netdev - [PATCH] ipv6: ensure ip6_dst_mtu_forward() returns at least IPV6_MIN

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Date:   Mon,  7 Jun 2021 16:43:07 -0700
From:   Maciej Żenczykowski <zenczykowski@...il.com>
To:     Maciej Żenczykowski <maze@...gle.com>
Cc:     Linux Network Development Mailing List <netdev@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>
Subject: [PATCH] ipv6: ensure ip6_dst_mtu_forward() returns at least IPV6_MIN_MTU

From: Maciej Żenczykowski <maze@...gle.com>

ip6_dst_mtu_forward() has just two call sites, one of which already
enforces the minimum mtu (which we can now remove), but it does
materially affect the other (presumably buggy) call site in:
  net/netfilter/nf_flow_table_core.c
  flow_offload_fill_route()

Cc: David S. Miller <davem@...emloft.net>
Cc: Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>
Cc: David Ahern <dsahern@...nel.org>
Cc: Jakub Kicinski <kuba@...nel.org>
Signed-off-by: Maciej Żenczykowski <maze@...gle.com>
---
 include/net/ip6_route.h | 4 ++--
 net/ipv6/ip6_output.c   | 2 --
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
index 9dd3c75a4d62..e01364c0821d 100644
--- a/include/net/ip6_route.h
+++ b/include/net/ip6_route.h
@@ -315,14 +315,14 @@ static inline unsigned int ip6_dst_mtu_forward(const struct dst_entry *dst)
 	unsigned int mtu = dst_metric_raw(dst, RTAX_MTU);
 
 	if (mtu)
-		return mtu;
+		return max_t(unsigned int, mtu, IPV6_MIN_MTU);
 
 	rcu_read_lock();
 	idev = __in6_dev_get(dst->dev);
 	mtu = idev ? idev->cnf.mtu6 : IPV6_MIN_MTU;
 	rcu_read_unlock();
 
-	return mtu;
+	return max_t(unsigned int, mtu, IPV6_MIN_MTU);
 }
 
 u32 ip6_mtu_from_fib6(const struct fib6_result *res,
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index ff4f9ebcf7f6..8e56378713cd 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -578,8 +578,6 @@ int ip6_forward(struct sk_buff *skb)
 	}
 
 	mtu = ip6_dst_mtu_forward(dst);
-	if (mtu < IPV6_MIN_MTU)
-		mtu = IPV6_MIN_MTU;
 
 	if (ip6_pkt_too_big(skb, mtu)) {
 		/* Again, force OUTPUT device used as source address */
-- 
2.32.0.rc1.229.g3e70b5a671-goog