[PATCH] mm, swap: free the cluster extend table on teardown

[David Carlier <devnexen@gmail.com>]

swap_cluster_free_table() frees every per-cluster side table but
ci->extend_table. That table is only released by
swap_extend_table_try_free(), which the teardown path never calls, so a
cluster can be freed with an extend table still attached.

It can also linger while the cluster is live. swap_dup_entries_cluster()
drops the lock to allocate an extend table when a slot reaches
SWP_TB_COUNT_MAX - 1, then retries. If the count dropped in the meantime,
the retry takes the normal path and leaves the table behind, all entries
zero; only the failure path frees it.

Since a swap_cluster_info is reused in place and swap_extend_table_alloc()
skips allocation when ci->extend_table is set, the next user of the
cluster inherits the stale table and its leftover counts, corrupting the
swap count of any slot that overflows. CONFIG_DEBUG_VM catches the
dangling table in swap_cluster_assert_empty(); otherwise it is silent.

Free it in swap_cluster_free_table(), and also on the
swap_dup_entries_cluster() success path to match the failure path.

Reported-by: syzbot+deedf22929084640666f@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=deedf22929084640666f
Fixes: 0d6af9bcf383 ("mm, swap: use the swap table to track the swap count")
Cc: <stable@vger.kernel.org>
Signed-off-by: David Carlier <devnexen@gmail.com>
---
 mm/swapfile.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 615d90867111..a69a26aec4c0 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -432,6 +432,9 @@ static void swap_cluster_free_table(struct swap_cluster_info *ci)
 	ci->zero_bitmap = NULL;
 #endif
 
+	kfree(ci->extend_table);
+	ci->extend_table = NULL;
+
 	table = (struct swap_table *)rcu_access_pointer(ci->table);
 	if (!table)
 		return;
@@ -1711,6 +1714,7 @@ static int swap_dup_entries_cluster(struct swap_info_struct *si,
 			goto failed;
 		}
 	} while (++ci_off < ci_end);
+	swap_extend_table_try_free(ci);
 	swap_cluster_unlock(ci);
 	return 0;
 failed:
-- 
2.53.0