From dd54bcf86c91a4455b1f95cbc8e9ac91205f3193 Mon Sep 17 00:00:00 2001 From: Nikolay Kuratov Date: Tue, 5 Aug 2025 16:09:17 +0300 Subject: [PATCH] vhost/net: Protect ubufs with rcu read lock in vhost_net_ubuf_put() When operating on struct vhost_net_ubuf_ref, the following execution sequence is theoretically possible: CPU0 is finalizing DMA operation CPU1 is doing VHOST_NET_SET_BACKEND // ubufs->refcount == 2 vhost_net_ubuf_put() vhost_net_ubuf_put_wait_and_free(oldubufs) vhost_net_ubuf_put_and_wait() vhost_net_ubuf_put() int r = atomic_sub_return(1, &ubufs->refcount); // r = 1 int r = atomic_sub_return(1, &ubufs->refcount); // r = 0 wait_event(ubufs->wait, !atomic_read(&ubufs->refcount)); // no wait occurs here because condition is already true kfree(ubufs); if (unlikely(!r)) wake_up(&ubufs->wait); // use-after-free This leads to use-after-free on ubufs access. This happens because CPU1 skips waiting for wake_up() when refcount is already zero. To prevent that use a read-side RCU critical section in vhost_net_ubuf_put(), as suggested by Hillf Danton. For this lock to take effect, free ubufs with kfree_rcu(). Cc: stable@vger.kernel.org Fixes: 0ad8b480d6ee9 ("vhost: fix ref cnt checking deadlock") Reported-by: Andrey Ryabinin Suggested-by: Hillf Danton Signed-off-by: Nikolay Kuratov Message-Id: <20250805130917.727332-1-kniv@yandex-team.ru> Signed-off-by: Michael S. Tsirkin --- drivers/vhost/net.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c index 6edac0c1ba9b..c6508fe0d5c8 100644 --- a/drivers/vhost/net.c +++ b/drivers/vhost/net.c @@ -99,6 +99,7 @@ struct vhost_net_ubuf_ref { atomic_t refcount; wait_queue_head_t wait; struct vhost_virtqueue *vq; + struct rcu_head rcu; }; #define VHOST_NET_BATCH 64 @@ -250,9 +251,13 @@ vhost_net_ubuf_alloc(struct vhost_virtqueue *vq, bool zcopy) static int vhost_net_ubuf_put(struct vhost_net_ubuf_ref *ubufs) { - int r = atomic_sub_return(1, &ubufs->refcount); + int r; + + rcu_read_lock(); + r = atomic_sub_return(1, &ubufs->refcount); if (unlikely(!r)) wake_up(&ubufs->wait); + rcu_read_unlock(); return r; } @@ -265,7 +270,7 @@ static void vhost_net_ubuf_put_and_wait(struct vhost_net_ubuf_ref *ubufs) static void vhost_net_ubuf_put_wait_and_free(struct vhost_net_ubuf_ref *ubufs) { vhost_net_ubuf_put_and_wait(ubufs); - kfree(ubufs); + kfree_rcu(ubufs, rcu); } static void vhost_net_clear_ubuf_info(struct vhost_net *n) -- 2.25.1