[net] ipv6: route: Fix return value of ip6_neigh_lookup() on neigh_create() error

In ip6_neigh_lookup(), we must not return errors coming from
neigh_create(): if creation of a neighbour entry fails, the lookup should
return NULL, in the same way as it's done in __neigh_lookup().

Otherwise, callers legitimately checking for a non-NULL return value of
the lookup function might dereference an invalid pointer.

For instance, on neighbour table overflow, ndisc_router_discovery()
crashes ndisc_update() by passing ERR_PTR(-ENOBUFS) as 'neigh' argument.

Reported-by: Jianlin Shi <jishi@redhat.com>
Fixes: f8a1b43b709d ("net/ipv6: Create a neigh_lookup for FIB entries")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 net/ipv6/route.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

On 1/2/19 5:29 AM, Stefano Brivio wrote:
> In ip6_neigh_lookup(), we must not return errors coming from
> neigh_create(): if creation of a neighbour entry fails, the lookup should
> return NULL, in the same way as it's done in __neigh_lookup().
> 
> Otherwise, callers legitimately checking for a non-NULL return value of
> the lookup function might dereference an invalid pointer.
> 
> For instance, on neighbour table overflow, ndisc_router_discovery()
> crashes ndisc_update() by passing ERR_PTR(-ENOBUFS) as 'neigh' argument.
> 
> Reported-by: Jianlin Shi <jishi@redhat.com>
> Fixes: f8a1b43b709d ("net/ipv6: Create a neigh_lookup for FIB entries")
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> ---
>  net/ipv6/route.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index a94e0b02a8ac..40b225f87d5e 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -210,7 +210,9 @@ struct neighbour *ip6_neigh_lookup(const struct in6_addr *gw,
>  	n = __ipv6_neigh_lookup(dev, daddr);
>  	if (n)
>  		return n;
> -	return neigh_create(&nd_tbl, daddr, dev);
> +
> +	n = neigh_create(&nd_tbl, daddr, dev);
> +	return IS_ERR(n) ? NULL : n;
>  }
>  
>  static struct neighbour *ip6_dst_neigh_lookup(const struct dst_entry *dst,
> 

I see now -- dst_neigh_lookup handles that check and I forgot to add the
same to the new ip6_neigh_lookup.

Thanks for the patch.

Reviewed-by: David Ahern <dsahern@gmail.com>

From: Stefano Brivio <sbrivio@redhat.com>
Date: Wed,  2 Jan 2019 13:29:27 +0100

> In ip6_neigh_lookup(), we must not return errors coming from
> neigh_create(): if creation of a neighbour entry fails, the lookup should
> return NULL, in the same way as it's done in __neigh_lookup().
> 
> Otherwise, callers legitimately checking for a non-NULL return value of
> the lookup function might dereference an invalid pointer.
> 
> For instance, on neighbour table overflow, ndisc_router_discovery()
> crashes ndisc_update() by passing ERR_PTR(-ENOBUFS) as 'neigh' argument.
> 
> Reported-by: Jianlin Shi <jishi@redhat.com>
> Fixes: f8a1b43b709d ("net/ipv6: Create a neigh_lookup for FIB entries")
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Applied and queued up for -stable, thank you.