Re: [PATCH net-next] net/mpls: fix WARNING in mpls_gso_segment

[Lizhi Xu]

On Thu, 22 Feb 2024 09:11:14 +0100, Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> > When the network header pointer is greater than the inner network header, the
> > difference between the two can cause mpls_hlen overflow.
> >
> > Reported-and-tested-by: syzbot+99d15fcdb0132a1e1a82@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx>
> > ---
> >  net/mpls/mpls_gso.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/mpls/mpls_gso.c b/net/mpls/mpls_gso.c
> > index 533d082f0701..2ab24b2fd90f 100644
> > --- a/net/mpls/mpls_gso.c
> > +++ b/net/mpls/mpls_gso.c
> > @@ -25,11 +25,11 @@ static struct sk_buff *mpls_gso_segment(struct sk_buff *skb,
> >         netdev_features_t mpls_features;
> >         u16 mac_len = skb->mac_len;
> >         __be16 mpls_protocol;
> > -       unsigned int mpls_hlen;
> > +       int mpls_hlen;
> >
> >         skb_reset_network_header(skb);
> >         mpls_hlen = skb_inner_network_header(skb) - skb_network_header(skb);
> > -       if (unlikely(!mpls_hlen || mpls_hlen % MPLS_HLEN))
> > +       if (unlikely(mpls_hlen <= 0 || mpls_hlen % MPLS_HLEN))
> >                 goto out;
> >         if (unlikely(!pskb_may_pull(skb, mpls_hlen)))
> >                 goto out;
> > --
> > 2.43.0
> >
> 
> I think Florian posted this patch, right ?
After you mentioned it, I discovered it. 
I forgot to check the email details before sending the patch.
> 
> We must add a Fixes: tag
> 
> Also we should ask ourselves :
> Why are we even looking at skb_inner_network_header(skb) if this was not set ?

__sys_sendto()->
   __sock_sendmsg()->
      netlink_sendmsg()->
         netlink_broadcast_filtered()->
            netlink_trim()->
	       1323         pskb_expand_head(skb, 0, -delta, 
                  1                          (allocation & ~__GFP_DIRECT_RECLAIM) |
                  2                          __GFP_NOWARN | __GFP_NORETRY);
               pskb_expand_head()->
                 skb_headers_offset_update()->
		 1977         skb->inner_network_header += off;

The root cause is: 
Initialize inner_network_header to 0 in network_trim(), and without any other
assignment operations.