[PATCH AUTOSEL 5.6 14/79] SUNRPC/cache: Fix unsafe traverse caused double-free in cache_purge

[Sasha Levin <sashal@kernel.org>]

From: Yihao Wu <wuyihao@linux.alibaba.com>

[ Upstream commit 43e33924c38e8faeb0c12035481cb150e602e39d ]

Deleting list entry within hlist_for_each_entry_safe is not safe unless
next pointer (tmp) is protected too. It's not, because once hash_lock
is released, cache_clean may delete the entry that tmp points to. Then
cache_purge can walk to a deleted entry and tries to double free it.

Fix this bug by holding only the deleted entry's reference.

Suggested-by: NeilBrown <neilb@suse.de>
Signed-off-by: Yihao Wu <wuyihao@linux.alibaba.com>
Reviewed-by: NeilBrown <neilb@suse.de>
[ cel: removed unused variable ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/sunrpc/cache.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
index bd843a81afa0b..d36cea4e270de 100644
--- a/net/sunrpc/cache.c
+++ b/net/sunrpc/cache.c
@@ -521,7 +521,6 @@ void cache_purge(struct cache_detail *detail)
 {
 	struct cache_head *ch = NULL;
 	struct hlist_head *head = NULL;
-	struct hlist_node *tmp = NULL;
 	int i = 0;
 
 	spin_lock(&detail->hash_lock);
@@ -533,7 +532,9 @@ void cache_purge(struct cache_detail *detail)
 	dprintk("RPC: %d entries in %s cache\n", detail->entries, detail->name);
 	for (i = 0; i < detail->hash_size; i++) {
 		head = &detail->hash_table[i];
-		hlist_for_each_entry_safe(ch, tmp, head, cache_list) {
+		while (!hlist_empty(head)) {
+			ch = hlist_entry(head->first, struct cache_head,
+					 cache_list);
 			sunrpc_begin_cache_remove_entry(ch, detail);
 			spin_unlock(&detail->hash_lock);
 			sunrpc_end_cache_remove_entry(ch, detail);
-- 
2.20.1