 UTC

head link

[PATCH] x86/paravirt: Add missing noinstr to arch_local*() helpers

On 07.08.20 13:38, Marco Elver wrote:> On Fri, Aug 07, 2020 at 12:35PM +0200, J?rgen Gro? wrote:
>> On 07.08.20 11:50, Marco Elver wrote:
>>> On Fri, Aug 07, 2020 at 11:24AM +0200, J?rgen Gro? wrote:
>>>> On 07.08.20 11:01, Marco Elver wrote:
>>>>> On Thu, 6 Aug 2020 at 18:06, Marco Elver <elver at
google.com> wrote:
>>>>>> On Thu, 6 Aug 2020 at 15:17, Marco Elver <elver at
google.com> wrote:
>>>>>>> On Thu, Aug 06, 2020 at 01:32PM +0200, peterz at
infradead.org wrote:
>>>>>>>> On Thu, Aug 06, 2020 at 09:47:23AM +0200, Marco
Elver wrote:
>>>>>>>>> Testing my hypothesis that raw then nested
non-raw
>>>>>>>>> local_irq_save/restore() breaks IRQ state
tracking -- see the reproducer
>>>>>>>>> below. This is at least 1 case I can think
of that we're bound to hit.
>>>>>>> ...
>>>>>>>>
>>>>>>>> /me goes ponder things...
>>>>>>>>
>>>>>>>> How's something like this then?
>>>>>>>>
>>>>>>>> ---
>>>>>>>>     include/linux/sched.h |  3 ---
>>>>>>>>     kernel/kcsan/core.c   | 62
++++++++++++++++++++++++++++++++++++---------------
>>>>>>>>     2 files changed, 44 insertions(+), 21
deletions(-)
>>>>>>>
>>>>>>> Thank you! That approach seems to pass syzbot (also
with
>>>>>>> CONFIG_PARAVIRT) and kcsan-test tests.
>>>>>>>
>>>>>>> I had to modify it some, so that report.c's use
of the restore logic
>>>>>>> works and not mess up the IRQ trace printed on
KCSAN reports (with
>>>>>>> CONFIG_KCSAN_VERBOSE).
>>>>>>>
>>>>>>> I still need to fully convince myself all is well
now and we don't end
>>>>>>> up with more fixes. :-) If it passes further
testing, I'll send it as a
>>>>>>> real patch (I want to add you as Co-developed-by,
but would need your
>>>>>>> Signed-off-by for the code you pasted, I think.)
>>>>>
>>>>> I let it run on syzbot through the night, and it's fine
without
>>>>> PARAVIRT (see below). I have sent the patch (need your
Signed-off-by
>>>>> as it's based on your code, thank you!):
>>>>> https://lkml.kernel.org/r/20200807090031.3506555-1-elver at
google.com
>>>>>
>>>>>> With CONFIG_PARAVIRT=y (without the notrace->noinstr
patch), I still
>>>>>> get lockdep
DEBUG_LOCKS_WARN_ON(!lockdep_hardirqs_enabled()), although
>>>>>> it takes longer for syzbot to hit them. But I think
that's expected
>>>>>> because we can still get the recursion that I pointed
out, and will
>>>>>> need that patch.
>>>>>
>>>>> Never mind, I get these warnings even if I don't turn
on KCSAN
>>>>> (CONFIG_KCSAN=n). Something else is going on with
PARAVIRT=y that
>>>>> throws off IRQ state tracking. :-/
>>>>
>>>> What are the settings of CONFIG_PARAVIRT_XXL and
>>>> CONFIG_PARAVIRT_SPINLOCKS in this case?
>>>
>>> I attached a config.
>>>
>>> 	$> grep PARAVIRT .config
>>> 	CONFIG_PARAVIRT=y
>>> 	CONFIG_PARAVIRT_XXL=y
>>> 	# CONFIG_PARAVIRT_DEBUG is not set
>>> 	CONFIG_PARAVIRT_SPINLOCKS=y
>>> 	# CONFIG_PARAVIRT_TIME_ACCOUNTING is not set
>>> 	CONFIG_PARAVIRT_CLOCK=y
>>
>> Anything special I need to do to reproduce the problem? Or would you be
>> willing to do some more rounds with different config settings?
> 
> I can only test it with syzkaller, but that probably doesn't help if
you
> don't already have it set up. It can't seem to find a C reproducer.
> 
> I did some more rounds with different configs.
> 
>> I think CONFIG_PARAVIRT_XXL shouldn't matter, but I'm not
completely
>> sure about that. CONFIG_PARAVIRT_SPINLOCKS would be my primary suspect.
> 
> Yes, PARAVIRT_XXL doesn't make a different. When disabling
> PARAVIRT_SPINLOCKS, however, the warnings go away.
Thanks for testing!

