Re: Regression for ip6-in-ip4 IPsec tunnel in 4.14.16

But also it could seem unfortunate that the code with my fix does not
look at actual packet size, but instead only looks at the MTU and then
fails, even if no packet was going to be so large.  The intention of
my patch was to prevent a negative number while calculating the
maxfraglen in  __ip6_append_data().  An alternative fix maybe to
instead return an error only if the mtu is less than or equal to the
fragheaderlen.   Something like:

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 3763dc01e374..5d912a289b95 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c

@@ -1214,8 +1214,6 @@ static int ip6_setup_cork(struct sock *sk,

struct inet_cork_full *cork,
                if (np->frag_size)
                        mtu = np->frag_size;
        }
-       if (mtu < IPV6_MIN_MTU)
-               return -EINVAL;
        cork->base.fragsize = mtu;
        if (dst_allfrag(rt->dst.path))
                cork->base.flags |= IPCORK_ALLFRAG;

@@ -1264,6 +1262,8 @@ static int __ip6_append_data(struct sock *sk,

        fragheaderlen = sizeof(struct ipv6hdr) + rt->rt6i_nfheader_len +
                        (opt ? opt->opt_nflen : 0);
+       if (mtu < fragheaderlen + 8)
+               return -EINVAL;
        maxfraglen = ((mtu - fragheaderlen) & ~7) + fragheaderlen -
                     sizeof(struct frag_hdr);
                        (opt ? opt->opt_nflen : 0);

But then we also have to convince ourselves that maxfraglen can never
be <= 0.  I'd have to think about that.

I am not sure if others have thoughts on supporting MTUs configured
below the min in the spec.