netdev - Re: [PATCH net] tcp: fix slab-out-of-bounds in tcp_zerocopy

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Date:   Thu, 3 Oct 2019 02:46:11 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] tcp: fix slab-out-of-bounds in tcp_zerocopy_receive()

On Wed, Oct 02, 2019 at 08:19:59PM -0700, Eric Dumazet wrote:
> Apparently a refactoring patch brought a bug, that was caught
> by syzbot [1]

That wasn't refactoring.  As you know (because we talked about it at
LSFMM), this is an enabling patch for supporting hch's work to fix
get_user_pages().

> Original code was correct, do not try to be smarter than the
> compiler :/

That wasn't an attempt to be smarter than the compiler.  I was trying
to keep the line length below 80 columns.  Which you probably now see
that you haven't done.

I must have a blind spot here.  I can't see the difference between the
two versions.

> +++ b/net/ipv4/tcp.c
> @@ -1798,13 +1798,11 @@ static int tcp_zerocopy_receive(struct sock *sk,
>  		}
>  		if (skb_frag_size(frags) != PAGE_SIZE || skb_frag_off(frags)) {
>  			int remaining = zc->recv_skip_hint;
> -			int size = skb_frag_size(frags);
>  
> -			while (remaining && (size != PAGE_SIZE ||
> +			while (remaining && (skb_frag_size(frags) != PAGE_SIZE ||
>  					     skb_frag_off(frags))) {
> -				remaining -= size;
> +				remaining -= skb_frag_size(frags);
>  				frags++;
> -				size = skb_frag_size(frags);
>  			}
>  			zc->recv_skip_hint -= remaining;
>  			break;
> -- 
> 2.23.0.581.g78d2f28ef7-goog
>