qcow2: Don't rely on free_cluster_index in alloc_refcount_block() (CVE-2014-0147)

free_cluster_index is only correct if update_refcount() was called from
an allocation function, and even there it's brittle because it's used to
protect unfinished allocations which still have a refcount of 0 - if it
moves in the wrong place, the unfinished allocation can be corrupted.

So not using it any more seems to be a good idea. Instead, use the
first requested cluster to do the calculations. Return -EAGAIN if
unfinished allocations could become invalid and let the caller restart
its search for some free clusters.

The context of creating a snapsnot is one situation where
update_refcount() is called outside of a cluster allocation. For this
case, the change fixes a buffer overflow if a cluster is referenced in
an L2 table that cannot be represented by an existing refcount block.
(new_table[refcount_table_index] was out of bounds)

[Bump the qemu-iotests 026 refblock_alloc.write leak count from 10 to
11.
--Stefan]

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

4 files changed, 22 insertions, 4 deletions

diff --git a/tests/qemu-iotests/026.out b/tests/qemu-iotests/026.out
index 1504579..f7c78e7 100644
--- a/tests/qemu-iotests/026.out
+++ b/tests/qemu-iotests/026.out
@@ -475,7 +475,7 @@ Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1073741824
 Event: refblock_alloc.write_blocks; errno: 28; imm: off; once: off; write 
 write failed: No space left on device
 
-10 leaked clusters were found on the image.
+11 leaked clusters were found on the image.
 This means waste of disk space, but no harm to data.
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1073741824 
 
@@ -499,7 +499,7 @@ Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1073741824
 Event: refblock_alloc.write_table; errno: 28; imm: off; once: off; write 
 write failed: No space left on device
 
-10 leaked clusters were found on the image.
+11 leaked clusters were found on the image.
 This means waste of disk space, but no harm to data.
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1073741824 
 
@@ -523,7 +523,7 @@ Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1073741824
 Event: refblock_alloc.switch_table; errno: 28; imm: off; once: off; write 
 write failed: No space left on device
 
-10 leaked clusters were found on the image.
+11 leaked clusters were found on the image.
 This means waste of disk space, but no harm to data.
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1073741824 
 
diff --git a/tests/qemu-iotests/044.out b/tests/qemu-iotests/044.out
index 5c5aa92..4789a53 100644
--- a/tests/qemu-iotests/044.out
+++ b/tests/qemu-iotests/044.out
@@ -1,6 +1,6 @@
 No errors were found on the image.
 7292415/33554432 = 21.73% allocated, 0.00% fragmented, 0.00% compressed clusters
-Image end offset: 4296448000
+Image end offset: 4296152064
 .
 ----------------------------------------------------------------------
 Ran 1 tests
diff --git a/tests/qemu-iotests/080 b/tests/qemu-iotests/080
index f3091a9..56f8903 100755
--- a/tests/qemu-iotests/080
+++ b/tests/qemu-iotests/080
@@ -56,6 +56,8 @@ offset_header_size=100
 offset_ext_magic=$header_size
 offset_ext_size=$((header_size + 4))
 
+offset_l2_table_0=$((0x40000))
+
 echo
 echo "== Huge header size =="
 _make_test_img 64M
@@ -143,6 +145,15 @@ poke_file "$TEST_IMG" "$offset_backing_file_offset" "\x00\x00\x00\x00\x00\x00\x1
 poke_file "$TEST_IMG" "$offset_backing_file_size" "\xff\xff\xff\xff"
 { $QEMU_IO -c "read 0 512" $TEST_IMG; } 2>&1 | _filter_qemu_io | _filter_testdir
 
+echo
+echo "== Invalid L2 entry (huge physical offset) =="
+_make_test_img 64M
+{ $QEMU_IO -c "write 0 512" $TEST_IMG; } 2>&1 | _filter_qemu_io | _filter_testdir
+poke_file "$TEST_IMG" "$offset_l2_table_0" "\xbf\xff\xff\xff\xff\xff\x00\x00"
+{ $QEMU_IMG snapshot -c test $TEST_IMG; } 2>&1 | _filter_qemu_io | _filter_testdir
+poke_file "$TEST_IMG" "$offset_l2_table_0" "\x80\x00\x00\xff\xff\xff\x00\x00"
+{ $QEMU_IMG snapshot -c test $TEST_IMG; } 2>&1 | _filter_qemu_io | _filter_testdir
+
 # success, all done
 echo "*** done"
 rm -f $seq.full
diff --git a/tests/qemu-iotests/080.out b/tests/qemu-iotests/080.out
index 8103211..303d6c3 100644
--- a/tests/qemu-iotests/080.out
+++ b/tests/qemu-iotests/080.out
@@ -63,4 +63,11 @@ no file open, try 'help open'
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 
 qemu-io: can't open device TEST_DIR/t.qcow2: Backing file name too long
 no file open, try 'help open'
+
+== Invalid L2 entry (huge physical offset) ==
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 
+wrote 512/512 bytes at offset 0
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+qemu-img: Could not create snapshot 'test': -27 (File too large)
+qemu-img: Could not create snapshot 'test': -11 (Resource temporarily unavailable)
 *** done