vhost: Fix false positive out-of-bounds

QEMU uses vhost_svq_translate_addr() to translate addresses
between the QEMU's virtual address and the SVQ IOVA. In order
to validate this translation, QEMU checks whether the translated
range falls within the mapped range.

Yet the problem is that, the value of `needle_last`, which is calculated
by `needle.translated_addr + iovec[i].iov_len`, should represent the
exclusive boundary of the translated range, rather than the last
inclusive addresses of the range. Consequently, QEMU fails the check
when the translated range matches the size of the mapped range.

This patch solves this problem by fixing the `needle_last` value to
the last inclusive address of the translated range.

Note that this bug cannot be triggered at the moment, because QEMU
is unable to translate such a big range due to the truncation of
the CVQ command in vhost_vdpa_net_handle_ctrl_avail().

Fixes: 34e3c94 ("vdpa: Add custom IOTLB translations to SVQ")
Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
Message-Id: <ee31c5420ffc8e6a29705ddd30badb814ddbae1d.1688743107.git.yin31149@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/vhost-shadow-virtqueue.c

@@ -111,7 +111,7 @@ static bool vhost_svq_translate_addr(const VhostShadowVirtqueue *svq,
         addrs[i] = map->iova + off;
 
         needle_last = int128_add(int128_make64(needle.translated_addr),
-                                 int128_make64(iovec[i].iov_len));
+                                 int128_makes64(iovec[i].iov_len - 1));
         map_last = int128_make64(map->translated_addr + map->size);
         if (unlikely(int128_gt(needle_last, map_last))) {
             qemu_log_mask(LOG_GUEST_ERROR,