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

From: Simon Horman <[email-masked]>
Date: Tue, 9 May 2023 10:21:48 +0200

On Tue, May 09, 2023 at 10:19:24AM +0800, Dong Chenchen wrote:
> 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 <[email-masked]>

nit: As this is a fix it should probably be targeted at 'net'
     (as opposed to 'net-next'). This should be noted in the subject.

     Subject: [PATCH net v2]...

> ---
> 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;

nit: It is generally preferred to arrange local variable in networking code
     from shortest line to longest - reverse xmas tree order.

     This can be verified using.
     https://github.com/ecree-solarflare/xmastree/blob/master/README

>  	__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
> 
>