From: Stefan Hajnoczi <stefanha@redhat.com>

Date: Mon, 27 Jan 2020 19:02:05 +0000

Subject: [PATCH] virtiofsd: prevent fv_queue_thread() vs virtio_loop() races

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We call into libvhost-user from the virtqueue handler thread and the

vhost-user message processing thread without a lock.  There is nothing

protecting the virtqueue handler thread if the vhost-user message

processing thread changes the virtqueue or memory table while it is

running.

This patch introduces a read-write lock.  Virtqueue handler threads are

readers.  The vhost-user message processing thread is a writer.  This

will allow concurrency for multiqueue in the future while protecting

against fv_queue_thread() vs virtio_loop() races.

Note that the critical sections could be made smaller but it would be

more invasive and require libvhost-user changes.  Let's start simple and

improve performance later, if necessary.  Another option would be an

RCU-style approach with lighter-weight primitives.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

(cherry picked from commit e7b337326d594b71b07cd6dbb332c49c122c80a4)

---

 tools/virtiofsd/fuse_virtio.c | 34 +++++++++++++++++++++++++++++++++-

 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/tools/virtiofsd/fuse_virtio.c b/tools/virtiofsd/fuse_virtio.c

index fb8d6d1379..f6242f9338 100644

--- a/tools/virtiofsd/fuse_virtio.c

+++ b/tools/virtiofsd/fuse_virtio.c

@@ -58,6 +58,18 @@ struct fv_VuDev {

     VuDev dev;

     struct fuse_session *se;

+    /*

+     * Either handle virtqueues or vhost-user protocol messages.  Don't do

+     * both at the same time since that could lead to race conditions if

+     * virtqueues or memory tables change while another thread is accessing

+     * them.

+     *

+     * The assumptions are:

+     * 1. fv_queue_thread() reads/writes to virtqueues and only reads VuDev.

+     * 2. virtio_loop() reads/writes virtqueues and VuDev.

+     */

+    pthread_rwlock_t vu_dispatch_rwlock;

+

     /*

      * The following pair of fields are only accessed in the main

      * virtio_loop

@@ -415,6 +427,8 @@ static void *fv_queue_thread(void *opaque)

              qi->qidx, qi->kick_fd);

     while (1) {

         struct pollfd pf[2];

+        int ret;

+

         pf[0].fd = qi->kick_fd;

         pf[0].events = POLLIN;

         pf[0].revents = 0;

@@ -461,6 +475,9 @@ static void *fv_queue_thread(void *opaque)

             fuse_log(FUSE_LOG_ERR, "Eventfd_read for queue: %m\n");

             break;

         }

+        /* Mutual exclusion with virtio_loop() */

+        ret = pthread_rwlock_rdlock(&qi->virtio_dev->vu_dispatch_rwlock);

+        assert(ret == 0); /* there is no possible error case */

         /* out is from guest, in is too guest */

         unsigned int in_bytes, out_bytes;

         vu_queue_get_avail_bytes(dev, q, &in_bytes, &out_bytes, ~0, ~0);

@@ -469,6 +486,7 @@ static void *fv_queue_thread(void *opaque)

                  "%s: Queue %d gave evalue: %zx available: in: %u out: %u\n",

                  __func__, qi->qidx, (size_t)evalue, in_bytes, out_bytes);

+

         while (1) {

             bool allocated_bufv = false;

             struct fuse_bufvec bufv;

@@ -597,6 +615,8 @@ static void *fv_queue_thread(void *opaque)

             free(elem);

             elem = NULL;

         }

+

+        pthread_rwlock_unlock(&qi->virtio_dev->vu_dispatch_rwlock);

     }

 out:

     pthread_mutex_destroy(&ch.lock);

@@ -711,6 +731,8 @@ int virtio_loop(struct fuse_session *se)

     while (!fuse_session_exited(se)) {

         struct pollfd pf[1];

+        bool ok;

+        int ret;

         pf[0].fd = se->vu_socketfd;

         pf[0].events = POLLIN;

         pf[0].revents = 0;

@@ -735,7 +757,15 @@ int virtio_loop(struct fuse_session *se)

         }

         assert(pf[0].revents & POLLIN);

         fuse_log(FUSE_LOG_DEBUG, "%s: Got VU event\n", __func__);

-        if (!vu_dispatch(&se->virtio_dev->dev)) {

+        /* Mutual exclusion with fv_queue_thread() */

+        ret = pthread_rwlock_wrlock(&se->virtio_dev->vu_dispatch_rwlock);

+        assert(ret == 0); /* there is no possible error case */

+

+        ok = vu_dispatch(&se->virtio_dev->dev);

+

+        pthread_rwlock_unlock(&se->virtio_dev->vu_dispatch_rwlock);

+

+        if (!ok) {

             fuse_log(FUSE_LOG_ERR, "%s: vu_dispatch failed\n", __func__);

             break;

         }

@@ -877,6 +907,7 @@ int virtio_session_mount(struct fuse_session *se)

     se->vu_socketfd = data_sock;

     se->virtio_dev->se = se;

+    pthread_rwlock_init(&se->virtio_dev->vu_dispatch_rwlock, NULL);

     vu_init(&se->virtio_dev->dev, 2, se->vu_socketfd, fv_panic, fv_set_watch,

             fv_remove_watch, &fv_iface);

@@ -892,6 +923,7 @@ void virtio_session_close(struct fuse_session *se)

     }

     free(se->virtio_dev->qi);

+    pthread_rwlock_destroy(&se->virtio_dev->vu_dispatch_rwlock);

     free(se->virtio_dev);

     se->virtio_dev = NULL;

 }