netdev - [PATCH net 1/2] net: nexthop: dereference nh only once in nexthop_select

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Message-Id: <20200519110424.2397623-2-nikolay@cumulusnetworks.com>
Date:   Tue, 19 May 2020 14:04:23 +0300
From:   Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
To:     netdev@...r.kernel.org
Cc:     roopa@...ulusnetworks.com, dsahern@...il.com,
        Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
Subject: [PATCH net 1/2] net: nexthop: dereference nh only once in nexthop_select_path

the ->nh pointer might become suddenly null while we're selecting the
path and we may dereference it. Dereference it only once in the
beginning and use that if it's not null, we rely on the refcounting and
rcu to protect against use-after-free.

Fixes: 430a049190de ("nexthop: Add support for nexthop groups")
Signed-off-by: Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
---
 net/ipv4/nexthop.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
index 2a31c4af845e..a6ffdb067253 100644
--- a/net/ipv4/nexthop.c
+++ b/net/ipv4/nexthop.c
@@ -490,28 +490,33 @@ struct nexthop *nexthop_select_path(struct nexthop *nh, int hash)
 	nhg = rcu_dereference(nh->nh_grp);
 	for (i = 0; i < nhg->num_nh; ++i) {
 		struct nh_grp_entry *nhge = &nhg->nh_entries[i];
+		struct nexthop *nhge_nh;
 		struct nh_info *nhi;
 
 		if (hash > atomic_read(&nhge->upper_bound))
 			continue;
 
+		nhge_nh = READ_ONCE(nhge->nh);
+		if (unlikely(!nhge_nh))
+			continue;
+
 		/* nexthops always check if it is good and does
 		 * not rely on a sysctl for this behavior
 		 */
-		nhi = rcu_dereference(nhge->nh->nh_info);
+		nhi = rcu_dereference(nhge_nh->nh_info);
 		switch (nhi->family) {
 		case AF_INET:
 			if (ipv4_good_nh(&nhi->fib_nh))
-				return nhge->nh;
+				return nhge_nh;
 			break;
 		case AF_INET6:
 			if (ipv6_good_nh(&nhi->fib6_nh))
-				return nhge->nh;
+				return nhge_nh;
 			break;
 		}
 
 		if (!rc)
-			rc = nhge->nh;
+			rc = nhge_nh;
 	}
 
 	return rc;
-- 
2.25.2