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

From: David Ahern <dsahern@kernel.org>
To: Ido Schimmel <idosch@idosch.org>
Cc: kuba@kernel.org, davem@davemloft.net, pabeni@redhat.com,
	netdev@vger.kernel.org, Gwangun Jung <exsociety@gmail.com>
Subject: Re: [PATCH net] ipv4: Handle attempt to delete multipath route when fib_info contains an nh reference
Date: Wed, 5 Oct 2022 13:27:59 -0600	[thread overview]
Message-ID: <84202713-ec7c-1e5e-8d9f-d36e715c81e4@kernel.org> (raw)
In-Reply-To: <Yz3WE+cBd9YUj7Bp@shredder>

On 10/5/22 1:08 PM, Ido Schimmel wrote:
> 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

that's what I did as well.

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

I have one in my tree, but in my tests nothing blew up or threw an error
message. It requires KASAN to be enabled otherwise the test does not
trigger anything.

> 
> 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(+)
>>

> 
> 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;

that should work as well. I went with the simplest change that would
definitely not have a negative impact on backports.

> +
>  	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,