netdev - Re: [PATCH net] ipv4: Fix NULL pointer dereference in ipv4_neigh

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Message-Id: <20190704.122449.742393341056317443.davem@davemloft.net>
Date:   Thu, 04 Jul 2019 12:24:49 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     idosch@...sch.org
Cc:     netdev@...r.kernel.org, dsahern@...il.com, jiri@...lanox.com,
        shalomt@...lanox.com, mlxsw@...lanox.com, idosch@...lanox.com
Subject: Re: [PATCH net] ipv4: Fix NULL pointer dereference in
 ipv4_neigh_lookup()

From: Ido Schimmel <idosch@...sch.org>
Date: Thu,  4 Jul 2019 19:26:38 +0300

> Both ip_neigh_gw4() and ip_neigh_gw6() can return either a valid pointer
> or an error pointer, but the code currently checks that the pointer is
> not NULL.
 ...
> @@ -447,7 +447,7 @@ static struct neighbour *ipv4_neigh_lookup(const struct dst_entry *dst,
>  		n = ip_neigh_gw4(dev, pkey);
>  	}
>  
> -	if (n && !refcount_inc_not_zero(&n->refcnt))
> +	if (!IS_ERR(n) && !refcount_inc_not_zero(&n->refcnt))
>  		n = NULL;
>  
>  	rcu_read_unlock_bh();

Don't the callers expect only non-error pointers?

All of this stuff is so confusing and fragile...