netdev - [PATCH net] ip: fix triggering of 'icmp redirect'

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Date:   Mon, 29 Aug 2022 12:01:21 +0200
From:   Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        David Ahern <dsahern@...nel.org>
Cc:     netdev@...r.kernel.org, Heng Qi <hengqi@...ux.alibaba.com>,
        Edwin Brossette <edwin.brossette@...nd.com>,
        kernel test robot <lkp@...el.com>, lkp@...ts.01.org,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        stable@...r.kernel.org, kernel test robot <yujie.liu@...el.com>
Subject: [PATCH net] ip: fix triggering of 'icmp redirect'

__mkroute_input() uses fib_validate_source() to trigger an icmp redirect.
My understanding is that fib_validate_source() is used to know if the src
address and the gateway address are on the same link. For that,
fib_validate_source() returns 1 (same link) or 0 (not the same network).
__mkroute_input() is the only user of these positive values, all other
callers only look if the returned value is negative.

Since the below patch, fib_validate_source() didn't return anymore 1 when
both addresses are on the same network, because the route lookup returns
RT_SCOPE_LINK instead of RT_SCOPE_HOST. But this is, in fact, right.
Let's adapat the test to return 1 again when both addresses are on the same
link.

CC: stable@...r.kernel.org
Fixes: 747c14307214 ("ip: fix dflt addr selection for connected nexthop")
Reported-by: kernel test robot <yujie.liu@...el.com>
Reported-by: Heng Qi <hengqi@...ux.alibaba.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@...nd.com>
---

This code exists since more than two decades:
https://git.kernel.org/pub/scm/linux/kernel/git/davem/netdev-vger-cvs.git/commit/?id=0c2c94df8133f

Please, feel free to comment if my analysis seems wrong.

Regards,
Nicolas

 net/ipv4/fib_frontend.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index f361d3d56be2..943edf4ad4db 100644
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -389,7 +389,7 @@ static int __fib_validate_source(struct sk_buff *skb, __be32 src, __be32 dst,
 	dev_match = dev_match || (res.type == RTN_LOCAL &&
 				  dev == net->loopback_dev);
 	if (dev_match) {
-		ret = FIB_RES_NHC(res)->nhc_scope >= RT_SCOPE_HOST;
+		ret = FIB_RES_NHC(res)->nhc_scope >= RT_SCOPE_LINK;
 		return ret;
 	}
 	if (no_addr)
@@ -401,7 +401,7 @@ static int __fib_validate_source(struct sk_buff *skb, __be32 src, __be32 dst,
 	ret = 0;
 	if (fib_lookup(net, &fl4, &res, FIB_LOOKUP_IGNORE_LINKSTATE) == 0) {
 		if (res.type == RTN_UNICAST)
-			ret = FIB_RES_NHC(res)->nhc_scope >= RT_SCOPE_HOST;
+			ret = FIB_RES_NHC(res)->nhc_scope >= RT_SCOPE_LINK;
 	}
 	return ret;
 
-- 
2.33.0