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

[Julien Grall]

Hi,

On 27/09/2019 16:39, Masayoshi Mizuma wrote:
> From: Masayoshi Mizuma <m.mizuma@xxxxxxxxxxxxxx>
>
> 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.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: bc0ee4760364 ("arm64/sve: Core task context handling")

Looking at the log, it looks like THREAD_INFO_IN_TASK was selected before the 
bc0ee4760364. So it should be fine to backport for all the Linux tree contain 
this commit.

> Signed-off-by: Masayoshi Mizuma <m.mizuma@xxxxxxxxxxxxxx>
> Reported-by: Hidetoshi Seto <seto.hidetoshi@xxxxxxxxxxxxxx>
> Suggested-by: Dave Martin <Dave.Martin@xxxxxxx>

I have tested the patch and can confirm that double-free disappeared after the 
patch is applied:

Tested-by: Julien Grall <julien.grall@xxxxxxx>

See below for a few comments.

> ---
>   arch/arm64/kernel/process.c | 21 ++++-----------------
>   1 file changed, 4 insertions(+), 17 deletions(-)
>
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index f674f28df..6937f5935 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -323,22 +323,16 @@ void arch_release_task_struct(struct task_struct *tsk)
>   	fpsimd_release_task(tsk);
>   }
>   
> -/*
> - * src and dst may temporarily have aliased sve_state after task_struct
> - * is copied.  We cannot fix this properly here, because src may have
> - * live SVE state and dst's thread_info may not exist yet, so tweaking
> - * either src's or dst's TIF_SVE is not safe.
> - *
> - * The unaliasing is done in copy_thread() instead.  This works because
> - * dst is not schedulable or traceable until both of these functions
> - * have been called.
> - */

It would be good to explain in the commit message why tweaking "dst" in 
arch_dup_task_struct() is fine.

From my understanding, Arm64 used to have thread_info on the stack. So it would 
not be possible to clear TIF_SVE until the stack is initialized.

Now that the thread_info is part of the task, it should be valid to modify the 
flag from arch_dup_task_struct().

Note that technically, TIF_SVE does not need to be cleared from 
arch_dup_task_struct(). It could also be done from copy_thread(). But it is 
easier to keep the both changes together.

>   int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
>   {
>   	if (current->mm)
>   		fpsimd_preserve_current_state();
>   	*dst = *src;
>   
> +	BUILD_BUG_ON(!IS_ENABLED(CONFIG_THREAD_INFO_IN_TASK));

You may want to add a comment on top explaining why TIF_SVE is cleared here.

> +	dst->thread.sve_state = NULL;
> +	clear_tsk_thread_flag(dst, TIF_SVE);
> +
>   	return 0;
>   }
>   
> @@ -351,13 +345,6 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
>   
>   	memset(&p->thread.cpu_context, 0, sizeof(struct cpu_context));
>   
> -	/*
> -	 * Unalias p->thread.sve_state (if any) from the parent task
> -	 * and disable discard SVE state for p:
> -	 */
> -	clear_tsk_thread_flag(p, TIF_SVE);
> -	p->thread.sve_state = NULL;
> -
>   	/*
>   	 * In case p was allocated the same task_struct pointer as some
>   	 * other recently-exited task, make sure p is disassociated from

Cheers,

--
Julien Grall