Re: [PATCH v3] arm64/sve: Fix wrong free for task->thread.sve_state

[Dave Martin <Dave.Martin@arm.com>]

On Mon, Sep 30, 2019 at 04:56:00PM -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.
> There is no need to wait until copy_process() to clear TIF_SVE for
> dst, because 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.
> 
> As a note, arm64 used to have thread_info on the stack. So it
> would not be possible to clear TIF_SVE until the stack is initialized.
> From commit c02433dd6de3 ("arm64: split thread_info from task stack"),
> the thread_info is part of the task, so it should be valid to modify
> the flag from arch_dup_task_struct().
> 
> Cc: stable@vger.kernel.org # 4.15.x-
> Fixes: bc0ee4760364 ("arm64/sve: Core task context handling")
> Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
> Reported-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
> Suggested-by: Dave Martin <Dave.Martin@arm.com>
> Tested-by: Julien Grall <julien.grall@arm.com>

Looks OK to me:

Reviewed-by: Dave Martin <Dave.Martin@arm.com>

Cheers
---Dave

> ---
>  arch/arm64/kernel/process.c | 32 +++++++++++++++-----------------
>  1 file changed, 15 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index a47462def04b..ef7aa909bfda 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -332,22 +332,27 @@ 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.
> - */
>  int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
>  {
>  	if (current->mm)
>  		fpsimd_preserve_current_state();
>  	*dst = *src;
>  
> +	/* We rely on the above assignment to initialize dst's thread_flags: */
> +	BUILD_BUG_ON(!IS_ENABLED(CONFIG_THREAD_INFO_IN_TASK));
> +
> +	/*
> +	 * Detach src's sve_state (if any) from dst so that it does not
> +	 * get erroneously used or freed prematurely.  dst's sve_state
> +	 * will be allocated on demand later on if dst uses SVE.
> +	 * For consistency, also clear TIF_SVE here: this could be done
> +	 * later in copy_process(), but to avoid tripping up future
> +	 * maintainers it is best not to leave TIF_SVE and sve_state in
> +	 * an inconsistent state, even temporarily.
> +	 */
> +	dst->thread.sve_state = NULL;
> +	clear_tsk_thread_flag(dst, TIF_SVE);
> +
>  	return 0;
>  }
>  
> @@ -360,13 +365,6 @@ 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
> -- 
> 2.18.1
> 
> 
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