netdev - [PATCH 5.10 3/5] net: tighten bad gso csum offset check in virtio_net

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Message-ID: <20250131083127.4204-1-arefev@swemel.ru>
Date: Fri, 31 Jan 2025 11:31:25 +0300
From: Denis Arefev <arefev@...mel.ru>
To: stable@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: "Michael S. Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>,
	Willem de Bruijn <willemb@...gle.com>,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org,
	lvc-project@...uxtesting.org
Subject: [PATCH 5.10 3/5] net: tighten bad gso csum offset check in virtio_net_hdr

From: Willem de Bruijn <willemb@...gle.com>

commit 6513eb3d3191574b58859ef2d6dc26c0277c6f81 upstream. 

The referenced commit drops bad input, but has false positives.
Tighten the check to avoid these.

The check detects illegal checksum offload requests, which produce
csum_start/csum_off beyond end of packet after segmentation.

But it is based on two incorrect assumptions:

1. virtio_net_hdr_to_skb with VIRTIO_NET_HDR_GSO_TCP[46] implies GSO.
True in callers that inject into the tx path, such as tap.
But false in callers that inject into rx, like virtio-net.
Here, the flags indicate GRO, and CHECKSUM_UNNECESSARY or
CHECKSUM_NONE without VIRTIO_NET_HDR_F_NEEDS_CSUM is normal.

2. TSO requires checksum offload, i.e., ip_summed == CHECKSUM_PARTIAL.
False, as tcp[46]_gso_segment will fix up csum_start and offset for
all other ip_summed by calling __tcp_v4_send_check.

Because of 2, we can limit the scope of the fix to virtio_net_hdr
that do try to set these fields, with a bogus value.

Link: https://lore.kernel.org/netdev/20240909094527.GA3048202@port70.net/
Fixes: 89add40066f9 ("net: drop bad gso csum_start and offset in virtio_net_hdr")
Signed-off-by: Willem de Bruijn <willemb@...gle.com>
Acked-by: Jason Wang <jasowang@...hat.com>
Acked-by: Michael S. Tsirkin <mst@...hat.com>
Cc: stable@...r.kernel.org
Link: https://patch.msgid.link/20240910213553.839926-1-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@...nel.org>
[Denis: minor fix to resolve merge conflict.]                                           
Signed-off-by: Denis Arefev <arefev@...mel.ru>                                    
---
Backport fix for CVE-2024-43817
Link: https://nvd.nist.gov/vuln/detail/cve-2024-43817
---
 include/linux/virtio_net.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index 823e28042f41..62613d4d84b7 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -161,7 +161,8 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 			break;
 		case SKB_GSO_TCPV4:
 		case SKB_GSO_TCPV6:
-			if (skb->csum_offset != offsetof(struct tcphdr, check))
+			if (skb->ip_summed == CHECKSUM_PARTIAL &&
+			    skb->csum_offset != offsetof(struct tcphdr, check))
 				return -EINVAL;
 			break;
 		}
-- 
2.43.0