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Date:   Wed,  7 Oct 2020 21:12:50 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     netdev@...r.kernel.org
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        syzbot+e96a7ba46281824cc46a@...kaller.appspotmail.com,
        Xin Long <lucien.xin@...il.com>, Jon Maloy <jmaloy@...hat.com>,
        Ying Xue <ying.xue@...driver.com>
Subject: [Patch net] tipc: fix the skb_unshare() in tipc_buf_append()

skb_unshare() drops a reference count on the old skb unconditionally,
so in the failure case, we end up freeing the skb twice here.
And because the skb is allocated in fclone and cloned by caller
tipc_msg_reassemble(), the consequence is actually freeing the
original skb too, thus triggered the UAF by syzbot.

Fix this by replacing this skb_unshare() with skb_cloned()+skb_copy().

Fixes: ff48b6222e65 ("tipc: use skb_unshare() instead in tipc_buf_append()")
Reported-and-tested-by: syzbot+e96a7ba46281824cc46a@...kaller.appspotmail.com
Cc: Xin Long <lucien.xin@...il.com>
Cc: Jon Maloy <jmaloy@...hat.com>
Cc: Ying Xue <ying.xue@...driver.com>
Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
---
 net/tipc/msg.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/tipc/msg.c b/net/tipc/msg.c
index 52e93ba4d8e2..681224401871 100644
--- a/net/tipc/msg.c
+++ b/net/tipc/msg.c
@@ -150,7 +150,8 @@ int tipc_buf_append(struct sk_buff **headbuf, struct sk_buff **buf)
 	if (fragid == FIRST_FRAGMENT) {
 		if (unlikely(head))
 			goto err;
-		frag = skb_unshare(frag, GFP_ATOMIC);
+		if (skb_cloned(frag))
+			frag = skb_copy(frag, GFP_ATOMIC);
 		if (unlikely(!frag))
 			goto err;
 		head = *headbuf = frag;
-- 
2.28.0

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