[PATCH 40/47] vhost: avoid a potential use of an uninitialized variable in vhost_svq_poll()

[Michael Tokarev]

From: Carlos López <clopez@suse.de>

In vhost_svq_poll(), if vhost_svq_get_buf() fails due to a device
providing invalid descriptors, len is left uninitialized and returned
to the caller, potentally leaking stack data or causing undefined
behavior.

Fix this by initializing len to 0.

Found with GCC 13 and -fanalyzer (abridged):

../hw/virtio/vhost-shadow-virtqueue.c: In function ‘vhost_svq_poll’:
../hw/virtio/vhost-shadow-virtqueue.c:538:12: warning: use of uninitialized 
value ‘len’ [CWE-457] [-Wanalyzer-use-of-uninitialized-value]
  538 |     return len;
      |            ^~~
  ‘vhost_svq_poll’: events 1-4
    |
    |  522 | size_t vhost_svq_poll(VhostShadowVirtqueue *svq)
    |      |        ^~~~~~~~~~~~~~
    |      |        |
    |      |        (1) entry to ‘vhost_svq_poll’
    |......
    |  525 |     uint32_t len;
    |      |              ~~~
    |      |              |
    |      |              (2) region created on stack here
    |      |              (3) capacity: 4 bytes
    |......
    |  528 |         if (vhost_svq_more_used(svq)) {
    |      |             ~
    |      |             |
    |      |             (4) inlined call to ‘vhost_svq_more_used’ from 
‘vhost_svq_poll’

    (...)

    |  528 |         if (vhost_svq_more_used(svq)) {
    |      |            ^~~~~~~~~~~~~~~~~~~~~~~~~
    |      |            ||
    |      |            |(8) ...to here
    |      |            (7) following ‘true’ branch...
    |......
    |  537 |     vhost_svq_get_buf(svq, &len);
    |      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |     |
    |      |     (9) calling ‘vhost_svq_get_buf’ from ‘vhost_svq_poll’
    |
    +--> ‘vhost_svq_get_buf’: events 10-11
           |
           |  416 | static VirtQueueElement 
*vhost_svq_get_buf(VhostShadowVirtqueue *svq,
           |      |                          ^~~~~~~~~~~~~~~~~
           |      |                          |
           |      |                          (10) entry to ‘vhost_svq_get_buf’
           |......
           |  423 |     if (!vhost_svq_more_used(svq)) {
           |      |          ~
           |      |          |
           |      |          (11) inlined call to ‘vhost_svq_more_used’ from 
‘vhost_svq_get_buf’
           |

           (...)

           |
         ‘vhost_svq_get_buf’: event 14
           |
           |  423 |     if (!vhost_svq_more_used(svq)) {
           |      |        ^
           |      |        |
           |      |        (14) following ‘false’ branch...
           |
         ‘vhost_svq_get_buf’: event 15
           |
           |cc1:
           | (15): ...to here
           |
    <------+
    |
  ‘vhost_svq_poll’: events 16-17
    |
    |  537 |     vhost_svq_get_buf(svq, &len);
    |      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |     |
    |      |     (16) returning to ‘vhost_svq_poll’ from ‘vhost_svq_get_buf’
    |  538 |     return len;
    |      |            ~~~
    |      |            |
    |      |            (17) use of uninitialized value ‘len’ here

Note by  Laurent Vivier <lvivier@redhat.com>:

    The return value is only used to detect an error:

    vhost_svq_poll
        vhost_vdpa_net_cvq_add
            vhost_vdpa_net_load_cmd
                vhost_vdpa_net_load_mac
                  -> a negative return is only used to detect error
                vhost_vdpa_net_load_mq
                  -> a negative return is only used to detect error
            vhost_vdpa_net_handle_ctrl_avail
              -> a negative return is only used to detect error

Fixes: d368c0b052ad ("vhost: Do not depend on !NULL VirtQueueElement on 
vhost_svq_flush")
Signed-off-by: Carlos López <clopez@suse.de>
Message-Id: <20230213085747.19956-1-clopez@suse.de>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit e4dd39c699b7d63a06f686ec06ded8adbee989c1)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
---
 hw/virtio/vhost-shadow-virtqueue.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/virtio/vhost-shadow-virtqueue.c 
b/hw/virtio/vhost-shadow-virtqueue.c
index 5bd14cad96..a723073747 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c
@@ -522,7 +522,7 @@ static void vhost_svq_flush(VhostShadowVirtqueue *svq,
 size_t vhost_svq_poll(VhostShadowVirtqueue *svq)
 {
     int64_t start_us = g_get_monotonic_time();
-    uint32_t len;
+    uint32_t len = 0;
 
     do {
         if (vhost_svq_more_used(svq)) {
-- 
2.30.2