[PATCH v2 0/3] vhost_task: Fix a bug where KVM wakes an exited task

[PATCH v2 0/3] vhost_task: Fix a bug where KVM wakes an exited task

[Sean Christopherson]

Michael,

Do you want to take this through the vhost tree?  It technically fixes a KVM
bug, but this obviously touches far more vhost code than KVM code, and the
patch that needs to go into 6.17 doesn't touch KVM at all.


Fix a bug where KVM attempts to wake a vhost task that has already exited in
response to a fatal signal, and tack on a few cleanups to harden against
introducing similar bugs in the future.

The issue is firmly a KVM problem, but I opted to fix the bug by making
vhost_task_wake() safe against an exited task as doing so is far simpler and
cleaner than implementing the same functionality in KVM, and I suspect that
if there are other users of vhost_tasks in the future, then there's a good
chance they will want/expect vhost_task to handle that detail.

Note, this only started causing problems when commit 56180dd20c19 ("futex:
Use RCU-based per-CPU reference counting instead of rcuref_t") landed, so
the explosions are "new" in 6.17, but the bug has existed since KVM switched
to vhost_task back in 6.13.

v2:
 - Drop the "safe" postfix variant and make the "default" vhost_task_wake()
   safe. [Michael].
 - Use vhost_task_wake() and __vhost_task_wake() for the public APIs, and
   vhost_task_wake_up_process() for the local helper. [Michael]
 - Drag the signalas back from their Spanish holiday. [Sebastian]

v1: https://lore.kernel.org/all/20250826004012.3835150-1-seanjc@google.com

Sean Christopherson (3):
  vhost_task: Don't wake KVM x86's recovery thread if vhost task was
    killed
  vhost_task: Allow caller to omit handle_sigkill() callback
  KVM: x86/mmu: Don't register a sigkill callback for NX hugepage
    recovery tasks

 arch/x86/kvm/mmu/mmu.c           |  7 +---
 drivers/vhost/vhost.c            |  2 +-
 include/linux/sched/vhost_task.h |  1 +
 kernel/vhost_task.c              | 62 +++++++++++++++++++++++++++-----
 4 files changed, 56 insertions(+), 16 deletions(-)


base-commit: 1b237f190eb3d36f52dffe07a40b5eb210280e00
-- 
2.51.0.268.g9569e192d0-goog

[PATCH v2 1/3] vhost_task: Don't wake KVM x86's recovery thread if vhost task was killed

[Sean Christopherson]

Make the "default" API for waking a vhost task safe against the underlying
task exiting due to a fatal signal.  This fixes a bug in KVM x86 where KVM
attempts to wake an NX hugepage recovery task that exiting before being
explicitly stopped, resulting in a use-after-free and thus crashes, hangs,
and other badness.

  Oops: general protection fault, probably for non-canonical address 0xff0e899fa1566052: 0000 [#1] SMP
  CPU: 51 UID: 0 PID: 53807 Comm: tee Tainted: G S         O        6.17.0-smp--38183c31756a-next #826 NONE
  Tainted: [S]=CPU_OUT_OF_SPEC, [O]=OOT_MODULE
  Hardware name: Google LLC Indus/Indus_QC_03, BIOS 30.110.0 09/13/2024
  RIP: 0010:queued_spin_lock_slowpath+0x123/0x250
  Code: ... <48> 89 8c 02 c0 da 47 a2 83 79 08 00 75 08 f3 90 83 79 08 00 74 f8
  RSP: 0018:ffffbf55cffe7cf8 EFLAGS: 00010006
  RAX: ff0e899fff0e8562 RBX: 0000000000d00000 RCX: ffffa39b40aefac0
  RDX: 0000000000000030 RSI: fffffffffffffff8 RDI: ffffa39d0592e68c
  RBP: 0000000000d00000 R08: 00000000ffffff80 R09: 0000000400000000
  R10: ffffa36cce4fe401 R11: 0000000000000800 R12: 0000000000000003
  R13: 0000000000000000 R14: ffffa39d0592e68c R15: ffffa39b9e672000
  FS:  00007f233b2e9740(0000) GS:ffffa39b9e672000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 00007f233b39fda0 CR3: 00000004d031f002 CR4: 00000000007726f0
  PKRU: 55555554
  Call Trace:
   <TASK>
   _raw_spin_lock_irqsave+0x50/0x60
   try_to_wake_up+0x4f/0x5d0
   set_nx_huge_pages+0xe4/0x1c0 [kvm]
   param_attr_store+0x89/0xf0
   module_attr_store+0x1e/0x30
   kernfs_fop_write_iter+0xe4/0x160
   vfs_write+0x2cb/0x420
   ksys_write+0x7f/0xf0
   do_syscall_64+0x6f/0x1f0
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
  RIP: 0033:0x7f233b4178b3
  R13: 0000000000000002 R14: 00000000226ff3d0 R15: 0000000000000002
   </TASK>

