linux-kernel - [PATCH net, v4] net: nsh: Use correct mac_offset to unwind gso skb in nsh_gso

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Date:   Thu, 11 May 2023 20:54:40 +0800
From:   Dong Chenchen <dongchenchen2@...wei.com>
To:     <edumazet@...gle.com>, <kuba@...nel.org>, <davem@...emloft.net>,
        <pabeni@...hat.com>, <simon.horman@...igine.com>
CC:     <jbenc@...hat.com>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <yuehaibing@...wei.com>,
        <weiyongjun1@...wei.com>, Dong Chenchen <dongchenchen2@...wei.com>,
        <syzbot+632b5d9964208bfef8c0@...kaller.appspotmail.com>
Subject: [PATCH net, v4] net: nsh: Use correct mac_offset to unwind gso skb in nsh_gso_segment()

As the call trace shows, skb_panic was caused by wrong skb->mac_header
in nsh_gso_segment():

invalid opcode: 0000 [#1] PREEMPT SMP KASAN PTI
CPU: 3 PID: 2737 Comm: syz Not tainted 6.3.0-next-20230505 #1
RIP: 0010:skb_panic+0xda/0xe0
call Trace:
 skb_push+0x91/0xa0
 nsh_gso_segment+0x4f3/0x570
 skb_mac_gso_segment+0x19e/0x270
 __skb_gso_segment+0x1e8/0x3c0
 validate_xmit_skb+0x452/0x890
 validate_xmit_skb_list+0x99/0xd0
 sch_direct_xmit+0x294/0x7c0
 __dev_queue_xmit+0x16f0/0x1d70
 packet_xmit+0x185/0x210
 packet_snd+0xc15/0x1170
 packet_sendmsg+0x7b/0xa0
 sock_sendmsg+0x14f/0x160

The root cause is:
nsh_gso_segment() use skb->network_header - nhoff to reset mac_header
in skb_gso_error_unwind() if inner-layer protocol gso fails.
However, skb->network_header may be reset by inner-layer protocol
gso function e.g. mpls_gso_segment. skb->mac_header reset by the
inaccurate network_header will be larger than skb headroom.

nsh_gso_segment
    nhoff = skb->network_header - skb->mac_header;
    __skb_pull(skb,nsh_len)
    skb_mac_gso_segment
        mpls_gso_segment
            skb_reset_network_header(skb);//skb->network_header+=nsh_len
            return -EINVAL;
    skb_gso_error_unwind
        skb_push(skb, nsh_len);
        skb->mac_header = skb->network_header - nhoff;
        // skb->mac_header > skb->headroom, cause skb_push panic

Use correct mac_offset to restore mac_header and get rid of nhoff.

Fixes: c411ed854584 ("nsh: add GSO support")
Reported-by: syzbot+632b5d9964208bfef8c0@...kaller.appspotmail.com
Suggested-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Dong Chenchen <dongchenchen2@...wei.com>
---
v2:
  - Use skb->mac_header not skb->network_header-nhoff for mac_offset.

v3:
  - 'net' is noted in the subject.
  - arrange local variable following reverse xmas tree order

v4:
  - get rid of nhoff 
---
 net/nsh/nsh.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/net/nsh/nsh.c b/net/nsh/nsh.c
index e9ca007718b7..0f23e5e8e03e 100644
--- a/net/nsh/nsh.c
+++ b/net/nsh/nsh.c
@@ -77,13 +77,12 @@ static struct sk_buff *nsh_gso_segment(struct sk_buff *skb,
 				       netdev_features_t features)
 {
 	struct sk_buff *segs = ERR_PTR(-EINVAL);
+	u16 mac_offset = skb->mac_header;
 	unsigned int nsh_len, mac_len;
 	__be16 proto;
-	int nhoff;
 
 	skb_reset_network_header(skb);
 
-	nhoff = skb->network_header - skb->mac_header;
 	mac_len = skb->mac_len;
 
 	if (unlikely(!pskb_may_pull(skb, NSH_BASE_HDR_LEN)))
@@ -108,15 +107,14 @@ static struct sk_buff *nsh_gso_segment(struct sk_buff *skb,
 	segs = skb_mac_gso_segment(skb, features);
 	if (IS_ERR_OR_NULL(segs)) {
 		skb_gso_error_unwind(skb, htons(ETH_P_NSH), nsh_len,
-				     skb->network_header - nhoff,
-				     mac_len);
+				     mac_offset, mac_len);
 		goto out;
 	}
 
 	for (skb = segs; skb; skb = skb->next) {
 		skb->protocol = htons(ETH_P_NSH);
 		__skb_push(skb, nsh_len);
-		skb_set_mac_header(skb, -nhoff);
+		skb->mac_header = mac_offset;
 		skb->network_header = skb->mac_header + mac_len;
 		skb->mac_len = mac_len;
 	}
-- 
2.25.1