netdev - Re: [PATCH] tcp: take care of misalignments

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Date:	Sun, 04 Dec 2011 19:51:08 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	subramanian.vijay@...il.com, therbert@...gle.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH] tcp: take care of misalignments

Le dimanche 04 décembre 2011 à 13:21 -0500, David Miller a écrit :
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Sun, 04 Dec 2011 08:39:53 +0100
> 
> > [PATCH] tcp: take care of misalignments
> > 
> > We discovered that TCP stack could retransmit misaligned skbs if a
> > malicious peer acknowledged sub MSS frame. This currently can happen
> > only if output interface is non SG enabled : If SG is enabled, tcp
> > builds headless skbs (all payload is included in fragments), so the tcp
> > trimming process only removes parts of skb fragments, header stay
> > aligned.
> > 
> > Some arches cant handle misalignments, so force a head reallocation and
> > shrink headroom to MAX_TCP_HEADER.
> > 
> > Dont care about misaligments on x86 and PPC (or other arches setting
> > NET_IP_ALIGN to 0)
> > 
> > This patch introduces __pskb_copy() which can specify the headroom of
> > new head, and pskb_copy() becomes a wrapper on top of __pskb_copy()
> > 
> > Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> 
> Looks good, applied.

Thanks !

Here a respin of previous patch then ?

[PATCH net-next] tcp: fix tcp_trim_head()

commit f07d960df3 (tcp: avoid frag allocation for small frames)
breaked assumption in tcp stack that skb is either linear (skb->data_len
== 0), or fully fragged (skb->data_len == skb->len)

tcp_trim_head() made this assumption, we must fix it.

Thanks to Vijay for providing a very detailed explanation.

Reported-by: Vijay Subramanian <subramanian.vijay@...il.com>
Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
---
 net/ipv4/tcp_output.c |   13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 50788d6..cf30680 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1093,6 +1093,13 @@ static void __pskb_trim_head(struct sk_buff *skb, int len)
 {
 	int i, k, eat;
 
+	eat = min_t(int, len, skb_headlen(skb));
+	if (eat) {
+		__skb_pull(skb, eat);
+		len -= eat;
+		if (!len)
+			return;
+	}
 	eat = len;
 	k = 0;
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
@@ -1124,11 +1131,7 @@ int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
 	if (skb_cloned(skb) && pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
 		return -ENOMEM;
 
-	/* If len == headlen, we avoid __skb_pull to preserve alignment. */
-	if (unlikely(len < skb_headlen(skb)))
-		__skb_pull(skb, len);
-	else
-		__pskb_trim_head(skb, len - skb_headlen(skb));
+	__pskb_trim_head(skb, len);
 
 	TCP_SKB_CB(skb)->seq += len;
 	skb->ip_summed = CHECKSUM_PARTIAL;


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