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SubjectRe: [PATCH v2] arm64/sve: Fix wrong free for task->thread.sve_state
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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@jp.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@vger.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@vger.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
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