ACK: [PATCH 1/1] ipv4: keep skb->dst around in presence of IP options

[Stefan Bader]

On 25.08.2017 15:28, Kleber Sacilotto de Souza wrote:
> From: Eric Dumazet <edumazet at google.com>
> 
> CVE-2017-5970
> 
> Andrey Konovalov got crashes in __ip_options_echo() when a NULL skb->dst
> is accessed.
> 
> ipv4_pktinfo_prepare() should not drop the dst if (evil) IP options
> are present.
> 
> We could refine the test to the presence of ts_needtime or srr,
> but IP options are not often used, so let's be conservative.
> 
> Thanks to syzkaller team for finding this bug.
> 
> Fixes: d826eb14ecef ("ipv4: PKTINFO doesnt need dst reference")
> Signed-off-by: Eric Dumazet <edumazet at google.com>
> Reported-by: Andrey Konovalov <andreyknvl at google.com>
> Signed-off-by: David S. Miller <davem at davemloft.net>
> (cherry picked from commit 34b2cef20f19c87999fff3da4071e66937db9644)
> Signed-off-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>
Acked-by: Stefan Bader <stefan.bader at canonical.com>

> ---
>  net/ipv4/ip_sockglue.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
> index 4aeb2131242b..b86d12793328 100644
> --- a/net/ipv4/ip_sockglue.c
> +++ b/net/ipv4/ip_sockglue.c
> @@ -1080,7 +1080,14 @@ void ipv4_pktinfo_prepare(const struct sock *sk, struct sk_buff *skb)
>  		pktinfo->ipi_ifindex = 0;
>  		pktinfo->ipi_spec_dst.s_addr = 0;
>  	}
> -	skb_dst_drop(skb);
> +	/* We need to keep the dst for __ip_options_echo()
> +	 * We could restrict the test to opt.ts_needtime || opt.srr,
> +	 * but the following is good enough as IP options are not often used.
> +	 */
> +	if (unlikely(IPCB(skb)->opt.optlen))
> +		skb_dst_force(skb);
> +	else
> +		skb_dst_drop(skb);
>  }
>  
>  int ip_setsockopt(struct sock *sk, int level,
> 


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