Re: AW: [PATCH] amd64: Fix csum_partial_copy_generic()

[Al Viro]

On Thu, Oct 19, 2023 at 09:39:45AM +0200, Eric Dumazet wrote:

> I wonder if the csum_and_copy_...() helpers are really needed in modern days,
> with much bigger cpu caches.
> 
> Maybe we could remove them and use more standard copy + standard
> checksum over kernel buffers.

FWIW, the reason we don't hit that shit all the time is that on almost
all paths all-zeroes block of data would be rejected anyway/could not
happen.  Note that e.g. for ICMPv6 the csum includes the pseudo-header
and there's no way for that to be all-zeroes, etc.

Whatever we do long-term (and I'd really like to get that mess dealt
with properly - fuckup is definitely mine, and I should have checked
the users of that stuff properly back then), I don't believe that
it's doable this late in the cycle.

How about the following:

icmp_reply(): paper over idiocy in csum_partial_copy_nocheck()

csum-and-copy helpers got screwed back in 2020; attempt to
be clever about reporting faults in csum_and_copy_..._user()
had ended up with "all zeroes" being indistinguishable from
"rfc1071 checksum is 0xffff".

The thing is, it almost works - the values modulo 0xffff are
right in all cases, so for purposes of adding them up we
are fine.  And we are almost never asked to calculate the
csum when there's no non-zeroes somewhere in the data.

Unfortunately, ICMPv4 replies provide at least one exception.
It's too late in the cycle to fix that properly; for now
(and for backports) let's take care of that in icmp_reply()
itself.

X-paperbag: brown
Fucked-up-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Reported-by: gus Gusenleitner Klaus <gus@xxxxxxxx>
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
---
diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index b8607763d113..6da09157f722 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c
@@ -376,6 +376,7 @@ static void icmp_push_reply(struct sock *sk,
 	} else if ((skb = skb_peek(&sk->sk_write_queue)) != NULL) {
 		struct icmphdr *icmph = icmp_hdr(skb);
 		__wsum csum;
+		__sum16 folded;
 		struct sk_buff *skb1;
 
 		csum = csum_partial_copy_nocheck((void *)&icmp_param->data,
@@ -384,7 +385,8 @@ static void icmp_push_reply(struct sock *sk,
 		skb_queue_walk(&sk->sk_write_queue, skb1) {
 			csum = csum_add(csum, skb1->csum);
 		}
-		icmph->checksum = csum_fold(csum);
+		folded = csum_fold(csum);
+		icmph->checksum = folded ? folded : CSUM_MANGLED_0;
 		skb->ip_summed = CHECKSUM_NONE;
 		ip_push_pending_frames(sk, fl4);
 	}