dmg: prevent out-of-bounds array access on terminator

When a terminator is reached the base for offsets and sectors is stored.
The following records that are processed will use this base value.

If the first record we encounter is a terminator, then calculating the
base values would result in out-of-bounds array accesses.  Don't do
that.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

diff --git a/block/dmg.c b/block/dmg.c
index be2f26e85e86499a92c4dc40d2d1ef93af329f53..f4f3e8e9f2e3a6d247060c6788c18ead96194352 100644 (file)
--- a/block/dmg.c
+++ b/block/dmg.c
@@ -182,7 +182,7 @@ static int dmg_open(BlockDriverState *bs, QDict *options, int flags,
                 offset += 4;
                 if (s->types[i] != 0x80000005 && s->types[i] != 1 &&
                     s->types[i] != 2) {
-                    if (s->types[i] == 0xffffffff) {
+                    if (s->types[i] == 0xffffffff && i > 0) {
                         last_in_offset = s->offsets[i - 1] + s->lengths[i - 1];
                         last_out_offset = s->sectors[i - 1] +
                                           s->sectorcounts[i - 1];