linux-kernel - Re: [PATCH v2] arm64/sve: Fix wrong free for task->thread.sve

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Date:   Fri, 27 Sep 2019 19:56:45 +0000
From:   Julien Grall <Julien.Grall@....com>
To:     Dave P Martin <Dave.Martin@....com>,
        Masayoshi Mizuma <msys.mizuma@...il.com>
CC:     nd <nd@....com>, Catalin Marinas <Catalin.Marinas@....com>,
        Will Deacon <will@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Masayoshi Mizuma <m.mizuma@...fujitsu.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] arm64/sve: Fix wrong free for task->thread.sve_state

Hi Dave,

On 27/09/2019 17:15, Dave Martin wrote:
> On Fri, Sep 27, 2019 at 11:39:49AM -0400, Masayoshi Mizuma wrote:
>> From: Masayoshi Mizuma <m.mizuma@...fujitsu.com>
>>
>> The system which has SVE feature crashed because of
>> the memory pointed by task->thread.sve_state was destroyed
>> by someone.
>>
>> That is because sve_state is freed while the forking the
>> child process. The child process has the pointer of sve_state
>> which is same as the parent's because the child's task_struct
>> is copied from the parent's one. If the copy_process()
>> fails as an error on somewhere, for example, copy_creds(),
>> then the sve_state is freed even if the parent is alive.
>> The flow is as follows.
>>
>> copy_process
>>          p = dup_task_struct
>>              => arch_dup_task_struct
>>                  *dst = *src;  // copy the entire region.
>> :
>>          retval = copy_creds
>>          if (retval < 0)
>>                  goto bad_fork_free;
>> :
>> bad_fork_free:
>> ...
>>          delayed_free_task(p);
>>            => free_task
>>               => arch_release_task_struct
>>                  => fpsimd_release_task
>>                     => __sve_free
>>                        => kfree(task->thread.sve_state);
>>                           // free the parent's sve_state
>>
>> Move child's sve_state = NULL and clearing TIF_SVE flag
>> to arch_dup_task_struct() so that the child doesn't free the
>> parent's one.
> 
> You could also add:
> 
> --8<--
> There is no need to wait until copy_process() to clear TIF_SVE for
> dst, becuase the thread flags for dst are initialized already by
> copying the src task_struct.
> 
> This change simplifies the code, so get rid of comments that are no
> longer needed.
> -->8--
> 
>>
>> Cc: stable@...r.kernel.org
> 
> Since SVE only exists from v4.15, it may be helpful to specify that,
> i.e., replace that Cc line with:
> 
> Cc: <stable@...r.kernel.org> # 4.15.x-
> 
> 
> Otherwise, I'm happy to see this applied, but I'd like somebody to
> confirm that this change definitely fixes the bug.

I am working on a reproducer for this. So I should be able to test it.

Cheers,

-- 
Julien Grall