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 10:31:43 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: David Howells <dhowells@...hat.com>, 
 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)

David Howells wrote:
> 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.

That arithmetic makes assumptions that are specific to a particular
set of conditions (e.g., that pagedlen is non-zero).

Since the arithmetic is so complicated and error prone, I would try
to structure a fix that is easy to reason about to only change
behavior for the MSG_SPLICE_PAGES case.
 
> I'm also not entirely sure what 'paged' means in this function.  Should it
> actually be set in the MSG_SPLICE_PAGES context?

I introduced it with MSG_ZEROCOPY. It sets up pagedlen to capture the
length that is not copied.

If the existing code would affect MSG_ZEROCOPY too, I expect syzbot
to have reported that previously.

> ---
> 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);
>