netdev - [PATCH net-next] net: fix GSO for SG-enabled devices.

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Date:   Tue, 19 Jan 2021 13:30:32 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     netdev@...r.kernel.org
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexander Duyck <alexanderduyck@...com>,
        Xin Long <lucien.xin@...il.com>
Subject: [PATCH net-next] net: fix GSO for SG-enabled devices.

The commit dbd50f238dec ("net: move the hsize check to the else
block in skb_segment") introduced a data corruption for devices
supporting scatter-gather.

The problem boils down to signed/unsigned comparison given
unexpected results: if signed 'hsize' is negative, it will be
considered greater than a positive 'len', which is unsigned.

This commit addresses the issue explicitly casting 'len' to a
signed integer, so that the comparison gives the correct result.

Bisected-by: Matthieu Baerts <matthieu.baerts@...sares.net>
Fixes: dbd50f238dec ("net: move the hsize check to the else block in skb_segment")
Signed-off-by: Paolo Abeni <pabeni@...hat.com>
---
note: a possible more readable alternative would be moving the
	if (hsize < 0)

before 'if (hsize > len)', but that was explicitly discouraged
in a previous iteration of the blamed commit to save a comparison,
so I opted to preserve that optimization.
---
 net/core/skbuff.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index e835193cabcc3..27f69c0bd8393 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3938,7 +3938,7 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 			skb_release_head_state(nskb);
 			__skb_push(nskb, doffset);
 		} else {
-			if (hsize > len || !sg)
+			if (hsize > (int)len || !sg)
 				hsize = len;
 			else if (hsize < 0)
 				hsize = 0;
-- 
2.26.2