Re: [Qemu-devel] [PATCH v3] iotests: Test abnormally large size in compressed cluster descriptor

[Alberto Garcia]

ping

On Mon 26 Feb 2018 03:36:23 PM CET, Alberto Garcia wrote:
> L2 entries for compressed clusters have a field that indicates the
> number of sectors used to store the data in the image.
>
> That's however not the size of the compressed data itself, just the
> number of sectors where that data is located. The actual data size is
> usually not a multiple of the sector size, and therefore cannot be
> represented with this field.
>
> The way it works is that QEMU reads all the specified sectors and
> starts decompressing the data until there's enough to recover the
> original uncompressed cluster. If there are any bytes left that
> haven't been decompressed they are simply ignored.
>
> One consequence of this is that even if the size field is larger than
> it needs to be QEMU can handle it just fine: it will read more data
> from disk but it will ignore the extra bytes.
>
> This test creates an image with two compressed clusters that use 5
> sectors (2.5 KB) each, increases the size field to the maximum (8192
> sectors, or 4 MB) and verifies that the data can be read without
> problems.
>
> This test is important because while the decompressed data takes
> exactly one cluster, the maximum value allowed in the compressed size
> field is twice the cluster size. So although QEMU won't produce images
> with such large values we need to make sure that it can handle them.
>
> Another effect of increasing the size field is that it can make
> it include data from the following host cluster(s). In this case
> 'qemu-img check' will detect that the refcounts are not correct, and
> we'll need to rebuild them.
>
> Additionally, this patch also tests that decreasing the size corrupts
> the image since the original data can no longer be recovered. In this
> case QEMU returns an error when trying to read the compressed data,
> but 'qemu-img check' doesn't see anything wrong if the refcounts are
> consistent.
>
> One possible task for the future is to make 'qemu-img check' verify
> the sizes of the compressed clusters, by trying to decompress the data
> and checking that the size stored in the L2 entry is correct.
>
> Signed-off-by: Alberto Garcia 
> ---
>
> v3: Add TODO comment, as suggested by Eric.
>
> Corrupt the length of the second compressed cluster as well so the
> uncompressed data would span three host clusters.
>
> v2: We now have two scenarios where we make QEMU read data from the
> next host cluster and from beyond the end of the image. This
> version also runs qemu-img check on the corrupted image.
>
> If the size field is too small, reading fails but qemu-img check
> succeeds.
>
> If the size field is too large, reading succeeds but qemu-img
> check fails (this can be repaired, though).
>
> ---
>  tests/qemu-iotests/122 | 45 +
>  tests/qemu-iotests/122.out | 31 +++
>  2 files changed, 76 insertions(+)
>
> diff --git a/tests/qemu-iotests/122 b/tests/qemu-iotests/122
> index 45b359c2ba..5b9593016c 100755
> --- a/tests/qemu-iotests/122
> +++ b/tests/qemu-iotests/122
> @@ -130,6 +130,51 @@ $QEMU_IO -c "read -P 01024k 1022k" "$TEST_IMG" 2>&1 
> | _filter_qemu_io | _fil
>  
>  
>  echo
> +echo "=== Corrupted size field in compressed cluster descriptor ==="
> +echo
> +# Create an empty image, fill half of it with data and compress it.
> +# The L2 entries of the two compressed clusters are located at
> +# 0x80 and 0x88, their original values are 0x400800a0
> +# and 0x400800a00802 (5 sectors for compressed data each).
> +TEST_IMG="$TEST_IMG".1 _make_test_img 8M
> +$QEMU_IO -c "write -P 0x11 0 4M" "$TEST_IMG".1 2>&1 | _filter_qemu_io | 
> _filter_testdir
> +$QEMU_IMG convert -c -O qcow2 -o cluster_size=2M "$TEST_IMG".1 "$TEST_IMG"
> +
> +# Reduce size of compressed data to 4 sectors: this corrupts the image.
> +poke_file "$TEST_IMG" $((0x80)) "\x40\x06"
> +$QEMU_IO -c "read  -P 0x11 0 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | 
> _filter_testdir
> +
> +# 'qemu-img check' however doesn't see anything wrong because it
> +# doesn't try to decompress the data and the refcounts are consistent.
> +# TODO: update qemu-img so this can be detected
> +_check_test_img
> +
> +# Increase size of compressed data to the maximum (8192 sectors).
> +# This makes QEMU read more data (8192 sectors instead of 5, host
> +# addresses [0xa0, 0xdf]), but the decompression algorithm
> +# stops once we have enough to restore the uncompressed cluster, so
> +# the rest of the data is ignored.
> +poke_file "$TEST_IMG" $((0x80)) "\x7f\xfe"
> +# Do it also for the second compressed cluster (L2 entry at 0x88).
> +# In this case the compressed data would span 3 host clusters
> +# (host addresses: [0xa00802, 0xe00801])
> +poke_file "$TEST_IMG" $((0x88)) "\x7f\xfe"
> +
> +# Here the image is too small so we're asking QEMU to read beyond the
> +# end of the image.
> +$QEMU_IO -c "read 

Re: [Qemu-devel] [PATCH v3] iotests: Test abnormally large size in compressed cluster descriptor

[Alberto Garcia]

