qcow2: Fix NULL dereference in qcow2_open() error path (CVE-2014-0146)

The qcow2 code assumes that s->snapshots is non-NULL if s->nb_snapshots
!= 0. By having the initialisation of both fields separated in
qcow2_open(), any error occuring in between would cause the error path
to dereference NULL in qcow2_free_snapshots() if the image had any
snapshots.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/qcow2.c

@@ -637,9 +637,6 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
         goto fail;
     }
 
-    s->snapshots_offset = header.snapshots_offset;
-    s->nb_snapshots = header.nb_snapshots;
-
     /* read the level 1 table */
     if (header.l1_size > 0x2000000) {
         /* 32 MB L1 table is enough for 2 PB images at 64k cluster size
@@ -734,6 +731,10 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
         bs->backing_file[len] = '\0';
     }
 
+    /* Internal snapshots */
+    s->snapshots_offset = header.snapshots_offset;
+    s->nb_snapshots = header.nb_snapshots;
+
     ret = qcow2_read_snapshots(bs);
     if (ret < 0) {
         error_setg_errno(errp, -ret, "Could not read snapshots");

tests/qemu-iotests/080

@@ -138,6 +138,13 @@ poke_file "$TEST_IMG" "$offset_l1_table_offset" "\x12\x34\x56\x78\x90\xab\xcd\xe
 poke_file "$TEST_IMG" "$offset_l1_size" "\x00\x00\x00\x01"
 { $QEMU_IO -c "read 0 512" $TEST_IMG; } 2>&1 | _filter_qemu_io | _filter_testdir
 
+echo
+echo "== Invalid L1 table (with internal snapshot in the image) =="
+_make_test_img 64M
+{ $QEMU_IMG snapshot -c foo $TEST_IMG; } 2>&1 | _filter_qemu_io | _filter_testdir
+poke_file "$TEST_IMG" "$offset_l1_size" "\x00\x00\x00\x00"
+_img_info
+
 echo
 echo "== Invalid backing file size =="
 _make_test_img 64M

tests/qemu-iotests/080.out

@@ -59,6 +59,10 @@ no file open, try 'help open'
 qemu-io: can't open device TEST_DIR/t.qcow2: Invalid L1 table offset
 no file open, try 'help open'
 
+== Invalid L1 table (with internal snapshot in the image) ==
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 
+qemu-img: Could not open 'TEST_DIR/t.IMGFMT': L1 table is too small
+
 == Invalid backing file size ==
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 
 qemu-io: can't open device TEST_DIR/t.qcow2: Backing file name too long