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

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Message-ID: <20200415164628.2dgrj4ghvtev45sy@linutronix.de>
Date:   Wed, 15 Apr 2020 18:46:28 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Matt Fleming <matt@...eblueprint.co.uk>
Cc:     linux-rt@...r.kernel.org, linux-kernel@...r.kernel.org,
        Daniel Wagner <wagi@...om.org>
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