I take it you are doing the tests in a KVM guest?

If so I have a gut feeling that the use of local_irq_save() and
local_irq_restore() in kvm_wait() might be fishy. I might be completely
wrong here, though.

BTW, I think Xen's variant of pv spinlocks is fine (no playing with IRQ
on/off).

Hyper-V seems to do the same as KVM, and kicking another vcpu could be
problematic as well, as it is just using IPI.


Juergen

Jürgen Groß

2020-Aug-11 07:04 UTC

head link

[PATCH] x86/paravirt: Add missing noinstr to arch_local*() helpers

On 11.08.20 09:00, Marco Elver wrote:> On Fri, 7 Aug 2020 at 17:19, Marco Elver <elver at google.com> wrote:
>> On Fri, Aug 07, 2020 at 02:08PM +0200, Marco Elver wrote:
>>> On Fri, 7 Aug 2020 at 14:04, J?rgen Gro? <jgross at suse.com>
wrote:
>>>>
>>>> On 07.08.20 13:38, Marco Elver wrote:
>>>>> On Fri, Aug 07, 2020 at 12:35PM +0200, J?rgen Gro? wrote:
> ...
>>>>>> I think CONFIG_PARAVIRT_XXL shouldn't matter, but
I'm not completely
>>>>>> sure about that. CONFIG_PARAVIRT_SPINLOCKS would be my
primary suspect.
>>>>>
>>>>> Yes, PARAVIRT_XXL doesn't make a different. When
disabling
>>>>> PARAVIRT_SPINLOCKS, however, the warnings go away.
>>>>
>>>> Thanks for testing!
>>>>
>>>> I take it you are doing the tests in a KVM guest?
>>>
>>> Yes, correct.
>>>
>>>> If so I have a gut feeling that the use of local_irq_save() and
>>>> local_irq_restore() in kvm_wait() might be fishy. I might be
completely
>>>> wrong here, though.
>>>
>>> Happy to help debug more, although I might need patches or pointers
>>> what to play with.
>>>
>>>> BTW, I think Xen's variant of pv spinlocks is fine (no
playing with IRQ
>>>> on/off).
>>>>
>>>> Hyper-V seems to do the same as KVM, and kicking another vcpu
could be
>>>> problematic as well, as it is just using IPI.
>>
>> I experimented a bit more, and the below patch seems to solve the
>> warnings. However, that was based on your pointer about kvm_wait(), and
>> I can't quite tell if it is the right solution.
>>
>> My hypothesis here is simply that kvm_wait() may be called in a place
>> where we get the same case I mentioned to Peter,
>>
>>          raw_local_irq_save(); /* or other IRQs off without tracing */
>>          ...
>>          kvm_wait() /* IRQ state tracing gets confused */
>>          ...
>>          raw_local_irq_restore();
>>
>> and therefore, using raw variants in kvm_wait() works. It's also
safe
>> because it doesn't call any other libraries that would result in
corrupt
>> IRQ state AFAIK.
> 
> Just to follow-up, it'd still be nice to fix this. Suggestions?
> 
> I could send the below as a patch, but can only go off my above
> hypothesis and the fact that syzbot is happier, so not entirely
> convincing.
Peter has told me via IRC he will look soon further into this.

Your finding suggests that the pv-lock implementation for Hyper-V
needs some tweaking, too. For that purpose I'm adding Wei to Cc.


