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

Xiang Mei (Microsoft) <[email protected]> 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: [email protected]
> Signed-off-by: Xiang Mei (Microsoft) <[email protected]>
> ---
>  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?

See
ef0efcd3bd3f ("ipv6: Fix dangling pointer when ipv6 fragment")

Thanks!