Re: [PATCH net] ipv4: Handle attempt to delete multipath route when fib_info contains an nh reference

[Ido Schimmel <idosch@idosch.org>]

On Wed, Oct 05, 2022 at 12:12:57PM -0600, David Ahern wrote:
> Gwangun Jung reported a slab-out-of-bounds access in fib_nh_match:
>     fib_nh_match+0xf98/0x1130 linux-6.0-rc7/net/ipv4/fib_semantics.c:961
>     fib_table_delete+0x5f3/0xa40 linux-6.0-rc7/net/ipv4/fib_trie.c:1753
>     inet_rtm_delroute+0x2b3/0x380 linux-6.0-rc7/net/ipv4/fib_frontend.c:874
> 
> Separate nexthop objects are mutually exclusive with the legacy
> multipath spec. Fix fib_nh_match to return if the config for the
> to be deleted route contains a multipath spec while the fib_info
> is using a nexthop object.

Cool bug... Managed to reproduce with:

 # ip nexthop add id 1 blackhole
 # ip route add 192.0.2.0/24 nhid 1
 # ip route del 192.0.2.0/24 nexthop via 198.51.100.1 nexthop via 198.51.100.2

Maybe add to tools/testing/selftests/net/fib_nexthops.sh ?

Checked IPv6 and I don't think we can hit it there, but I will double
check tomorrow morning.

> 
> Fixes: 493ced1ac47c ("ipv4: Allow routes to use nexthop objects")
> Reported-by: Gwangun Jung <exsociety@gmail.com>
> Signed-off-by: David Ahern <dsahern@kernel.org>
> ---
>  net/ipv4/fib_semantics.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
> index 2dc97583d279..17caa73f57e6 100644
> --- a/net/ipv4/fib_semantics.c
> +++ b/net/ipv4/fib_semantics.c
> @@ -926,6 +926,10 @@ int fib_nh_match(struct net *net, struct fib_config *cfg, struct fib_info *fi,
>  	if (!cfg->fc_mp)
>  		return 0;
>  
> +	/* multipath spec and nexthop id are mutually exclusive */
> +	if (fi->nh)
> +		return 1;
> +
>  	rtnh = cfg->fc_mp;
>  	remaining = cfg->fc_mp_len;

There is already such a check above for the non-multipath check, maybe
we can just move it up to cover both cases? Something like:

diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index 2dc97583d279..e9a7f70a54df 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -888,13 +888,13 @@ int fib_nh_match(struct net *net, struct fib_config *cfg, struct fib_info *fi,
 		return 1;
 	}
 
+	/* cannot match on nexthop object attributes */
+	if (fi->nh)
+		return 1;
+
 	if (cfg->fc_oif || cfg->fc_gw_family) {
 		struct fib_nh *nh;
 
-		/* cannot match on nexthop object attributes */
-		if (fi->nh)
-			return 1;
-
 		nh = fib_info_nh(fi, 0);
 		if (cfg->fc_encap) {
 			if (fib_encap_match(net, cfg->fc_encap_type,