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Subject[PATCH 4.9 197/329] ipv4: lock mtu in fnhe when received PMTU < net.ipv4.route.min_pmtu
Date
4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Sabrina Dubroca <sd@queasysnail.net>

[ Upstream commit d52e5a7e7ca49457dd31fc8b42fb7c0d58a31221 ]

Prior to the rework of PMTU information storage in commit
2c8cec5c10bc ("ipv4: Cache learned PMTU information in inetpeer."),
when a PMTU event advertising a PMTU smaller than
net.ipv4.route.min_pmtu was received, we would disable setting the DF
flag on packets by locking the MTU metric, and set the PMTU to
net.ipv4.route.min_pmtu.

Since then, we don't disable DF, and set PMTU to
net.ipv4.route.min_pmtu, so the intermediate router that has this link
with a small MTU will have to drop the packets.

This patch reestablishes pre-2.6.39 behavior by splitting
rtable->rt_pmtu into a bitfield with rt_mtu_locked and rt_pmtu.
rt_mtu_locked indicates that we shouldn't set the DF bit on that path,
and is checked in ip_dont_fragment().

One possible workaround is to set net.ipv4.route.min_pmtu to a value low
enough to accommodate the lowest MTU encountered.

Fixes: 2c8cec5c10bc ("ipv4: Cache learned PMTU information in inetpeer.")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/net/ip.h | 11 +++++++++--
include/net/ip_fib.h | 1 +
include/net/route.h | 3 ++-
net/ipv4/route.c | 26 +++++++++++++++++++-------
net/ipv4/xfrm4_policy.c | 1 +
5 files changed, 32 insertions(+), 10 deletions(-)

--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -304,6 +304,13 @@ int ip_decrease_ttl(struct iphdr *iph)
return --iph->ttl;
}

+static inline int ip_mtu_locked(const struct dst_entry *dst)
+{
+ const struct rtable *rt = (const struct rtable *)dst;
+
+ return rt->rt_mtu_locked || dst_metric_locked(dst, RTAX_MTU);
+}
+
static inline
int ip_dont_fragment(const struct sock *sk, const struct dst_entry *dst)
{
@@ -311,7 +318,7 @@ int ip_dont_fragment(const struct sock *

return pmtudisc == IP_PMTUDISC_DO ||
(pmtudisc == IP_PMTUDISC_WANT &&
- !(dst_metric_locked(dst, RTAX_MTU)));
+ !ip_mtu_locked(dst));
}

static inline bool ip_sk_accept_pmtu(const struct sock *sk)
@@ -337,7 +344,7 @@ static inline unsigned int ip_dst_mtu_ma
struct net *net = dev_net(dst->dev);

if (net->ipv4.sysctl_ip_fwd_use_pmtu ||
- dst_metric_locked(dst, RTAX_MTU) ||
+ ip_mtu_locked(dst) ||
!forwarding)
return dst_mtu(dst);

--- a/include/net/ip_fib.h
+++ b/include/net/ip_fib.h
@@ -57,6 +57,7 @@ struct fib_nh_exception {
int fnhe_genid;
__be32 fnhe_daddr;
u32 fnhe_pmtu;
+ bool fnhe_mtu_locked;
__be32 fnhe_gw;
unsigned long fnhe_expires;
struct rtable __rcu *fnhe_rth_input;
--- a/include/net/route.h
+++ b/include/net/route.h
@@ -63,7 +63,8 @@ struct rtable {
__be32 rt_gateway;

/* Miscellaneous cached information */
- u32 rt_pmtu;
+ u32 rt_mtu_locked:1,
+ rt_pmtu:31;

u32 rt_table_id;

--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -618,6 +618,7 @@ static inline u32 fnhe_hashfun(__be32 da
static void fill_route_from_fnhe(struct rtable *rt, struct fib_nh_exception *fnhe)
{
rt->rt_pmtu = fnhe->fnhe_pmtu;
+ rt->rt_mtu_locked = fnhe->fnhe_mtu_locked;
rt->dst.expires = fnhe->fnhe_expires;

if (fnhe->fnhe_gw) {
@@ -628,7 +629,7 @@ static void fill_route_from_fnhe(struct
}

static void update_or_create_fnhe(struct fib_nh *nh, __be32 daddr, __be32 gw,
- u32 pmtu, unsigned long expires)
+ u32 pmtu, bool lock, unsigned long expires)
{
struct fnhe_hash_bucket *hash;
struct fib_nh_exception *fnhe;
@@ -665,8 +666,10 @@ static void update_or_create_fnhe(struct
fnhe->fnhe_genid = genid;
if (gw)
fnhe->fnhe_gw = gw;
- if (pmtu)
+ if (pmtu) {
fnhe->fnhe_pmtu = pmtu;
+ fnhe->fnhe_mtu_locked = lock;
+ }
fnhe->fnhe_expires = max(1UL, expires);
/* Update all cached dsts too */
rt = rcu_dereference(fnhe->fnhe_rth_input);
@@ -690,6 +693,7 @@ static void update_or_create_fnhe(struct
fnhe->fnhe_daddr = daddr;
fnhe->fnhe_gw = gw;
fnhe->fnhe_pmtu = pmtu;
+ fnhe->fnhe_mtu_locked = lock;
fnhe->fnhe_expires = expires;

