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

Cheers
---Dave

[...]

> 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>
> ---
> 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.
> - */
> 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));
> + 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
> --
> 2.18.1
>
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

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