qcow2: Prevent allocating L2 tables at offset 0

If the refcount data is corrupted then we can end up trying to
allocate a new L2 table at offset 0 in the image, triggering an
assertion in the qcow2 cache that would crash QEMU:

  qcow2_cache_entry_mark_dirty: Assertion `c->entries[i].offset != 0' failed

This patch adds an explicit check for this scenario and a new test
case.
Signed-off-by: NAlberto Garcia <berto@igalia.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Message-id: 92dac37191ae7844a2da22c122204eb493cc3133.1509718618.git.berto@igalia.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

block/qcow2-cluster.c

@@ -278,6 +278,14 @@ static int l2_allocate(BlockDriverState *bs, int l1_index, uint64_t **table)
         goto fail;
     }
 
+    /* If we're allocating the table at offset 0 then something is wrong */
+    if (l2_offset == 0) {
+        qcow2_signal_corruption(bs, true, -1, -1, "Preventing invalid "
+                                "allocation of L2 table at offset 0");
+        ret = -EIO;
+        goto fail;
+    }
+
     ret = qcow2_cache_flush(bs, s->refcount_block_cache);
     if (ret < 0) {
         goto fail;

tests/qemu-iotests/060

@@ -253,6 +253,13 @@ poke_file "$TEST_IMG" "$rt_offset"        "\x00\x00\x00\x00\x00\x00\x00\x00"
 # causing a refcount block to be allocated at offset 0
 $QEMU_IO -c "write 0 128k" "$TEST_IMG" | _filter_qemu_io
 
+echo
+echo "=== Testing empty refcount block ==="
+echo
+_make_test_img 64M
+poke_file "$TEST_IMG" "$rb_offset"        "\x00\x00\x00\x00\x00\x00\x00\x00"
+$QEMU_IO -c "write 0 64k" "$TEST_IMG" | _filter_qemu_io
+
 # success, all done
 echo "*** done"
 rm -f $seq.full

tests/qemu-iotests/060.out

@@ -189,4 +189,10 @@ wrote 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 qcow2: Marking image as corrupt: Preventing invalid allocation of refcount block at offset 0; further corruption events will be suppressed
 write failed: Input/output error
+
+=== Testing empty refcount block ===
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864
+qcow2: Marking image as corrupt: Preventing invalid allocation of L2 table at offset 0; further corruption events will be suppressed
+write failed: Input/output error
 *** done