Handle VHOST_TASK_FLAGS_KILLED in vhost_task_wake() instead of forcing KVM
to solve the problem, as KVM would literally just add an equivalent flag,
along with a new lock to protect said flag.  In general, forcing simple
usage of vhost task to care about signals _and_ take non-trivial action to
do the right thing isn't developer friendly, and is likely to lead to
similar bugs in the future.

Keep the existing behavior for vhost (by calling __vhost_task_wake()
instead of vhost_task_wake()), as vhost_worker_killed() takes extra care
to stop and flush all workers, i.e. doesn't need the extra protection, and
because  vhost_vq_work_queue() calls

  vhost_worker_queue()
  |
  -> worker->ops->wakeup(worker)
     |
     -> vhost_task_wakeup()
        |
        -> vhost_task_wake()

while holding RCU and so can't sleep, i.e. can't take exit_mutex.

        rcu_read_lock();
        worker = rcu_dereference(vq->worker);
        if (worker) {
                queued = true;
                vhost_worker_queue(worker, work);
        }
        rcu_read_unlock();

Debugged-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/all/aKkLEtoDXKxAAWju@google.com
Link: https://lore.kernel.org/all/aJ_vEP2EHj6l0xRT@google.com
Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Fixes: d96c77bd4eeb ("KVM: x86: switch hugepage recovery thread to vhost_task")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 drivers/vhost/vhost.c            |  2 +-
 include/linux/sched/vhost_task.h |  1 +
 kernel/vhost_task.c              | 52 +++++++++++++++++++++++++++-----
 3 files changed, 46 insertions(+), 9 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 8570fdf2e14a..dafce01a9c0d 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -744,7 +744,7 @@ static void vhost_workers_free(struct vhost_dev *dev)
 
 static void vhost_task_wakeup(struct vhost_worker *worker)
 {
-	return vhost_task_wake(worker->vtsk);
+	return __vhost_task_wake(worker->vtsk);
 }
 
 static void vhost_kthread_wakeup(struct vhost_worker *worker)
diff --git a/include/linux/sched/vhost_task.h b/include/linux/sched/vhost_task.h
index 25446c5d3508..1a9c2ac65c9a 100644
--- a/include/linux/sched/vhost_task.h
+++ b/include/linux/sched/vhost_task.h
@@ -10,5 +10,6 @@ struct vhost_task *vhost_task_create(bool (*fn)(void *),
 void vhost_task_start(struct vhost_task *vtsk);
 void vhost_task_stop(struct vhost_task *vtsk);
 void vhost_task_wake(struct vhost_task *vtsk);
+void __vhost_task_wake(struct vhost_task *vtsk);
 
 #endif /* _LINUX_SCHED_VHOST_TASK_H */
diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c
index bc738fa90c1d..bd213d0b6da3 100644
--- a/kernel/vhost_task.c
+++ b/kernel/vhost_task.c
@@ -67,16 +67,52 @@ static int vhost_task_fn(void *data)
 	do_exit(0);
 }
 
