[Devel] [PATCH hci-8.1 3/3] dm-qcow2: fix SEEK_HOLE/SEEK_DATA using an image freed by a merge

["Denis V. Lunev" <den@openvz.org>]

From: Denis V. Lunev <den@openvz.org>

qcow2_llseek_hole() starts at tgt->top and follows qcow2->lower,
parsing the metadata of each image, without an inflight ref. dm core
holds only the table SRCU around the call, which does not keep a
target message out. merge_backward complete unlinks the merged image,
waits in qcow2_inflight_ref_switch() for the qios holding refs and
destroys the image, so a seek already walking it goes on with a freed
struct qcow2, its md pages and its file. A SEEK_DATA scan of a large
sparse image keeps the walk running long enough to hit it.

Hold an inflight ref for the whole walk and take the top image from
qcow2_ref_inc() under it.

Fixes: 412d5a4f7bf1 ("dm/dm-qcow2: add llseek_hole")
https://virtuozzo.atlassian.net/browse/VSTOR-153050
Feature: dm-qcow2: block device over QCOW2 files driver
Signed-off-by: Denis V. Lunev <den@openvz.org>
---
 drivers/md/dm-qcow2-map.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/md/dm-qcow2-map.c b/drivers/md/dm-qcow2-map.c
index 8ab7cb472560..358a71f75cb4 100644
--- a/drivers/md/dm-qcow2-map.c
+++ b/drivers/md/dm-qcow2-map.c
@@ -4841,18 +4841,25 @@ static int qcow2_llseek_hole_qio(struct qio *qio, int whence, loff_t *result)
 
 loff_t qcow2_llseek_hole(struct dm_target *ti, loff_t offset, int whence)
 {
-	struct qcow2 *qcow2 = to_qcow2_target(ti)->top;
+	struct qcow2_target *tgt = to_qcow2_target(ti);
 	loff_t result = -EINVAL;
+	struct qcow2 *qcow2;
 	struct qio *qio;
+	u8 ref_index;
 	int ret;
 
+	/* Keep merge_backward complete from freeing images under the walk */
+	qcow2 = qcow2_ref_inc(tgt, &ref_index);
 	qio = alloc_seek_qio(qcow2, NULL, qcow2->hdr.size);
-	if (!qio)
+	if (!qio) {
+		qcow2_ref_dec(tgt, ref_index);
 		return -ENOMEM;
+	}
 
 	seek_qio_set_sector(qio, to_sector(offset));
 
 	ret = qcow2_llseek_hole_qio(qio, whence, &result);
+	qcow2_ref_dec(tgt, ref_index);
 	/* In case of error remap ENXIO as it have special meaning for llseek */
 	if (ret < 0)
 		return (ret == -ENXIO) ? -EINVAL : ret;
-- 
2.53.0

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