[PATCH next, v2] net: nsh: Use correct mac_offset to unwind gso skb in nsh_gso_segment()

[Dong Chenchen <dongchenchen2@huawei.com>]

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 to fix it.

Fixes: c411ed854584 ("nsh: add GSO support")
Signed-off-by: Dong Chenchen <dongchenchen2@huawei.com>
---
v2:
  - Use skb->mac_header not skb->network_header-nhoff for mac_offset.
---
 net/nsh/nsh.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/nsh/nsh.c b/net/nsh/nsh.c
index e9ca007718b7..7eb536a9677f 100644
--- a/net/nsh/nsh.c
+++ b/net/nsh/nsh.c
@@ -78,6 +78,7 @@ static struct sk_buff *nsh_gso_segment(struct sk_buff *skb,
 {
 	struct sk_buff *segs = ERR_PTR(-EINVAL);
 	unsigned int nsh_len, mac_len;
+	u16 mac_offset = skb->mac_header;
 	__be16 proto;
 	int nhoff;
 
@@ -108,8 +109,7 @@ 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;
 	}
 
-- 
2.25.1