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

[Eric Dumazet]

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.


>  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.

> +                       kfree_skb(skb);
> +                       return ERR_PTR(-EINPROGRESS);
> +               }
>                 __skb_push(skb, skb_gro_offset(skb));
>
>                 ops = rcu_dereference(inet6_offloads[proto]);
> --
> 2.25.1
>