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.
Cc: [email protected]
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]>
---
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
On Fri, Sep 27, 2019 at 11:39:49AM -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.
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: [email protected]
Since SVE only exists from v4.15, it may be helpful to specify that,
i.e., replace that Cc line with:
Cc: <[email protected]> # 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 <[email protected]>
> Reported-by: Hidetoshi Seto <[email protected]>
> Suggested-by: Dave Martin <[email protected]>
> ---
> 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
> [email protected]
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
Hi Dave,
On 27/09/2019 17:15, Dave Martin wrote:
> On Fri, Sep 27, 2019 at 11:39:49AM -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.
>
> 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: [email protected]
>
> Since SVE only exists from v4.15, it may be helpful to specify that,
> i.e., replace that Cc line with:
>
> Cc: <[email protected]> # 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.
I am working on a reproducer for this. So I should be able to test it.
Cheers,
--
Julien Grall
Hi,
On 27/09/2019 16:39, 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.
>
> Cc: [email protected]
> 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 <[email protected]>
> Reported-by: Hidetoshi Seto <[email protected]>
> Suggested-by: Dave Martin <[email protected]>
I have tested the patch and can confirm that double-free disappeared after the
patch is applied:
Tested-by: Julien Grall <[email protected]>
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
On Mon, Sep 30, 2019 at 01:23:18PM +0100, Julien Grall wrote:
> Hi,
>
> On 27/09/2019 16:39, 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.
> >
> >Cc: [email protected]
> >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 <[email protected]>
> >Reported-by: Hidetoshi Seto <[email protected]>
> >Suggested-by: Dave Martin <[email protected]>
>
> I have tested the patch and can confirm that double-free disappeared after
> the patch is applied:
>
> Tested-by: Julien Grall <[email protected]>
Good to have that confirmed -- thanks for verifying.
[...]
> >---
> > 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;
Ack, some more explanation would be a good idea here.
Maybe the following comments are sufficient?
/* We rely on the above assingment to initialise dst's thread_flags: */
> >+ BUILD_BUG_ON(!IS_ENABLED(CONFIG_THREAD_INFO_IN_TASK));
>
and
/*
* 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);
(TIF_SVE should not usually be set in the first place of course, since
we are in a fork() or clone() syscall in src. This may not be true if
a task is created using kernel_thread() while running in the context of
some user task that entered the kernel due to a trap or syscall --
though probably nobody should be doing that.)
[...]
Cheers
---DavE
Hi Julien and Dave,
On Mon, Sep 30, 2019 at 02:02:46PM +0100, Dave Martin wrote:
> On Mon, Sep 30, 2019 at 01:23:18PM +0100, Julien Grall wrote:
> > Hi,
> >
> > On 27/09/2019 16:39, 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.
> > >
> > >Cc: [email protected]
> > >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.
I think this patch is needed for the kernel has SVE support.
I'll add the Cc tag as Dave said:
Cc: [email protected] # 4.15+
So, I suppose this patch will be backported to stables 5.3.X,
5.2.X and longterm 4.19.X.
Does this make sense?
> >
> > >Signed-off-by: Masayoshi Mizuma <[email protected]>
> > >Reported-by: Hidetoshi Seto <[email protected]>
> > >Suggested-by: Dave Martin <[email protected]>
> >
> > I have tested the patch and can confirm that double-free disappeared after
> > the patch is applied:
> >
> > Tested-by: Julien Grall <[email protected]>
Thank you so much!
>
> Good to have that confirmed -- thanks for verifying.
>
> [...]
>
> > >---
> > > 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.
Thanks, let me add some comments to the commit log.
> >
> > > int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
> > > {
> > > if (current->mm)
> > > fpsimd_preserve_current_state();
> > > *dst = *src;
>
> Ack, some more explanation would be a good idea here.
>
> Maybe the following comments are sufficient?
>
> /* We rely on the above assingment to initialise dst's thread_flags: */
Thanks, I'll add this comment.
>
> > >+ BUILD_BUG_ON(!IS_ENABLED(CONFIG_THREAD_INFO_IN_TASK));
> >
>
> and
>
> /*
> * 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.
> */
I'll add this comments.
>
> > >+ dst->thread.sve_state = NULL;
> > >+ clear_tsk_thread_flag(dst, TIF_SVE);
>
> (TIF_SVE should not usually be set in the first place of course, since
> we are in a fork() or clone() syscall in src. This may not be true if
> a task is created using kernel_thread() while running in the context of
> some user task that entered the kernel due to a trap or syscall --
> though probably nobody should be doing that.)
Thanks!
Masa
On Mon, Sep 30, 2019 at 10:29:53AM -0400, Masayoshi Mizuma wrote:
> Hi Julien and Dave,
>
> On Mon, Sep 30, 2019 at 02:02:46PM +0100, Dave Martin wrote:
> > On Mon, Sep 30, 2019 at 01:23:18PM +0100, Julien Grall wrote:
> > > Hi,
> > >
> > > On 27/09/2019 16:39, 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.
> > > >
> > > >Cc: [email protected]
> > > >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.
>
> I think this patch is needed for the kernel has SVE support.
> I'll add the Cc tag as Dave said:
>
> Cc: [email protected] # 4.15+
Based on running git log --grep='stable@vger.*#.*-', the most common
syntax for this situation seems to be
Cc: <[email protected]> # 4.15.x-
> So, I suppose this patch will be backported to stables 5.3.X,
> 5.2.X and longterm 4.19.X.
> Does this make sense?
Yes, I think so. It's up to the stable maintainers to decide which
trees the patch actually gets applied too.
> > > >Signed-off-by: Masayoshi Mizuma <[email protected]>
> > > >Reported-by: Hidetoshi Seto <[email protected]>
> > > >Suggested-by: Dave Martin <[email protected]>
> > >
> > > I have tested the patch and can confirm that double-free disappeared after
> > > the patch is applied:
> > >
> > > Tested-by: Julien Grall <[email protected]>
>
> Thank you so much!
>
> >
> > Good to have that confirmed -- thanks for verifying.
> >
> > [...]
> >
> > > >---
> > > > 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.
>
> Thanks, let me add some comments to the commit log.
>
> > >
> > > > int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
> > > > {
> > > > if (current->mm)
> > > > fpsimd_preserve_current_state();
> > > > *dst = *src;
> >
> > Ack, some more explanation would be a good idea here.
> >
> > Maybe the following comments are sufficient?
> >
> > /* We rely on the above assingment to initialise dst's thread_flags: */
>
> Thanks, I'll add this comment.
>
> >
> > > >+ BUILD_BUG_ON(!IS_ENABLED(CONFIG_THREAD_INFO_IN_TASK));
> > >
> >
> > and
> >
> > /*
> > * 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.
> > */
>
> I'll add this comments.
OK, if these make sense to you, feel free to add them.
[...]
Cheers
---Dave