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SubjectRe: [PATCH 1/1] arm64/sve: Fix wrong free for task->thread.sve_state
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(+ Dave)

Hi,

Thank you for the patch.

On 26/09/2019 20:08, 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

The flow makes sense to me and I agree you would end up to free the parent's state.

>
> Add a flag in task->thread which shows the fork is in progress.
> If the fork is in progress, that means the child has the pointer
> to the parent's sve_state, doesn't free the sve_state.

I haven't fully investigate it yet but I was wondering if we could just clear
task->thread.sve_state for the child in arch_dup_task_struct().

I saw the comment on top of function mentioning potential issue to do it there.
I understand that you may not be able to clear TIF_SVE in the function, but I
don't understand why clearing just task->thread.sve_state would be an issue.

The only risk I can see is TIF_SVE may be set with task->thread.sve_state to be
NULL. But this is a new task, so I don't think there are risk here to have it
unsync. Dave?

>
> Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
> Reported-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
> ---
> arch/arm64/include/asm/processor.h | 1 +
> arch/arm64/kernel/fpsimd.c | 6 ++++--
> arch/arm64/kernel/process.c | 2 ++
> 3 files changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
> index 5623685c7d13..3ca3e350145a 100644
> --- a/arch/arm64/include/asm/processor.h
> +++ b/arch/arm64/include/asm/processor.h
> @@ -143,6 +143,7 @@ struct thread_struct {
> unsigned long fault_address; /* fault info */
> unsigned long fault_code; /* ESR_EL1 value */
> struct debug_info debug; /* debugging */
> + unsigned int fork_in_progress;
> #ifdef CONFIG_ARM64_PTR_AUTH
> struct ptrauth_keys keys_user;
> #endif
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index 37d3912cfe06..8641db4cb062 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -202,8 +202,10 @@ static bool have_cpu_fpsimd_context(void)
> */
> static void __sve_free(struct task_struct *task)
> {
> - kfree(task->thread.sve_state);
> - task->thread.sve_state = NULL;
> + if (!task->thread.fork_in_progress) {
> + kfree(task->thread.sve_state);
> + task->thread.sve_state = NULL;
> + }
> }
>
> static void sve_free(struct task_struct *task)
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index a47462def04b..8ac0ee4e5f76 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -347,6 +347,7 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
> if (current->mm)
> fpsimd_preserve_current_state();
> *dst = *src;
> + dst->thread.fork_in_progress = 1;
>
> return 0;
> }
> @@ -365,6 +366,7 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
> * and disable discard SVE state for p:
> */
> clear_tsk_thread_flag(p, TIF_SVE);
> + p->thread.fork_in_progress = 0;
> p->thread.sve_state = NULL;
>
> /*
>

Cheers,

--
Julien Grall

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