2020-04-07 09:55:24

by Matt Fleming

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

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 <[email protected]>
Reported-by: Daniel Wagner <[email protected]>
---
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


2020-04-07 14:48:04

by Markus Elfring

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

> refcount_t: underflow; use-after-free.

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

Regards,
Markus

2020-04-16 00:59:54

by Daniel Wagner

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

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

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

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

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

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