[PATCH 0/1] arm64/sve: Fix wrong free for task->thread.sve_state

[PATCH 0/1] arm64/sve: Fix wrong free for task->thread.sve_state

[Masayoshi Mizuma]

From: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>

The system which has SVE feature crashed by unknown
reason. According to the memory dump, the panic happened
because the memory data pointed by task->thread.sve_state was
destroyed by someone.

We tried to reproduce the crash, however, it's hard to do that.
But, we found a potential issue by reviewing the code.

In copy_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

To fix that, 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.

Masayoshi Mizuma (1):
  arm64/sve: Fix wrong free for task->thread.sve_state

 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(-)

-- 
2.18.1

[PATCH 1/1] arm64/sve: Fix wrong free for task->thread.sve_state

[Masayoshi Mizuma]

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

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.

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;
 
 	/*
-- 
2.18.1

Re: [PATCH 1/1] arm64/sve: Fix wrong free for task->thread.sve_state

[Julien Grall]

(+ 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

Re: [PATCH 1/1] arm64/sve: Fix wrong free for task->thread.sve_state

[Dave Martin]

On Thu, Sep 26, 2019 at 03:08:46PM -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
> 
> 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.
> 
> 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;

There's definitely a problem here, but a simpler fix is probably to
NULL sve_state and clear TIF_SVE for dst at the same time.

Once upon a time, I had to cope with the thread_flags not being copied
as part of task_struct here, which is one reason why the code is the
(broken) way it is, but this is ancient history...

Commit c02433dd6de3 ("arm64: split thread_info from task stack") was
merged in v4.10 and predates SVE support anyway.


So can just do the following (untested) and delete the confusing
comments?

Cheers
---Dave

--8<--


diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index f674f28..6937f59 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;
 }
 
@@ -352,13 +346,6 @@ 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
 	 * any cpu that may have run that now-exited task recently.

Re: [PATCH 1/1] arm64/sve: Fix wrong free for task->thread.sve_state

[Masayoshi Mizuma]

Hi Julien and Dave,

Thank you for your comments!
Dave's suggestion looks good for me, many thanks!
I'll post it as v2.

- Masa

On Fri, Sep 27, 2019 at 01:52:30PM +0100, Dave Martin wrote:
> On Thu, Sep 26, 2019 at 03:08:46PM -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
> > 
> > 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.
> > 
> > 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;
> 
> There's definitely a problem here, but a simpler fix is probably to
> NULL sve_state and clear TIF_SVE for dst at the same time.
> 
> Once upon a time, I had to cope with the thread_flags not being copied
> as part of task_struct here, which is one reason why the code is the
> (broken) way it is, but this is ancient history...
> 
> Commit c02433dd6de3 ("arm64: split thread_info from task stack") was
> merged in v4.10 and predates SVE support anyway.
> 
> 
> So can just do the following (untested) and delete the confusing
> comments?
> 
> Cheers
> ---Dave
> 
> --8<--
> 
> 
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index f674f28..6937f59 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;
>  }
>  
> @@ -352,13 +346,6 @@ 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
>  	 * any cpu that may have run that now-exited task recently.

Re: [PATCH 1/1] arm64/sve: Fix wrong free for task->thread.sve_state

[Dave Martin]

On Fri, Sep 27, 2019 at 10:38:02AM -0400, Masayoshi Mizuma wrote:
> Hi Julien and Dave,
> 
> Thank you for your comments!
> Dave's suggestion looks good for me, many thanks!
> I'll post it as v2.

Please add Fixes and Cc: stable tags as appropriate.

(I'm happy not to push this patch myself, but I would expect to see a
Suggested-by tag...)

See Documentation/process/submitting-patches.rst,
Documentation/process/stable-kernel-rules.txt if unsure.

[...]

Cheers
---Dave