From: Cong Wang <xiyou.wangcong@gmail.com>
Date: Wed, 7 Oct 2020 21:12:50 -0700
Subject: tipc: fix the skb_unshare() in tipc_buf_append()
Git-commit: ed42989eab57d619667d7e87dfbd8fe207db54fe
Patch-mainline: v5.11-rc1
References: git-fixes

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@syzkaller.appspotmail.com
Cc: Jon Maloy <jmaloy@redhat.com>
Cc: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 net/tipc/msg.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/net/tipc/msg.c
+++ b/net/tipc/msg.c
@@ -140,7 +140,8 @@ int tipc_buf_append(struct sk_buff **hea
 	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;