vhost: Take a reference on the task that is reference in struct vhost_task.

[PATCH] vhost: Take a reference on the task that is reference in struct vhost_task.

Posted by Sebastian Andrzej Siewior 1 week, 6 days ago

vhost_task_create() creates a task and keeps a reference to its
task_struct. That task may exit early via a signal and its task_struct
will be released.
A pending vhost_task_wake() will then attempt to wake the task and
access a task_struct which is no longer there.

Acquire a reference on the task_struct while creating the thread and
release the reference while the struct vhost_task itself is removed.
If the task exits early due to a signal, then the vhost_task_wake() will
still access a valid task_struct. The wake is safe and will be skipped
in this case.

Fixes: f9010dbdce911 ("fork, vhost: Use CLONE_THREAD to fix freezer/ps regression")
Reported-by: Sean Christopherson <seanjc@google.com>
Closes: https://lore.kernel.org/all/aKkLEtoDXKxAAWju@google.com/
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/vhost_task.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c
index bc738fa90c1d6..27107dcc1cbfe 100644
--- a/kernel/vhost_task.c
+++ b/kernel/vhost_task.c
@@ -100,6 +100,7 @@ void vhost_task_stop(struct vhost_task *vtsk)
 	 * freeing it below.
 	 */
 	wait_for_completion(&vtsk->exited);
+	put_task_struct(vtsk->task);
 	kfree(vtsk);
 }
 EXPORT_SYMBOL_GPL(vhost_task_stop);
@@ -148,7 +149,7 @@ struct vhost_task *vhost_task_create(bool (*fn)(void *),
 		return ERR_CAST(tsk);
 	}
 
-	vtsk->task = tsk;
+	vtsk->task = get_task_struct(tsk);
 	return vtsk;
 }
 EXPORT_SYMBOL_GPL(vhost_task_create);
-- 
2.51.0

Re: [PATCH] vhost: Take a reference on the task that is reference in struct vhost_task.

Posted by Michael S. Tsirkin 1 week, 3 days ago

Subject: that is reference -> that is referenced

On Thu, Sep 18, 2025 at 08:11:44PM +0200, Sebastian Andrzej Siewior wrote:
> vhost_task_create() creates a task and keeps a reference to its
> task_struct. That task may exit early via a signal and its task_struct
> will be released.
> A pending vhost_task_wake() will then attempt to wake the task and
> access a task_struct which is no longer there.
> 
> Acquire a reference on the task_struct while creating the thread and
> release the reference while the struct vhost_task itself is removed.
> If the task exits early due to a signal, then the vhost_task_wake() will
> still access a valid task_struct. The wake is safe and will be skipped
> in this case.
> 
> Fixes: f9010dbdce911 ("fork, vhost: Use CLONE_THREAD to fix freezer/ps regression")
> Reported-by: Sean Christopherson <seanjc@google.com>
> Closes: https://lore.kernel.org/all/aKkLEtoDXKxAAWju@google.com/
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  kernel/vhost_task.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c
> index bc738fa90c1d6..27107dcc1cbfe 100644
> --- a/kernel/vhost_task.c
> +++ b/kernel/vhost_task.c
> @@ -100,6 +100,7 @@ void vhost_task_stop(struct vhost_task *vtsk)
>  	 * freeing it below.
>  	 */
>  	wait_for_completion(&vtsk->exited);
> +	put_task_struct(vtsk->task);
>  	kfree(vtsk);
>  }
>  EXPORT_SYMBOL_GPL(vhost_task_stop);
> @@ -148,7 +149,7 @@ struct vhost_task *vhost_task_create(bool (*fn)(void *),
>  		return ERR_CAST(tsk);
>  	}
>  
> -	vtsk->task = tsk;
> +	vtsk->task = get_task_struct(tsk);
>  	return vtsk;
>  }
>  EXPORT_SYMBOL_GPL(vhost_task_create);
> -- 
> 2.51.0

Re: [PATCH] vhost: Take a reference on the task that is reference in struct vhost_task.

Posted by Michael S. Tsirkin 1 week, 3 days ago

On Sun, Sep 21, 2025 at 04:56:20PM -0400, Michael S. Tsirkin wrote:
> Subject: that is reference -> that is referenced

to note i fixed it for now. just dropped "that is referenced"
completely. shorter.

> On Thu, Sep 18, 2025 at 08:11:44PM +0200, Sebastian Andrzej Siewior wrote:
> > vhost_task_create() creates a task and keeps a reference to its
> > task_struct. That task may exit early via a signal and its task_struct
> > will be released.
> > A pending vhost_task_wake() will then attempt to wake the task and
> > access a task_struct which is no longer there.
> > 
> > Acquire a reference on the task_struct while creating the thread and
> > release the reference while the struct vhost_task itself is removed.
> > If the task exits early due to a signal, then the vhost_task_wake() will
> > still access a valid task_struct. The wake is safe and will be skipped
> > in this case.
> > 
> > Fixes: f9010dbdce911 ("fork, vhost: Use CLONE_THREAD to fix freezer/ps regression")
> > Reported-by: Sean Christopherson <seanjc@google.com>
> > Closes: https://lore.kernel.org/all/aKkLEtoDXKxAAWju@google.com/
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > ---
> >  kernel/vhost_task.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c
> > index bc738fa90c1d6..27107dcc1cbfe 100644
> > --- a/kernel/vhost_task.c
> > +++ b/kernel/vhost_task.c
> > @@ -100,6 +100,7 @@ void vhost_task_stop(struct vhost_task *vtsk)
> >  	 * freeing it below.
> >  	 */
> >  	wait_for_completion(&vtsk->exited);
> > +	put_task_struct(vtsk->task);
> >  	kfree(vtsk);
> >  }
> >  EXPORT_SYMBOL_GPL(vhost_task_stop);
> > @@ -148,7 +149,7 @@ struct vhost_task *vhost_task_create(bool (*fn)(void *),
> >  		return ERR_CAST(tsk);
> >  	}
> >  
> > -	vtsk->task = tsk;
> > +	vtsk->task = get_task_struct(tsk);
> >  	return vtsk;
> >  }
> >  EXPORT_SYMBOL_GPL(vhost_task_create);
> > -- 
> > 2.51.0

Re: [PATCH] vhost: Take a reference on the task that is reference in struct vhost_task.

Posted by Sean Christopherson 1 week, 5 days ago

On Thu, Sep 18, 2025, Sebastian Andrzej Siewior wrote:
> vhost_task_create() creates a task and keeps a reference to its
> task_struct. That task may exit early via a signal and its task_struct
> will be released.
> A pending vhost_task_wake() will then attempt to wake the task and
> access a task_struct which is no longer there.
> 
> Acquire a reference on the task_struct while creating the thread and
> release the reference while the struct vhost_task itself is removed.
> If the task exits early due to a signal, then the vhost_task_wake() will
> still access a valid task_struct. The wake is safe and will be skipped
> in this case.
> 
> Fixes: f9010dbdce911 ("fork, vhost: Use CLONE_THREAD to fix freezer/ps regression")
> Reported-by: Sean Christopherson <seanjc@google.com>
> Closes: https://lore.kernel.org/all/aKkLEtoDXKxAAWju@google.com/
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---

Tested-by: Sean Christopherson <seanjc@google.com>