On Mon 26 Feb 2018 04:12:22 PM CET, Eric Blake wrote:
>> +# Create an empty image, fill half of it with data and compress it.
>> +# The L2 entries of the two compressed clusters are located at
>> +# 0x80 and 0x88, their original values are 0x400800a0
>> +# and 0x400800a00802 (5 sectors for compressed data each).
>> +TEST_IMG="$TEST_IMG".1 _make_test_img 8M
>> +$QEMU_IO -c "write -P 0x11 0 4M" "$TEST_IMG".1 2>&1 | _filter_qemu_io | 
>> _filter_testdir
>> +$QEMU_IMG convert -c -O qcow2 -o cluster_size=2M "$TEST_IMG".1 "$TEST_IMG"
>
> Is it worth a $QEMU_IO -c "read -v 0x80 16" so that our .out file
> validates that we indeed see the values we expect (if some other qcow2
> change causes us to stick the L2 table at a different offset, the
> verbose read will make it a bit more obvious why this test starts
> failing).

I don't think it's necessary, the success of the rest of the tests after
this one requires that we modify these exact L2 entries. If we're
touching something else then one of them will fail.

Berto

Re: [Qemu-devel] [PATCH v3] iotests: Test abnormally large size in compressed cluster descriptor

[Eric Blake]

On 02/26/2018 08:36 AM, Alberto Garcia wrote:

L2 entries for compressed clusters have a field that indicates the
number of sectors used to store the data in the image.





One consequence of this is that even if the size field is larger than
it needs to be QEMU can handle it just fine: it will read more data
from disk but it will ignore the extra bytes.


Modulo any ref count checks, of course ;)




Signed-off-by: Alberto Garcia 
---

v3: Add TODO comment, as suggested by Eric.

 Corrupt the length of the second compressed cluster as well so the
 uncompressed data would span three host clusters.


Rather, it is the 'claimed' size of the compressed data that spans three 
host clusters.  I don't know if our refcount repair code is geared for 
that (it IS prepared for a compressed data cluster that spans two host 
clusters, but spanning three is only possible for externally-produced 
images, as in this test).




  echo
+echo "=== Corrupted size field in compressed cluster descriptor ==="
+echo
+# Create an empty image, fill half of it with data and compress it.
+# The L2 entries of the two compressed clusters are located at
+# 0x80 and 0x88, their original values are 0x400800a0
+# and 0x400800a00802 (5 sectors for compressed data each).
+TEST_IMG="$TEST_IMG".1 _make_test_img 8M
+$QEMU_IO -c "write -P 0x11 0 4M" "$TEST_IMG".1 2>&1 | _filter_qemu_io | 
_filter_testdir
+$QEMU_IMG convert -c -O qcow2 -o cluster_size=2M "$TEST_IMG".1 "$TEST_IMG"


Is it worth a $QEMU_IO -c "read -v 0x80 16" so that our .out file 
validates that we indeed see the values we expect (if some other qcow2 
change causes us to stick the L2 table at a different offset, the 
verbose read will make it a bit more obvious why this test starts 
failing).  But that's an extra layer of paranoia, I'm fairly confident 
this test would start failing even without that extra read, so it's not 
a reason for a respin.



+
+# Reduce size of compressed data to 4 sectors: this corrupts the image.
+poke_file "$TEST_IMG" $((0x80)) "\x40\x06"
+$QEMU_IO -c "read  -P 0x11 0 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | 
_filter_testdir
+
+# 'qemu-img check' however doesn't see anything wrong because it
+# doesn't try to decompress the data and the refcounts are consistent.
+# TODO: update qemu-img so this can be detected
+_check_test_img
+
+# Increase size of compressed data to the maximum (8192 sectors).
+# This makes QEMU read more data (8192 sectors instead of 5, host
+# addresses [0xa0, 0xdf]), but the decompression algorithm
+# stops once we have enough to restore the uncompressed cluster, so
+# the rest of the data is ignored.
+poke_file "$TEST_IMG" $((0x80)) "\x7f\xfe"
+# Do it also for the second compressed cluster (L2 entry at 0x88).
+# In this case the compressed data would span 3 host clusters
+# (host addresses: [0xa00802, 0xe00801])
+poke_file "$TEST_IMG" $((0x88)) "\x7f\xfe"
+
+# Here the image is too small so we're asking QEMU to read beyond the
+# end of the image.
+$QEMU_IO -c "read  -P 0x11  0 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | 
_filter_testdir
+# But if we grow the image we won't be reading beyond its end anymore.
+$QEMU_IO -c "write -P 0x22 4M 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | 
_filter_testdir
+$QEMU_IO -c "read  -P 0x11  0 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | 
_filter_testdir
+
+# The refcount data is however wrong because due to the increased size
+# of the compressed data it now reaches the following host clusters.
+# This can be repaired by qemu-img check.
+_check_test_img -r all
+$QEMU_IO -c "read  -P 0x11  0 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | 
_filter_testdir
+$QEMU_IO -c "read  -P 0x22 4M 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | 
_filter_testdir


Looks good.  Thanks for adding this in v3.

Reviewed-by: Eric Blake 

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.   +1-919-301-3266
Virtualization:  qemu.org | libvirt.org