[Qemu-devel] [PATCH 1/2] libvhost-user: fix vu_queue_get_avail_bytes() crash

[Marc-André Lureau]

Calling vu_queue_get_avail_bytes() when the queue doesn't yet have
addresses will result in the following crash:

Program received signal SIGSEGV, Segmentation fault.
0x000055c414112ce4 in vring_avail_idx (vq=0x55c41582fd68, vq=0x55c41582fd68)
    at /home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:940
940            vq->shadow_avail_idx = vq->vring.avail->idx;
(gdb) p vq
$1 = (VuVirtq *) 0x55c41582fd68
(gdb) p vq->vring
$2 = {num = 0, desc = 0x0, avail = 0x0, used = 0x0, log_guest_addr = 0, flags = 
0}

    at /home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:940
No locals.
    at /home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:960
        num_heads = <optimized out>
    address@hidden, address@hidden,
    address@hidden) at 
/home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:1034

Check if vring.avail != null before accessing it.

Signed-off-by: Marc-André Lureau <address@hidden>
---
 contrib/libvhost-user/libvhost-user.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/contrib/libvhost-user/libvhost-user.c 
b/contrib/libvhost-user/libvhost-user.c
index af4faad60b..f9680b6279 100644
--- a/contrib/libvhost-user/libvhost-user.c
+++ b/contrib/libvhost-user/libvhost-user.c
@@ -1031,6 +1031,10 @@ vu_queue_get_avail_bytes(VuDev *dev, VuVirtq *vq, 
unsigned int *in_bytes,
     idx = vq->last_avail_idx;
 
     total_bufs = in_total = out_total = 0;
+    if (!vq->vring.avail) {
+        goto done;
+    }
+
     while ((rc = virtqueue_num_heads(dev, vq, idx)) > 0) {
         unsigned int max, num_bufs, indirect = 0;
         struct vring_desc *desc;
-- 
2.12.0.191.gc5d8de91d