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

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>
Message-Id: <20250918181144.Ygo8BZ-R@linutronix.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Sean Christopherson <seanjc@google.com>

diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c
index bc738fa90c1d6875344beca7813aafb03dff4762..27107dcc1cbfebab9aa03042ee4e02701ef7bdc2 100644 (file)
--- 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);