-/**
- * vhost_task_wake - wakeup the vhost_task
- * @vtsk: vhost_task to wake
- *
- * wake up the vhost_task worker thread
- */
-void vhost_task_wake(struct vhost_task *vtsk)
+static void vhost_task_wake_up_process(struct vhost_task *vtsk)
 {
 	wake_up_process(vtsk->task);
 }
+
+/**
+ * __vhost_task_wake - wakeup the vhost_task
+ * @vtsk: vhost_task to wake
+ *
+ * Wake up the vhost_task worker thread.  The caller is responsible for ensuring
+ * that the task hasn't exited.
+ */
+void __vhost_task_wake(struct vhost_task *vtsk)
+{
+	/*
+	 * Checking VHOST_TASK_FLAGS_KILLED can race with signal delivery, but
+	 * a race can only result in false negatives and this is just a sanity
+	 * check, i.e. if KILLED is set, the caller is buggy no matter what.
+	 */
+	if (WARN_ON_ONCE(test_bit(VHOST_TASK_FLAGS_KILLED, &vtsk->flags)))
+		return;
+
+	vhost_task_wake_up_process(vtsk);
+}
+EXPORT_SYMBOL_GPL(__vhost_task_wake);
+
+/**
+ * vhost_task_wake - wakeup the vhost_task if it hasn't been killed
+ * @vtsk: vhost_task to wake
+ *
+ * Wake up the vhost_task worker thread if the task hasn't exited, e.g. due to
+ * a signal.
+ */
+void vhost_task_wake(struct vhost_task *vtsk)
+{
+	guard(mutex)(&vtsk->exit_mutex);
+
+	/* Attempting to wake a task that has been explicitly stopped is a bug. */
+	if (WARN_ON_ONCE(test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags)))
+		return;
+
+	if (test_bit(VHOST_TASK_FLAGS_KILLED, &vtsk->flags))
+		return;
+
+	vhost_task_wake_up_process(vtsk);
+}
 EXPORT_SYMBOL_GPL(vhost_task_wake);
 
 /**
@@ -91,7 +127,7 @@ void vhost_task_stop(struct vhost_task *vtsk)
 	mutex_lock(&vtsk->exit_mutex);
 	if (!test_bit(VHOST_TASK_FLAGS_KILLED, &vtsk->flags)) {
 		set_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags);
-		vhost_task_wake(vtsk);
+		vhost_task_wake_up_process(vtsk);
 	}
 	mutex_unlock(&vtsk->exit_mutex);
 
-- 
2.51.0.268.g9569e192d0-goog

[PATCH v2 2/3] vhost_task: Allow caller to omit handle_sigkill() callback

[Sean Christopherson]

Now that vhost_task provides an API to safely wake a task without relying
on the caller to react to signals, make handle_sigkill() optional and
WARN if the "unsafe" __vhost_task_wake() is used without hooking sigkill.
Requiring the user to react to sigkill adds no meaningful value, e.g. it
didn't help KVM do the right thing with respect to signals, and adding a
sanity check in __vhost_task_wake() gives developers a hint as to what
needs to be done in response to sigkill.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 kernel/vhost_task.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c
index bd213d0b6da3..01bf7b0e2c5b 100644
--- a/kernel/vhost_task.c
+++ b/kernel/vhost_task.c
@@ -59,7 +59,8 @@ static int vhost_task_fn(void *data)
 	 */
 	if (!test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags)) {
 		set_bit(VHOST_TASK_FLAGS_KILLED, &vtsk->flags);
-		vtsk->handle_sigkill(vtsk->data);
+		if (vtsk->handle_sigkill)
+			vtsk->handle_sigkill(vtsk->data);
 	}
 	mutex_unlock(&vtsk->exit_mutex);
 	complete(&vtsk->exited);
@@ -81,6 +82,13 @@ static void vhost_task_wake_up_process(struct vhost_task *vtsk)
  */
 void __vhost_task_wake(struct vhost_task *vtsk)
 {
+	/*
+	 * Waking the task without taking exit_mutex is safe if and only if the
+	 * implementation hooks sigkill, as that's the only way the caller can
+	 * know if the task has exited prematurely due to a signal.
+	 */
+	WARN_ON_ONCE(!vtsk->handle_sigkill);
+
 	/*
 	 * Checking VHOST_TASK_FLAGS_KILLED can race with signal delivery, but
 	 * a race can only result in false negatives and this is just a sanity
-- 
2.51.0.268.g9569e192d0-goog

