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Message-ID: <20250827194107.4142164-2-seanjc@google.com>
Date: Wed, 27 Aug 2025 12:41:05 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>,
"Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>
Cc: kvm@...r.kernel.org, virtualization@...ts.linux.dev,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH v2 1/3] vhost_task: Don't wake KVM x86's recovery thread if
vhost task was killed
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@...utronix.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@...utronix.de>
Fixes: d96c77bd4eeb ("KVM: x86: switch hugepage recovery thread to vhost_task")
Cc: stable@...r.kernel.org
Signed-off-by: Sean Christopherson <seanjc@...gle.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
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