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Tsirkin" To: Hillf Danton Cc: Nikolay Kuratov , linux-kernel@vger.kernel.org, virtualization@lists.linux.dev, Jason Wang , Eugenio Perez , Lei Yang , Andrey Ryabinin , Andrey Smetanin Subject: Re: [PATCH v2] vhost/net: Replace wait_queue with completion in ubufs reference Message-ID: <20250720121256-mutt-send-email-mst@kernel.org> References: <20250718110355.1550454-1-kniv@yandex-team.ru> <20250718230356.2459-1-hdanton@sina.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250718230356.2459-1-hdanton@sina.com> On Sat, Jul 19, 2025 at 07:03:23AM +0800, Hillf Danton wrote: > On Fri, 18 Jul 2025 14:03:55 +0300 Nikolay Kuratov wrote: > > 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 completion instead of wait_queue as the ubufs > > notification mechanism. wait_for_completion() guarantees that there will > > be complete() call prior to its return. > > > Alternatively rcu helps. > > --- x/drivers/vhost/net.c > +++ y/drivers/vhost/net.c > @@ -96,6 +96,7 @@ 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 > @@ -247,9 +248,13 @@ vhost_net_ubuf_alloc(struct vhost_virtqu > > 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; > } > > @@ -262,7 +267,7 @@ static void vhost_net_ubuf_put_and_wait( > 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) I like that. -- MST