Subject: Re: [PATCH net] ipv6: fix uninit-value in ip6_multipath_l3_keys()

From: Jakub Sitnicki <[email-masked]>
Date: Mon, 30 Apr 2018 11:59:37 +0200

On Sun, 29 Apr 2018 09:54:59 -0700
Eric Dumazet <[email-masked]> wrote:

> syzbot/KMSAN reported an uninit-value in ip6_multipath_l3_keys(),
> root caused to a bad assumption of ICMP header being already
> pulled in skb->head
> 
> ip_multipath_l3_keys() does the correct thing, so it is an IPv6 only bug.
> 
> BUG: KMSAN: uninit-value in ip6_multipath_l3_keys net/ipv6/route.c:1830 [inline]
> BUG: KMSAN: uninit-value in rt6_multipath_hash+0x5c4/0x640 net/ipv6/route.c:1858
> CPU: 0 PID: 4507 Comm: syz-executor661 Not tainted 4.16.0+ #87
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:17 [inline]
>  dump_stack+0x185/0x1d0 lib/dump_stack.c:53
>  kmsan_report+0x142/0x240 mm/kmsan/kmsan.c:1067
>  __msan_warning_32+0x6c/0xb0 mm/kmsan/kmsan_instr.c:683
>  ip6_multipath_l3_keys net/ipv6/route.c:1830 [inline]
>  rt6_multipath_hash+0x5c4/0x640 net/ipv6/route.c:1858
>  ip6_route_input+0x65a/0x920 net/ipv6/route.c:1884
>  ip6_rcv_finish+0x413/0x6e0 net/ipv6/ip6_input.c:69
>  NF_HOOK include/linux/netfilter.h:288 [inline]
>  ipv6_rcv+0x1e16/0x2340 net/ipv6/ip6_input.c:208
>  __netif_receive_skb_core+0x47df/0x4a90 net/core/dev.c:4562
>  __netif_receive_skb net/core/dev.c:4627 [inline]
>  netif_receive_skb_internal+0x49d/0x630 net/core/dev.c:4701
>  netif_receive_skb+0x230/0x240 net/core/dev.c:4725
>  tun_rx_batched drivers/net/tun.c:1555 [inline]
>  tun_get_user+0x740f/0x7c60 drivers/net/tun.c:1962
>  tun_chr_write_iter+0x1d4/0x330 drivers/net/tun.c:1990
>  call_write_iter include/linux/fs.h:1782 [inline]
>  new_sync_write fs/read_write.c:469 [inline]
>  __vfs_write+0x7fb/0x9f0 fs/read_write.c:482
>  vfs_write+0x463/0x8d0 fs/read_write.c:544
>  SYSC_write+0x172/0x360 fs/read_write.c:589
>  SyS_write+0x55/0x80 fs/read_write.c:581
>  do_syscall_64+0x309/0x430 arch/x86/entry/common.c:287
>  entry_SYSCALL_64_after_hwframe+0x3d/0xa2
> 
> Fixes: 23aebdacb05d ("ipv6: Compute multipath hash for ICMP errors from offending packet")
> Signed-off-by: Eric Dumazet <[email-masked]>
> Reported-by: syzbot <[email-masked]>
> Cc: Jakub Sitnicki <[email-masked]>
> ---
>  net/ipv6/route.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index cde7d8251377c1a115e02c46843d361d3c0b4313..f4d61736c41abe8cd7f439c4a37100e90c1eacca 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -1835,11 +1835,16 @@ static void ip6_multipath_l3_keys(const struct sk_buff *skb,
>  	const struct ipv6hdr *inner_iph;
>  	const struct icmp6hdr *icmph;
>  	struct ipv6hdr _inner_iph;
> +	struct icmp6hdr _icmph;
>  
>  	if (likely(outer_iph->nexthdr != IPPROTO_ICMPV6))
>  		goto out;
>  
> -	icmph = icmp6_hdr(skb);
> +	icmph = skb_header_pointer(skb, skb_transport_offset(skb),
> +				   sizeof(_icmph), &_icmph);
> +	if (!icmph)
> +		goto out;
> +
>  	if (icmph->icmp6_type != ICMPV6_DEST_UNREACH &&
>  	    icmph->icmp6_type != ICMPV6_PKT_TOOBIG &&
>  	    icmph->icmp6_type != ICMPV6_TIME_EXCEED &&

Thank you for the fix. I was not being careful there.

Acked-by: Jakub Sitnicki <[email-masked]>