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From:   Yihao Wu <wuyihao@linux.alibaba.com>
To:     "J . Bruce Fields" <bfields@fieldses.org>,
        Chuck Lever <chuck.lever@oracle.com>,
        NeilBrown <neilb@suse.de>, Sasha Levin <sashal@kernel.org>
Cc:     linux-nfs@vger.kernel.org
Subject: [PATCH v2] SUNRPC/cache: Fix unsafe traverse caused double-free in cache_purge
Date:   Mon,  6 Apr 2020 01:38:16 +0800
Message-Id: <4568a7cf87f110b8e59fda6f53fda34c550ab403.1586108200.git.wuyihao@linux.alibaba.com>
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Sender: linux-nfs-owner@vger.kernel.org
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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>
---
 net/sunrpc/cache.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
index af0ddd28b081..9649c7fcccd2 100644
--- a/net/sunrpc/cache.c
+++ b/net/sunrpc/cache.c
@@ -541,7 +541,8 @@ 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