/* Exception created; mark the cached routes for the nexthop
@@ -771,7 +775,8 @@ static void __ip_do_redirect(struct rtab
struct fib_nh *nh = &FIB_RES_NH(res);

update_or_create_fnhe(nh, fl4->daddr, new_gw,
- 0, jiffies + ip_rt_gc_timeout);
+ 0, false,
+ jiffies + ip_rt_gc_timeout);
}
if (kill_route)
rt->dst.obsolete = DST_OBSOLETE_KILL;
@@ -983,15 +988,18 @@ static void __ip_rt_update_pmtu(struct r
{
struct dst_entry *dst = &rt->dst;
struct fib_result res;
+ bool lock = false;

- if (dst_metric_locked(dst, RTAX_MTU))
+ if (ip_mtu_locked(dst))
return;

if (ipv4_mtu(dst) < mtu)
return;

- if (mtu < ip_rt_min_pmtu)
+ if (mtu < ip_rt_min_pmtu) {
+ lock = true;
mtu = ip_rt_min_pmtu;
+ }

if (rt->rt_pmtu == mtu &&
time_before(jiffies, dst->expires - ip_rt_mtu_expires / 2))
@@ -1001,7 +1009,7 @@ static void __ip_rt_update_pmtu(struct r
if (fib_lookup(dev_net(dst->dev), fl4, &res, 0) == 0) {
struct fib_nh *nh = &FIB_RES_NH(res);

- update_or_create_fnhe(nh, fl4->daddr, 0, mtu,
+ update_or_create_fnhe(nh, fl4->daddr, 0, mtu, lock,
jiffies + ip_rt_mtu_expires);
}
rcu_read_unlock();
@@ -1256,7 +1264,7 @@ static unsigned int ipv4_mtu(const struc

mtu = READ_ONCE(dst->dev->mtu);

- if (unlikely(dst_metric_locked(dst, RTAX_MTU))) {
+ if (unlikely(ip_mtu_locked(dst))) {
if (rt->rt_uses_gateway && mtu > 576)
mtu = 576;
}
@@ -1481,6 +1489,7 @@ struct rtable *rt_dst_alloc(struct net_d
rt->rt_is_input = 0;
rt->rt_iif = 0;
rt->rt_pmtu = 0;
+ rt->rt_mtu_locked = 0;
rt->rt_gateway = 0;
rt->rt_uses_gateway = 0;
rt->rt_table_id = 0;
@@ -2403,6 +2412,7 @@ struct dst_entry *ipv4_blackhole_route(s
rt->rt_is_input = ort->rt_is_input;
rt->rt_iif = ort->rt_iif;
rt->rt_pmtu = ort->rt_pmtu;
+ rt->rt_mtu_locked = ort->rt_mtu_locked;

rt->rt_genid = rt_genid_ipv4(net);
rt->rt_flags = ort->rt_flags;
@@ -2505,6 +2515,8 @@ static int rt_fill_info(struct net *net,
memcpy(metrics, dst_metrics_ptr(&rt->dst), sizeof(metrics));
if (rt->rt_pmtu && expires)
metrics[RTAX_MTU - 1] = rt->rt_pmtu;
+ if (rt->rt_mtu_locked && expires)
+ metrics[RTAX_LOCK - 1] |= BIT(RTAX_MTU);
if (rtnetlink_put_metrics(skb, metrics) < 0)
goto nla_put_failure;

--- a/net/ipv4/xfrm4_policy.c
+++ b/net/ipv4/xfrm4_policy.c
@@ -97,6 +97,7 @@ static int xfrm4_fill_dst(struct xfrm_ds
xdst->u.rt.rt_gateway = rt->rt_gateway;
xdst->u.rt.rt_uses_gateway = rt->rt_uses_gateway;
xdst->u.rt.rt_pmtu = rt->rt_pmtu;
+ xdst->u.rt.rt_mtu_locked = rt->rt_mtu_locked;
xdst->u.rt.rt_table_id = rt->rt_table_id;
INIT_LIST_HEAD(&xdst->u.rt.rt_uncached);


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