net: ipv4: don't allow setting net.ipv4.route.min_pmtu below 68

According to RFC 1191 sections 3 and 4, ICMP frag-needed messages
indicating an MTU below 68 should be rejected:

    A host MUST never reduce its estimate of the Path MTU below 68
    octets.

and (talking about ICMP frag-needed's Next-Hop MTU field):

    This field will never contain a value less than 68, since every
    router "must be able to forward a datagram of 68 octets without
    fragmentation".

Furthermore, by letting net.ipv4.route.min_pmtu be set to negative
values, we can end up with a very large PMTU when (-1) is cast into u32.

Let's also make ip_rt_min_pmtu a u32, since it's only ever compared to
unsigned ints.
Reported-by: NJianlin Shi <jishi@redhat.com>
Signed-off-by: NSabrina Dubroca <sd@queasysnail.net>
Reviewed-by: NStefano Brivio <sbrivio@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -128,10 +128,13 @@ static int ip_rt_redirect_silence __read_mostly	= ((HZ / 50) << (9 + 1));
 static int ip_rt_error_cost __read_mostly	= HZ;
 static int ip_rt_error_burst __read_mostly	= 5 * HZ;
 static int ip_rt_mtu_expires __read_mostly	= 10 * 60 * HZ;
-static int ip_rt_min_pmtu __read_mostly		= 512 + 20 + 20;
+static u32 ip_rt_min_pmtu __read_mostly		= 512 + 20 + 20;
 static int ip_rt_min_advmss __read_mostly	= 256;
 
 static int ip_rt_gc_timeout __read_mostly	= RT_GC_TIMEOUT;
+
+static int ip_min_valid_pmtu __read_mostly	= IPV4_MIN_MTU;
+
 /*
  *	Interface to generic destination cache.
  */
@@ -2933,7 +2936,8 @@ static struct ctl_table ipv4_route_table[] = {
 		.data		= &ip_rt_min_pmtu,
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
-		.proc_handler	= proc_dointvec,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &ip_min_valid_pmtu,
 	},
 	{
 		.procname	= "min_adv_mss",