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

["dongchenchen (A)" <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>
> 
> 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
> 
Thank you very much for your suggestions!
v3 will be sent.

Dong Chenchen
> >  	__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
> > 
> >