[PATCH 5.10.y 2/2] dm-thin: fix metadata refcount underflow

[Sasha Levin <sashal@kernel.org>]

From: Mikulas Patocka <mpatocka@redhat.com>

[ Upstream commit 09a65adc7d8bbfce06392cb6d375468e2728ead5 ]

There's a bug in dm-thin in the function rebalance_children. If the
internal btree node has one entry, the code tries to copy all btree
entries from the node's child to the node itself and then decrement the
child's reference count.

If the child node is shared (it has reference count > 1), we won't free
it, so there would be two pointers to each of the grandchildren nodes.
But the reference counts of the grandchildren is not increased, thus the
reference count doesn't match the number of pointers that point to the
grandchildren. This results in "device mapper: space map common: unable
to decrement block" errors.

Fix this bug by incrementing reference counts on the grandchildren if the
btree node is shared.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: 3241b1d3e0aa ("dm: add persistent data library")
Cc: stable@vger.kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/md/persistent-data/dm-btree-remove.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/md/persistent-data/dm-btree-remove.c b/drivers/md/persistent-data/dm-btree-remove.c
index 63f2baed3c8a6..1cf31861020b4 100644
--- a/drivers/md/persistent-data/dm-btree-remove.c
+++ b/drivers/md/persistent-data/dm-btree-remove.c
@@ -415,12 +415,20 @@ static int rebalance_children(struct shadow_spine *s,
 
 	if (le32_to_cpu(n->header.nr_entries) == 1) {
 		struct dm_block *child;
+		int is_shared;
 		dm_block_t b = value64(n, 0);
 
+		r = dm_tm_block_is_shared(info->tm, b, &is_shared);
+		if (r)
+			return r;
+
 		r = dm_tm_read_lock(info->tm, b, &btree_node_validator, &child);
 		if (r)
 			return r;
 
+		if (is_shared)
+			inc_children(info->tm, dm_block_data(child), vt);
+
 		memcpy(n, dm_block_data(child),
 		       dm_bm_block_size(dm_tm_get_bm(info->tm)));
 
-- 
2.53.0