From: Masayoshi Mizuma <[email protected]>
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: [email protected] # 4.15.x-
Fixes: bc0ee4760364 ("arm64/sve: Core task context handling")
Signed-off-by: Masayoshi Mizuma <[email protected]>
Reported-by: Hidetoshi Seto <[email protected]>
Suggested-by: Dave Martin <[email protected]>
Tested-by: Julien Grall <[email protected]>
---
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
On Mon, Sep 30, 2019 at 04:56:00PM -0400, Masayoshi Mizuma wrote:
> From: Masayoshi Mizuma <[email protected]>
>
> 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: [email protected] # 4.15.x-
> Fixes: bc0ee4760364 ("arm64/sve: Core task context handling")
> Signed-off-by: Masayoshi Mizuma <[email protected]>
> Reported-by: Hidetoshi Seto <[email protected]>
> Suggested-by: Dave Martin <[email protected]>
> Tested-by: Julien Grall <[email protected]>
Looks OK to me:
Reviewed-by: Dave Martin <[email protected]>
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
>
>
> _______________________________________________
> linux-arm-kernel mailing list
> [email protected]
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel