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qcow2: Forbid discard in qcow2 v2 images with backing files

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A discard request deallocates the selected clusters so they read back
as zeroes. This is done by clearing the cluster offset field and
setting QCOW_OFLAG_ZERO in the L2 entry.

This flag is however only supported when qcow_version >= 3. In older
images the cluster is simply deallocated, exposing any possible stale
data from the backing file.

Since discard is an advisory operation it's safer to simply forbid it
in this scenario.

Note that we are adding this check to qcow2_co_pdiscard() and not to
qcow2_cluster_discard() or discard_in_l2_slice() because the last
two are also used by qcow2_snapshot_create() to discard the clusters
used by the VM state. In this case there's no risk of exposing stale
data to the guest and we really want that the clusters are always
discarded.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-Id: <20200331114345.29993-1-berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Alberto Garcia 2020-03-31 13:43:45 +02:00 • committed by Max Reitz
parent 53ef8a92eb
commit 80f5c01183
9 changed files with 187 additions and 19 deletions
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2
tests/qemu-iotests/060.out
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@ -105,8 +105,6 @@ wrote 65536/65536 bytes at offset 0
64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
wrote 65536/65536 bytes at offset 536870912
64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
discard 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 write on metadata (overlaps with active L2 table); further corruption events will be suppressed
blkdebug: Suspended request '0'
write failed: Input/output error