[PATCH v2 3/3] KVM: x86/mmu: Don't register a sigkill callback for NX hugepage recovery tasks

[Sean Christopherson]

Don't register a sigkill callback with vhost_task when creating NX hugepage
recovery threads now that said callback is optional.  In addition to
removing what is effectively dead code, not registering a sigkill "handler"
also guards against improper use of __vhost_task_wake().

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu/mmu.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 6e838cb6c9e1..ace302137533 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -7677,10 +7677,6 @@ static void kvm_recover_nx_huge_pages(struct kvm *kvm)
 	srcu_read_unlock(&kvm->srcu, rcu_idx);
 }
 
-static void kvm_nx_huge_page_recovery_worker_kill(void *data)
-{
-}
-
 static bool kvm_nx_huge_page_recovery_worker(void *data)
 {
 	struct kvm *kvm = data;
@@ -7713,8 +7709,7 @@ static int kvm_mmu_start_lpage_recovery(struct once *once)
 	struct vhost_task *nx_thread;
 
 	kvm->arch.nx_huge_page_last = get_jiffies_64();
-	nx_thread = vhost_task_create(kvm_nx_huge_page_recovery_worker,
-				      kvm_nx_huge_page_recovery_worker_kill,
+	nx_thread = vhost_task_create(kvm_nx_huge_page_recovery_worker, NULL,
 				      kvm, "kvm-nx-lpage-recovery");
 
 	if (IS_ERR(nx_thread))
-- 
2.51.0.268.g9569e192d0-goog

Re: [PATCH v2 0/3] vhost_task: Fix a bug where KVM wakes an exited task

[Sebastian Andrzej Siewior]

On 2025-08-27 12:41:04 [-0700], Sean Christopherson wrote:
> Michael,

Sean,

would the bellow work by chance? It is a quick shot but it looks
symmetrical…

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);

Sebastian

Re: [PATCH v2 0/3] vhost_task: Fix a bug where KVM wakes an exited task

[Sean Christopherson]

On Wed, Aug 27, 2025, Sebastian Andrzej Siewior wrote:
> On 2025-08-27 12:41:04 [-0700], Sean Christopherson wrote:
> > Michael,
> 
> Sean,
> 
> would the bellow work by chance? It is a quick shot but it looks
> symmetrical…

Gah, sorry, I flagged your earlier mail and then forgot to circle back to it
(for whatever reason, I didn't entirely grok what you were suggesting).

> 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);

Nice!  This fixes things too.  Either solution works for me.  Or maybe do both?
Attempting to wake a task that vhost_task knows has exited (is exiting?) is a
bit gross, but even with that hardening, guarding against UAF is very nice to
have too.

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

Re: [PATCH v2 0/3] vhost_task: Fix a bug where KVM wakes an exited task

[Lei Yang]

Tested this series of patches's v2 again with vhost-net regression
tests, everything works fine.

Tested-by: Lei Yang <leiyang@redhat.com>

On Thu, Aug 28, 2025 at 4:11 AM Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
>
> On 2025-08-27 12:41:04 [-0700], Sean Christopherson wrote:
> > Michael,
>
> Sean,
>
> would the bellow work by chance? It is a quick shot but it looks
> symmetrical…
>
> 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);
>
> Sebastian
>

Re: [PATCH v2 0/3] vhost_task: Fix a bug where KVM wakes an exited task

[Sebastian Andrzej Siewior]

On 2025-08-27 17:16:35 [-0700], Sean Christopherson wrote:
> Nice!  This fixes things too.  Either solution works for me.  Or maybe do both?
> Attempting to wake a task that vhost_task knows has exited (is exiting?) is a
> bit gross, but even with that hardening, guarding against UAF is very nice to
> have too.

I don't mind either way.
If this is requested I can submit a proper patch.

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

Sebastian