Re: [PATCH net] ipv6/gro: fix an out of bounds memory bug in ipv6_gro_receive()

[Eric Dumazet]

On Thu, Oct 27, 2022 at 6:01 AM Ziyang Xuan (William)
<william.xuanziyang@xxxxxxxxxx> wrote:
>
> > On Thu, Oct 27, 2022 at 3:25 AM Ziyang Xuan
> > <william.xuanziyang@xxxxxxxxxx> wrote:
> >>
> >> IPv6 packets without NEXTHDR_NONE extension header can make continuous
> >> __skb_pull() until pskb_may_pull() failed in ipv6_gso_pull_exthdrs().
> >> That results in a big value of skb_gro_offset(), and after __skb_push()
> >> in ipv6_gro_receive(), skb->data will less than skb->head, an out of
> >> bounds memory bug occurs. That will trigger the problem as following:
> >>
> >> ==================================================================
> >> BUG: KASAN: use-after-free in eth_type_trans+0x100/0x260
> >> ...
> >> Call trace:
> >>  dump_backtrace+0xd8/0x130
> >>  show_stack+0x1c/0x50
> >>  dump_stack_lvl+0x64/0x7c
> >>  print_address_description.constprop.0+0xbc/0x2e8
> >>  print_report+0x100/0x1e4
> >>  kasan_report+0x80/0x120
> >>  __asan_load8+0x78/0xa0
> >>  eth_type_trans+0x100/0x260
> >
> > Crash happens from eth_type_trans() , this should happen before
> > ipv6_gro_receive() ?
> >
> > It seems your patch is unrelated.
> >
> > Please provide a repro.
>
> C repro put in attachment.

This seems to be a bug in tun device.

Please take more time to root cause this issue, instead of adding work
arounds all over the place.

Thanks.

>
> >
> >
> >>  napi_gro_frags+0x164/0x550
> >>  tun_get_user+0xda4/0x1270
> >>  tun_chr_write_iter+0x74/0x130
> >>  do_iter_readv_writev+0x130/0x1ec
> >>  do_iter_write+0xbc/0x1e0
> >>  vfs_writev+0x13c/0x26c
> >>
> >> Add comparison between skb->data - skb_gro_offset() and skb->head
> >> and exception handler before __skb_push() to fix the bug.
> >>
> >> Fixes: 86911732d399 ("gro: Avoid copying headers of unmerged packets")
> >> Signed-off-by: Ziyang Xuan <william.xuanziyang@xxxxxxxxxx>
> >> ---
> >>  net/ipv6/ip6_offload.c | 4 ++++
> >>  1 file changed, 4 insertions(+)
> >>
> >> diff --git a/net/ipv6/ip6_offload.c b/net/ipv6/ip6_offload.c
> >> index 3ee345672849..6659ccf25387 100644
> >> --- a/net/ipv6/ip6_offload.c
> >> +++ b/net/ipv6/ip6_offload.c
> >> @@ -237,6 +237,10 @@ INDIRECT_CALLABLE_SCOPE struct sk_buff *ipv6_gro_receive(struct list_head *head,
> >>                 proto = ipv6_gso_pull_exthdrs(skb, proto);
> >>                 skb_gro_pull(skb, -skb_transport_offset(skb));
> >>                 skb_reset_transport_header(skb);
> >> +               if (unlikely(skb_headroom(skb) < skb_gro_offset(skb))) {
> >
> > This makes no sense to me.
> >
> > If there is a bug, it should be fixed earlier.
>
> Maybe it is good to validate IPv6 packet earlier in ipv6_gro_receive() or more earlier?
>
> >
> >> +                       kfree_skb(skb);
> >> +                       return ERR_PTR(-EINPROGRESS);
> >> +               }
> >>                 __skb_push(skb, skb_gro_offset(skb));
> >>
> >>                 ops = rcu_dereference(inet6_offloads[proto]);
> >> --
> >> 2.25.1
> >>
> > .
> >