[PATCH v3] SUNRPC/cache: Fix unsafe traverse caused double-free in cache_purge

[Yihao Wu <wuyihao@linux.alibaba.com>]

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.

Signed-off-by: Yihao Wu <wuyihao@linux.alibaba.com>
---
v1->v2: Use Neil's better solution
v2->v3: Fix a checkscript warning

 net/sunrpc/cache.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
index af0ddd28b081..b445874e8e2f 100644
--- a/net/sunrpc/cache.c
+++ b/net/sunrpc/cache.c
@@ -541,7 +541,9 @@ 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);
-- 
2.20.1.2432.ga663e714