ivshmem: validate incoming_posn value from server

Check incoming_posn to avoid out-of-bounds array accesses if the ivshmem
server on the host sends invalid values.

Cc: Cam Macdonell <cam@cs.ualberta.ca>
Reported-by: NSebastian Krahmer <krahmer@suse.de>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
[AF: Tighten upper bound check for posn in close_guest_eventfds()]
Cc: qemu-stable@nongnu.org
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/misc/ivshmem.c

@@ -389,6 +389,9 @@ static void close_guest_eventfds(IVShmemState *s, int posn)
     if (!ivshmem_has_feature(s, IVSHMEM_IOEVENTFD)) {
         return;
     }
+    if (posn < 0 || posn >= s->nb_peers) {
+        return;
+    }
 
     guest_curr_max = s->peers[posn].nb_eventfds;
 
@@ -455,6 +458,11 @@ static void ivshmem_read(void *opaque, const uint8_t *buf, int size)
         }
     }
 
+    if (incoming_posn < -1) {
+        IVSHMEM_DPRINTF("invalid incoming_posn %ld\n", incoming_posn);
+        return;
+    }
+
     /* pick off s->server_chr->msgfd and store it, posn should accompany msg */
     tmp_fd = qemu_chr_fe_get_msgfd(s->server_chr);
     IVSHMEM_DPRINTF("posn is %ld, fd is %d\n", incoming_posn, tmp_fd);