[PATCH RT] signal: Prevent double-free of user struct

[PATCH RT] signal: Prevent double-free of user struct

[Matt Fleming]

The way user struct reference counting works changed significantly with,

  fda31c50292a ("signal: avoid double atomic counter increments for user accounting")

Now user structs are only freed once the last pending signal is
dequeued. Make sigqueue_free_current() follow this new convention to
avoid freeing the user struct multiple times and triggering this
warning:

 refcount_t: underflow; use-after-free.
 WARNING: CPU: 0 PID: 6794 at lib/refcount.c:288 refcount_dec_not_one+0x45/0x50
 Call Trace:
  refcount_dec_and_lock_irqsave+0x16/0x60
  free_uid+0x31/0xa0
  ? schedule_hrtimeout_range_clock+0x104/0x110
  __dequeue_signal+0x17c/0x190
  dequeue_signal+0x5a/0x1b0
  do_sigtimedwait+0x208/0x250
  __x64_sys_rt_sigtimedwait+0x6f/0xd0
  do_syscall_64+0x72/0x200
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Reported-by: Daniel Wagner <wagi@monom.org>
---
 kernel/signal.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 267fce07df5d..3651483bd4d8 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -494,8 +494,8 @@ static void sigqueue_free_current(struct sigqueue *q)
 
 	up = q->user;
 	if (rt_prio(current->normal_prio) && !put_task_cache(current, q)) {
-		atomic_dec(&up->sigpending);
-		free_uid(up);
+		if (atomic_dec_and_test(&up->sigpending))
+			free_uid(up);
 	} else
 		  __sigqueue_free(q);
 }
-- 
2.16.4

Re: [PATCH RT] signal: Prevent double-free of user struct

[Sebastian Andrzej Siewior]

On 2020-04-07 10:54:13 [+0100], Matt Fleming wrote:
> The way user struct reference counting works changed significantly with,
> 
>   fda31c50292a ("signal: avoid double atomic counter increments for user accounting")
> 
> Now user structs are only freed once the last pending signal is
> dequeued. Make sigqueue_free_current() follow this new convention to
> avoid freeing the user struct multiple times and triggering this
> warning:
> 
>  refcount_t: underflow; use-after-free.
>  WARNING: CPU: 0 PID: 6794 at lib/refcount.c:288 refcount_dec_not_one+0x45/0x50
>  Call Trace:
>   refcount_dec_and_lock_irqsave+0x16/0x60
>   free_uid+0x31/0xa0
>   ? schedule_hrtimeout_range_clock+0x104/0x110
>   __dequeue_signal+0x17c/0x190
>   dequeue_signal+0x5a/0x1b0
>   do_sigtimedwait+0x208/0x250
>   __x64_sys_rt_sigtimedwait+0x6f/0xd0
>   do_syscall_64+0x72/0x200
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe

While all this sounds reasonable, may I ask what did you do to trigger
this?
This is v5.6 only, correct?

Sebastian

Re: [PATCH RT] signal: Prevent double-free of user struct

[Daniel Wagner]

Hi Sebastian,

On 15.04.20 18:46, Sebastian Andrzej Siewior wrote:
> On 2020-04-07 10:54:13 [+0100], Matt Fleming wrote:
>> The way user struct reference counting works changed significantly with,
>>
>>    fda31c50292a ("signal: avoid double atomic counter increments for user accounting")
>>
>> Now user structs are only freed once the last pending signal is
>> dequeued. Make sigqueue_free_current() follow this new convention to
>> avoid freeing the user struct multiple times and triggering this
>> warning:
>>
>>   refcount_t: underflow; use-after-free.
>>   WARNING: CPU: 0 PID: 6794 at lib/refcount.c:288 refcount_dec_not_one+0x45/0x50
>>   Call Trace:
>>    refcount_dec_and_lock_irqsave+0x16/0x60
>>    free_uid+0x31/0xa0
>>    ? schedule_hrtimeout_range_clock+0x104/0x110
>>    __dequeue_signal+0x17c/0x190
>>    dequeue_signal+0x5a/0x1b0
>>    do_sigtimedwait+0x208/0x250
>>    __x64_sys_rt_sigtimedwait+0x6f/0xd0
>>    do_syscall_64+0x72/0x200
>>    entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> While all this sounds reasonable, may I ask what did you do to trigger
> this?

This can be triggered by running sigwaittest.

  # sigwaittest -t -a -p 98

a few seconds should be enough to get the splat.

> This is v5.6 only, correct?

I've seen this also with a frankstein version of 5.2-rt...v5-4-rt :)

Thanks,
Daniel

Re: [PATCH RT] signal: Prevent double-free of user struct

[Sebastian Andrzej Siewior]

On 2020-04-15 21:05:14 [+0200], Daniel Wagner wrote:
> Hi Sebastian,
Hi Daniel,

> This can be triggered by running sigwaittest.
> 
>  # sigwaittest -t -a -p 98
> 
> a few seconds should be enough to get the splat.

thank you.

> > This is v5.6 only, correct?
> 
> I've seen this also with a frankstein version of 5.2-rt...v5-4-rt :)

…

> Thanks,
> Daniel

Sebastian

Re: [PATCH RT] signal: Prevent double-free of user struct

[Markus Elfring]

>  refcount_t: underflow; use-after-free.

I suggest to add the tag “Fixes” to the final change description.

Regards,
Markus