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

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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>
This commit is contained in:
Kevin Wolf 2014-03-28 18:06:31 +01:00 • committed by Stefan Hajnoczi
parent 6d33e8e7dc
commit b106ad9185
6 changed files with 65 additions and 44 deletions
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11
tests/qemu-iotests/080
View file

@ -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