tipc: fix the skb_unshare() in tipc_buf_append()

[ Upstream commit ed42989eab57d619667d7e87dfbd8fe207db54fe ]

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/net/tipc/msg.c b/net/tipc/msg.c
index d048ec6dab12a9b7ef329be5ffef9b0b4aaf282e..e61657a0438a979e023691c9a46069684d999150 100644 (file)
--- a/net/tipc/msg.c
+++ b/net/tipc/msg.c
@@ -140,7 +140,8 @@ 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;