Re: [PATCH net] netfilter: bridge: fix stale prevhdr pointer in br_ip6_fragment()

[Xiang Mei]

On Wed, Jul 8, 2026 at 4:46 AM Florian Westphal <fw@xxxxxxxxx> wrote:
>
> Xiang Mei (Microsoft) <xmei5@xxxxxxx> wrote:
> > br_ip6_fragment() gets prevhdr, a pointer into the skb head, from
> > ip6_find_1stfragopt(), then calls skb_checksum_help().  For a cloned skb
> > skb_checksum_help() reallocates the head via pskb_expand_head(), leaving
> > prevhdr dangling.  It is later dereferenced in ip6_frag_next(), causing a
> > use-after-free write.
> >
> > Re-find prevhdr after skb_checksum_help() so it points into the current
> > head.
> >
> >   BUG: KASAN: slab-use-after-free in ip6_frag_next (net/ipv6/ip6_output.c:857)
> >   Write of size 1 at addr ffff888013ff5016 by task exploit/141
> >   Call Trace:
> >    ...
> >    kasan_report (mm/kasan/report.c:595)
> >    ip6_frag_next (net/ipv6/ip6_output.c:857)
> >    br_ip6_fragment (net/ipv6/netfilter.c:212)
> >    nf_ct_bridge_post (net/bridge/netfilter/nf_conntrack_bridge.c:407)
> >    nf_hook_slow (net/netfilter/core.c:619)
> >    br_forward_finish (net/bridge/br_forward.c:66)
> >    __br_forward (net/bridge/br_forward.c:115)
> >    maybe_deliver (net/bridge/br_forward.c:191)
> >    br_flood (net/bridge/br_forward.c:245)
> >    br_handle_frame_finish (net/bridge/br_input.c:229)
> >    br_handle_frame (net/bridge/br_input.c:442)
> >    ...
> >    packet_sendmsg (net/packet/af_packet.c:3114)
> >    ...
> >    do_syscall_64 (arch/x86/entry/syscall_64.c:94)
> >    entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
> >   Kernel panic - not syncing: Fatal exception in interrupt
> >
> > Fixes: 764dd163ac92 ("netfilter: nf_conntrack_bridge: add support for IPv6")
> > Reported-by: AutonomousCodeSecurity@xxxxxxxxxxxxx
> > Signed-off-by: Xiang Mei (Microsoft) <xmei5@xxxxxxx>
> > ---
> >  net/ipv6/netfilter.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/net/ipv6/netfilter.c b/net/ipv6/netfilter.c
> > index 6d80f85e55fa..547879da9532 100644
> > --- a/net/ipv6/netfilter.c
> > +++ b/net/ipv6/netfilter.c
> > @@ -147,6 +147,10 @@ int br_ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
> >           (err = skb_checksum_help(skb)))
> >               goto blackhole;
> >
> > +     err = ip6_find_1stfragopt(skb, &prevhdr);
> > +     if (err < 0)
> > +             goto blackhole;
>
> Would you mind sending a v2 that solves this the same way that it was
> fixed in ipv6 output engine?
>

Thanks for pointing me to the better fix. We have tested v2 on the poc
and sent v2.

Xiang
> See
> ef0efcd3bd3f ("ipv6: Fix dangling pointer when ipv6 fragment")
>
> Thanks!