Patch "net: ipv4: don't allow setting net.ipv4.route.min_pmtu below 68" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:

     net-ipv4-don-t-allow-setting-net.ipv4.route.min_pmtu-below-68.patch

and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Sabrina Dubroca <[email protected]>
Date: Mon, 26 Feb 2018 16:13:43 +0100
Subject: net: ipv4: don't allow setting net.ipv4.route.min_pmtu below 68

From: Sabrina Dubroca <[email protected]>

[ Upstream commit c7272c2f1229125f74f22dcdd59de9bbd804f1c8 ]

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.

---
 net/ipv4/route.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

@@ -126,10 +126,13 @@ static int ip_rt_redirect_silence __read
 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.
  */

@@ -2765,7 +2768,8 @@ 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",

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/net-ipv4-don-t-allow-setting-net.ipv4.route.min_pmtu-below-68.patch