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Date:   Wed, 23 Sep 2020 10:46:50 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Maciej Żenczykowski <zenczykowski@...il.com>,
        Maciej Żenczykowski <maze@...gle.com>,
        "David S . Miller" <davem@...emloft.net>
Cc:     Linux Network Development Mailing List <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Lorenzo Colitti <lorenzo@...gle.com>,
        Sunmeet Gill <sgill@...cinc.com>,
        Vinay Paradkar <vparadka@....qualcomm.com>,
        Tyler Wear <twear@...cinc.com>,
        David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH v2] net/ipv4: always honour route mtu during forwarding



On 9/23/20 6:51 AM, Maciej Żenczykowski wrote:
> From: Maciej Żenczykowski <maze@...gle.com>
> 
> Documentation/networking/ip-sysctl.txt:46 says:
>   ip_forward_use_pmtu - BOOLEAN
>     By default we don't trust protocol path MTUs while forwarding
>     because they could be easily forged and can lead to unwanted
>     fragmentation by the router.
>     You only need to enable this if you have user-space software
>     which tries to discover path mtus by itself and depends on the
>     kernel honoring this information. This is normally not the case.
>     Default: 0 (disabled)
>     Possible values:
>     0 - disabled
>     1 - enabled
> 
> Which makes it pretty clear that setting it to 1 is a potential
> security/safety/DoS issue, and yet it is entirely reasonable to want
> forwarded traffic to honour explicitly administrator configured
> route mtus (instead of defaulting to device mtu).
> 
> Indeed, I can't think of a single reason why you wouldn't want to.
> Since you configured a route mtu you probably know better...
> 
> It is pretty common to have a higher device mtu to allow receiving
> large (jumbo) frames, while having some routes via that interface
> (potentially including the default route to the internet) specify
> a lower mtu.
> 
> Note that ipv6 forwarding uses device mtu unless the route is locked
> (in which case it will use the route mtu).
> 
> This approach is not usable for IPv4 where an 'mtu lock' on a route
> also has the side effect of disabling TCP path mtu discovery via
> disabling the IPv4 DF (don't frag) bit on all outgoing frames.
> 
> I'm not aware of a way to lock a route from an IPv6 RA, so that also
> potentially seems wrong.
> 
> Signed-off-by: Maciej Żenczykowski <maze@...gle.com>
> Cc: Eric Dumazet <maze@...gle.com>

Note that my email address is more like : <edumazet@...gle.com>

> Cc: Willem de Bruijn <willemb@...gle.com>
> Cc: Lorenzo Colitti <lorenzo@...gle.com>
> Cc: Sunmeet Gill (Sunny) <sgill@...cinc.com>
> Cc: Vinay Paradkar <vparadka@....qualcomm.com>
> Cc: Tyler Wear <twear@...cinc.com>
> Cc: David Ahern <dsahern@...nel.org>
> ---
>  include/net/ip.h | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/include/net/ip.h b/include/net/ip.h
> index b09c48d862cc..c2188bebbc54 100644
> --- a/include/net/ip.h
> +++ b/include/net/ip.h
> @@ -436,12 +436,17 @@ static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
>  						    bool forwarding)
>  {
>  	struct net *net = dev_net(dst->dev);
> +	unsigned int mtu;
>  
>  	if (net->ipv4.sysctl_ip_fwd_use_pmtu ||
>  	    ip_mtu_locked(dst) ||
>  	    !forwarding)
>  		return dst_mtu(dst);
>  
> +	/* 'forwarding = true' case should always honour route mtu */
> +	mtu = dst_metric_raw(dst, RTAX_MTU);
> +	if (mtu) return mtu;


        if (mtu)
                return mtu;

Apparently route mtu are capped to 65520, not sure where it is done exactly IP_MAX_MTU being 65535)

# ip ro add 1.1.1.4 dev wlp2s0 mtu 100000
# ip ro get 1.1.1.4
1.1.1.4 dev wlp2s0 src 192.168.8.147 uid 0 
    cache mtu 65520 




> +
>  	return min(READ_ONCE(dst->dev->mtu), IP_MAX_MTU);
>  }
>  
> 

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