Juergen
> 
> Thanks,
> -- Marco
> 
>> ------ >8 ------
>>
>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> index 233c77d056c9..1d412d1466f0 100644
>> --- a/arch/x86/kernel/kvm.c
>> +++ b/arch/x86/kernel/kvm.c
>> @@ -797,7 +797,7 @@ static void kvm_wait(u8 *ptr, u8 val)
>>          if (in_nmi())
>>                  return;
>>
>> -       local_irq_save(flags);
>> +       raw_local_irq_save(flags);
>>
>>          if (READ_ONCE(*ptr) != val)
>>                  goto out;
>> @@ -810,10 +810,10 @@ static void kvm_wait(u8 *ptr, u8 val)
>>          if (arch_irqs_disabled_flags(flags))
>>                  halt();
>>          else
>> -               safe_halt();
>> +               raw_safe_halt();
>>
>>   out:
>> -       local_irq_restore(flags);
>> +       raw_local_irq_restore(flags);
>>   }
>>
>>   #ifdef CONFIG_X86_32
>

Peter Zijlstra

2020-Aug-11 07:41 UTC

head link

[PATCH] x86/paravirt: Add missing noinstr to arch_local*() helpers

On Fri, Aug 07, 2020 at 05:19:03PM +0200, Marco Elver wrote:
> My hypothesis here is simply that kvm_wait() may be called in a place
> where we get the same case I mentioned to Peter,
> 
> 	raw_local_irq_save(); /* or other IRQs off without tracing */
> 	...
> 	kvm_wait() /* IRQ state tracing gets confused */
> 	...
> 	raw_local_irq_restore();
> 
> and therefore, using raw variants in kvm_wait() works. It's also safe
> because it doesn't call any other libraries that would result in
corrupt
Yes, this is definitely an issue.

Tracing, we also musn't call into tracing when using raw_local_irq_*().
Because then we re-intoduce this same issue all over again.

Both halt() and safe_halt() are more paravirt calls, but given we're in
a KVM paravirt call already, I suppose we can directly use native_*()
here.

Something like so then... I suppose, but then the Xen variants need TLC
too.

---
 arch/x86/include/asm/irqflags.h |  4 ++--
 arch/x86/include/asm/kvm_para.h | 18 +++++++++---------
 arch/x86/kernel/kvm.c           | 14 +++++++-------
 3 files changed, 18 insertions(+), 18 deletions(-)

diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
index 02a0cf547d7b..7c614db25274 100644
--- a/arch/x86/include/asm/irqflags.h
+++ b/arch/x86/include/asm/irqflags.h
@@ -54,13 +54,13 @@ static __always_inline void native_irq_enable(void)
 	asm volatile("sti": : :"memory");
 }

-static inline __cpuidle void native_safe_halt(void)
+static __always_inline __cpuidle void native_safe_halt(void)
 {
 	mds_idle_clear_cpu_buffers();
 	asm volatile("sti; hlt": : :"memory");
 }

-static inline __cpuidle void native_halt(void)
+static __always_inline __cpuidle void native_halt(void)
 {
 	mds_idle_clear_cpu_buffers();
 	asm volatile("hlt": : :"memory");
diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
index 49d3a9edb06f..90f7ea58ebb0 100644
--- a/arch/x86/include/asm/kvm_para.h
+++ b/arch/x86/include/asm/kvm_para.h
@@ -30,7 +30,7 @@ static inline bool kvm_check_and_clear_guest_paused(void)
  * noted by the particular hypercall.
  */

-static inline long kvm_hypercall0(unsigned int nr)
+static __always_inline long kvm_hypercall0(unsigned int nr)
 {
 	long ret;
 	asm volatile(KVM_HYPERCALL
@@ -40,7 +40,7 @@ static inline long kvm_hypercall0(unsigned int nr)
 	return ret;
 }

-static inline long kvm_hypercall1(unsigned int nr, unsigned long p1)
+static __always_inline long kvm_hypercall1(unsigned int nr, unsigned long p1)
 {
 	long ret;
 	asm volatile(KVM_HYPERCALL
@@ -50,8 +50,8 @@ static inline long kvm_hypercall1(unsigned int nr, unsigned
long p1)
 	return ret;
 }

-static inline long kvm_hypercall2(unsigned int nr, unsigned long p1,
-				  unsigned long p2)
+static __always_inline long kvm_hypercall2(unsigned int nr, unsigned long p1,
+					   unsigned long p2)
 {
 	long ret;
 	asm volatile(KVM_HYPERCALL
@@ -61,8 +61,8 @@ static inline long kvm_hypercall2(unsigned int nr, unsigned
long p1,
 	return ret;
 }

