From 5efd97e6f0c745d5cb34e4e3b709200e23d4d7b7 Mon Sep 17 00:00:00 2001 From: Andrey Drobyshev Date: Fri, 24 Jul 2026 14:45:41 +0300 Subject: [PATCH] vhost: synchronize with RCU readers when freeing workers vhost_vq_work_queue() only holds the RCU read lock while it dereferences vq->worker and queues work on it. vhost_workers_free() however clears the vq->worker pointers and immediately frees the workers, without waiting for a grace period. A caller that fetched the worker right before the pointer was cleared can therefore still be queueing work on it while it is freed. And even when the queueing itself wins the race, the work is never run, so its VHOST_WORK_QUEUED bit stays set and all future attempts to queue it are silently skipped. None of the current callers can actually hit this: net and scsi stop their virtqueues before the workers are freed, and vsock unhashes the device and does synchronize_rcu() of its own in vhost_vsock_dev_release() before the workers go away. But the upcoming VHOST_RESET_OWNER support in vhost-vsock keeps the device hashed while its workers are freed, so the lockless send/cancel paths become able to race with the teardown. Fix this by threading a bool 'sync' param through the chain: vhost_dev_reset_owner() vhost_dev_cleanup() vhost_workers_free() When set, after clearing the vq->worker pointers, wait for a grace period and flush the workers so any work the last RCU readers queued runs (clearing VHOST_WORK_QUEUED) before the workers are freed. Only the vsock RESET_OWNER path (added in the next patch) passes sync=true; every other teardown has already quiesced and passes false, so they do not need to wait the grace period. Suggested-by: Stefano Garzarella Suggested-by: Michael S. Tsirkin Signed-off-by: Andrey Drobyshev Reviewed-by: Stefano Garzarella Acked-by: Michael S. Tsirkin Signed-off-by: Michael S. Tsirkin Message-ID: <20260724114542.734623-5-andrey.drobyshev@virtuozzo.com> --- drivers/vhost/net.c | 4 ++-- drivers/vhost/scsi.c | 2 +- drivers/vhost/test.c | 4 ++-- drivers/vhost/vdpa.c | 4 ++-- drivers/vhost/vhost.c | 26 +++++++++++++++++++++----- drivers/vhost/vhost.h | 5 +++-- drivers/vhost/vsock.c | 2 +- 7 files changed, 32 insertions(+), 15 deletions(-) diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c index 2cc730729e08..4570a4ffbab2 100644 --- a/drivers/vhost/net.c +++ b/drivers/vhost/net.c @@ -1452,7 +1452,7 @@ static int vhost_net_release(struct inode *inode, struct file *f) vhost_net_stop(n, &tx_sock, &rx_sock); vhost_net_flush(n); vhost_dev_stop(&n->dev); - vhost_dev_cleanup(&n->dev); + vhost_dev_cleanup(&n->dev, false); vhost_net_vq_reset(n); if (tx_sock) sockfd_put(tx_sock); @@ -1661,7 +1661,7 @@ static long vhost_net_reset_owner(struct vhost_net *n) vhost_net_stop(n, &tx_sock, &rx_sock); vhost_net_flush(n); vhost_dev_stop(&n->dev); - vhost_dev_reset_owner(&n->dev, umem); + vhost_dev_reset_owner(&n->dev, umem, false); vhost_net_vq_reset(n); done: mutex_unlock(&n->dev.mutex); diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c index 4f8c0260bc9e..885fac5ed042 100644 --- a/drivers/vhost/scsi.c +++ b/drivers/vhost/scsi.c @@ -2390,7 +2390,7 @@ static int vhost_scsi_release(struct inode *inode, struct file *f) mutex_unlock(&vs->dev.mutex); vhost_scsi_clear_endpoint(vs, &t); vhost_dev_stop(&vs->dev); - vhost_dev_cleanup(&vs->dev); + vhost_dev_cleanup(&vs->dev, false); kfree(vs->dev.vqs); kvfree(vs->vqs); kfree(vs->old_inflight); diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c index 24514c8fdee4..84ed9ea81219 100644 --- a/drivers/vhost/test.c +++ b/drivers/vhost/test.c @@ -163,7 +163,7 @@ static int vhost_test_release(struct inode *inode, struct file *f) vhost_test_stop(n, &private); vhost_test_flush(n); vhost_dev_stop(&n->dev); - vhost_dev_cleanup(&n->dev); + vhost_dev_cleanup(&n->dev, false); kfree(n->dev.vqs); kfree(n); return 0; @@ -238,7 +238,7 @@ static long vhost_test_reset_owner(struct vhost_test *n) vhost_test_stop(n, &priv); vhost_test_flush(n); vhost_dev_stop(&n->dev); - vhost_dev_reset_owner(&n->dev, umem); + vhost_dev_reset_owner(&n->dev, umem, false); done: