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bh=YEOGyJmGKUKsyGlXa7iWyGR4KW1LgZOGoEcIsMrdP3w=; b=M2MyrqgkDr4p2wDm7lfRGR+Nkf7doXodr23Ay5TKU3QBISQXyuylcu5NtPrPadvV718/Jg AsYkAJLHnS8/jCsqoFYiomYAQf5N8cxyN2poCsLKbYT4lbD40TLbgM7lXXCxhElBKOnzDf OHOo6EV2b3IHgREseOJtV7PuGQIhZ/U= X-MC-Unique: nKA3qtLaMVaHm3oT25aP5w-1 From: Maxim Levitsky To: qemu-devel@nongnu.org Subject: [PATCH 1/1] Fix qcow2 corruption on discard Date: Mon, 23 Nov 2020 17:49:29 +0200 Message-Id: <20201123154929.330338-2-mlevitsk@redhat.com> In-Reply-To: <20201123154929.330338-1-mlevitsk@redhat.com> References: <20201123154929.330338-1-mlevitsk@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mlevitsk@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; 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charset="utf-8" Commit 205fa50750 ("qcow2: Add subcluster support to zero_in_l2_slice()") introduced a subtle change to code in zero_in_l2_slice: It swapped the order of 1. qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice); 2. set_l2_entry(s, l2_slice, l2_index + i, QCOW_OFLAG_ZERO); 3. qcow2_free_any_clusters(bs, old_offset, 1, QCOW2_DISCARD_REQUEST); To 1. qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice); 2. qcow2_free_any_clusters(bs, old_offset, 1, QCOW2_DISCARD_REQUEST); 3. set_l2_entry(s, l2_slice, l2_index + i, QCOW_OFLAG_ZERO); It seems harmless, however the call to qcow2_free_any_clusters can trigger a cache flush which can mark the L2 table as clean, and assuming that this was the last write to it, a stale version of it will remain on the disk. Now we have a valid L2 entry pointing to a freed cluster. Oops. Fixes: 205fa50750 ("qcow2: Add subcluster support to zero_in_l2_slice()") Signed-off-by: Maxim Levitsky --- block/qcow2-cluster.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c index 485b4cb92e..267b46a4ca 100644 --- a/block/qcow2-cluster.c +++ b/block/qcow2-cluster.c @@ -2010,11 +2010,11 @@ static int zero_in_l2_slice(BlockDriverState *bs, u= int64_t offset, continue; } =20 - qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice); if (unmap) { qcow2_free_any_cluster(bs, old_l2_entry, QCOW2_DISCARD_REQUEST= ); } set_l2_entry(s, l2_slice, l2_index + i, new_l2_entry); + qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice); if (has_subclusters(s)) { set_l2_bitmap(s, l2_slice, l2_index + i, new_l2_bitmap); } --=20 2.26.2