-static inline long kvm_hypercall3(unsigned int nr, unsigned long p1,
-				  unsigned long p2, unsigned long p3)
+static __always_inline long kvm_hypercall3(unsigned int nr, unsigned long p1,
+					   unsigned long p2, unsigned long p3)
 {
 	long ret;
 	asm volatile(KVM_HYPERCALL
@@ -72,9 +72,9 @@ static inline long kvm_hypercall3(unsigned int nr, unsigned
long p1,
 	return ret;
 }

-static inline long kvm_hypercall4(unsigned int nr, unsigned long p1,
-				  unsigned long p2, unsigned long p3,
-				  unsigned long p4)
+static __always_inline long kvm_hypercall4(unsigned int nr, unsigned long p1,
+					   unsigned long p2, unsigned long p3,
+					   unsigned long p4)
 {
 	long ret;
 	asm volatile(KVM_HYPERCALL
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 233c77d056c9..15f8dfd8812d 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -779,7 +779,7 @@ arch_initcall(kvm_alloc_cpumask);
 #ifdef CONFIG_PARAVIRT_SPINLOCKS

 /* Kick a cpu by its apicid. Used to wake up a halted vcpu */
-static void kvm_kick_cpu(int cpu)
+static notrace kvm_kick_cpu(int cpu)
 {
 	int apicid;
 	unsigned long flags = 0;
@@ -790,14 +790,14 @@ static void kvm_kick_cpu(int cpu)

 #include <asm/qspinlock.h>

-static void kvm_wait(u8 *ptr, u8 val)
+static notrace kvm_wait(u8 *ptr, u8 val)
 {
 	unsigned long flags;

 	if (in_nmi())
 		return;

-	local_irq_save(flags);
+	raw_local_irq_save(flags);

 	if (READ_ONCE(*ptr) != val)
 		goto out;
@@ -808,16 +808,16 @@ static void kvm_wait(u8 *ptr, u8 val)
 	 * in irq spinlock slowpath and no spurious interrupt occur to save us.
 	 */
 	if (arch_irqs_disabled_flags(flags))
-		halt();
+		native_halt();
 	else
-		safe_halt();
+		native_safe_halt();

 out:
-	local_irq_restore(flags);
+	raw_local_irq_restore(flags);
 }

 #ifdef CONFIG_X86_32
-__visible bool __kvm_vcpu_is_preempted(long cpu)
+__visible notrace bool __kvm_vcpu_is_preempted(long cpu)
 {
 	struct kvm_steal_time *src = &per_cpu(steal_time, cpu);

Jürgen Groß

2020-Aug-11 07:57 UTC

head link

[PATCH] x86/paravirt: Add missing noinstr to arch_local*() helpers

On 11.08.20 09:41, Peter Zijlstra wrote:> On Fri, Aug 07, 2020 at 05:19:03PM +0200, Marco Elver wrote:
> 
>> My hypothesis here is simply that kvm_wait() may be called in a place
>> where we get the same case I mentioned to Peter,
>>
>> 	raw_local_irq_save(); /* or other IRQs off without tracing */
>> 	...
>> 	kvm_wait() /* IRQ state tracing gets confused */
>> 	...
>> 	raw_local_irq_restore();
>>
>> and therefore, using raw variants in kvm_wait() works. It's also
safe
>> because it doesn't call any other libraries that would result in
corrupt
> 
> Yes, this is definitely an issue.
> 
> Tracing, we also musn't call into tracing when using raw_local_irq_*().
> Because then we re-intoduce this same issue all over again.
> 
> Both halt() and safe_halt() are more paravirt calls, but given we're in
> a KVM paravirt call already, I suppose we can directly use native_*()
> here.
> 
> Something like so then... I suppose, but then the Xen variants need TLC
> too.
Just to be sure I understand you correct:

You mean that xen_qlock_kick() and xen_qlock_wait() and all functions
called by those should gain the "notrace" attribute, right?

I am not sure why the kick variants need it, though. IMO those are
called only after the lock has been released, so they should be fine
without notrace.

And again: we shouldn't forget the Hyper-V variants.


Juergen

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