mutex_unlock(&n->dev.mutex); return err; diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c index a31786796d4c..ff7d90719171 100644 --- a/drivers/vhost/vdpa.c +++ b/drivers/vhost/vdpa.c @@ -948,7 +948,7 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep, case VHOST_SET_OWNER: r = vhost_vdpa_bind_mm(v); if (r) - vhost_dev_reset_owner(d, NULL); + vhost_dev_reset_owner(d, NULL, false); break; } out: @@ -1453,7 +1453,7 @@ static void vhost_vdpa_cleanup(struct vhost_vdpa *v) } vhost_vdpa_free_domain(v); - vhost_dev_cleanup(&v->vdev); + vhost_dev_cleanup(&v->vdev, false); kfree(v->vdev.vqs); v->vdev.vqs = NULL; } diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index 44cac11b68d2..4d5d32e68d35 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -731,7 +731,7 @@ static void vhost_worker_destroy(struct vhost_dev *dev, kfree(worker); } -static void vhost_workers_free(struct vhost_dev *dev) +static void vhost_workers_free(struct vhost_dev *dev, bool sync) { struct vhost_worker *worker; unsigned long i; @@ -741,6 +741,21 @@ static void vhost_workers_free(struct vhost_dev *dev) for (i = 0; i < dev->nvqs; i++) rcu_assign_pointer(dev->vqs[i]->worker, NULL); + + /* + * This path can be reached by lockless work queuers. In this case + * vhost_vq_work_queue() reads vq->worker under rcu_read_lock(), so a + * RCU reader that fetched a worker before we cleared the pointers above + * may still be queueing work on it. Wait for those readers to finish, + * then flush so any work they queued runs (clearing VHOST_WORK_QUEUED) + * before the workers are freed. Notably, that is the case for the + * vsock RESET_OWNER path. + */ + if (sync) { + synchronize_rcu(); + vhost_dev_flush(dev); + } + /* * Free the default worker we created and cleanup workers userspace * created but couldn't clean up (it forgot or crashed). @@ -1163,11 +1178,12 @@ struct vhost_iotlb *vhost_dev_reset_owner_prepare(void) EXPORT_SYMBOL_GPL(vhost_dev_reset_owner_prepare); /* Caller should have device mutex */ -void vhost_dev_reset_owner(struct vhost_dev *dev, struct vhost_iotlb *umem) +void vhost_dev_reset_owner(struct vhost_dev *dev, struct vhost_iotlb *umem, + bool sync) { int i; - vhost_dev_cleanup(dev); + vhost_dev_cleanup(dev, sync); dev->fork_owner = fork_from_owner_default; dev->umem = umem; @@ -1227,7 +1243,7 @@ void vhost_clear_msg(struct vhost_dev *dev) } EXPORT_SYMBOL_GPL(vhost_clear_msg); -void vhost_dev_cleanup(struct vhost_dev *dev) +void vhost_dev_cleanup(struct vhost_dev *dev, bool sync) { int i; @@ -1251,7 +1267,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev) dev->iotlb = NULL; vhost_clear_msg(dev); wake_up_interruptible_poll(&dev->wait, EPOLLIN | EPOLLRDNORM); - vhost_workers_free(dev); + vhost_workers_free(dev, sync); vhost_detach_mm(dev); } EXPORT_SYMBOL_GPL(vhost_dev_cleanup); diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h index 39e6121f7525..5a684ba5e389 100644 --- a/drivers/vhost/vhost.h +++ b/drivers/vhost/vhost.h @@ -219,8 +219,9 @@ long vhost_dev_set_owner(struct vhost_dev *dev); bool vhost_dev_has_owner(struct vhost_dev *dev); long vhost_dev_check_owner(struct vhost_dev *); struct vhost_iotlb *vhost_dev_reset_owner_prepare(void); -void vhost_dev_reset_owner(struct vhost_dev *dev, struct vhost_iotlb *iotlb); -void vhost_dev_cleanup(struct vhost_dev *); +void vhost_dev_reset_owner(struct vhost_dev *dev, struct vhost_iotlb *iotlb, + bool sync); +void vhost_dev_cleanup(struct vhost_dev *dev, bool sync); void vhost_dev_stop(struct vhost_dev *); long vhost_dev_ioctl(struct vhost_dev *, unsigned int ioctl, void __user *argp); long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp); diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c index 30d4b16a5e50..e6ac9b41c2a5 100644 --- a/drivers/vhost/vsock.c +++ b/drivers/vhost/vsock.c @@ -824,7 +824,7 @@ static int vhost_vsock_dev_release(struct inode *inode, struct file *file) virtio_vsock_skb_queue_purge(&vsock->send_pkt_queue); - vhost_dev_cleanup(&vsock->dev); + vhost_dev_cleanup(&vsock->dev, false); put_net_track(vsock->net, &vsock->ns_tracker); kfree(vsock->dev.vqs); vhost_vsock_free(vsock);