netdev - Re: Endless loop in udp with MSG_SPLICE_READ - Re: [syzbot] [fs?] INFO: task hung in pipe

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Date: Tue, 01 Aug 2023 15:19:06 +0100
From: David Howells <dhowells@...hat.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: dhowells@...hat.com, Jakub Kicinski <kuba@...nel.org>,
    syzbot <syzbot+f527b971b4bdc8e79f9e@...kaller.appspotmail.com>,
    bpf@...r.kernel.org, brauner@...nel.org, davem@...emloft.net,
    dsahern@...nel.org, edumazet@...gle.com,
    linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
    netdev@...r.kernel.org, pabeni@...hat.com,
    syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk
Subject: Re: Endless loop in udp with MSG_SPLICE_READ - Re: [syzbot] [fs?] INFO: task hung in pipe_release (4)

The attached seems to work.  I still think copy isn't correctly calculated in
some circumstances - as I showed, several terms in the maths cancel out,
including the length of the data.

I'm also not entirely sure what 'paged' means in this function.  Should it
actually be set in the MSG_SPLICE_PAGES context?

---
udp: Fix __ip_addend_data()

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 6e70839257f7..54675a4f2c9f 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -1157,7 +1157,7 @@ static int __ip_append_data(struct sock *sk,
 				pskb_trim_unique(skb_prev, maxfraglen);
 			}
 
-			copy = datalen - transhdrlen - fraggap - pagedlen;
+			copy = max_t(int, datalen - transhdrlen - fraggap - pagedlen, 0);
 			if (copy > 0 && getfrag(from, data + transhdrlen, offset, copy, fraggap, skb) < 0) {
 				err = -EFAULT;
 				kfree_skb(skb);