ipv6: route: enforce RCU protection in ip6_route_check_nh_onlink() [Linux 5.0]

This Linux kernel change "ipv6: route: enforce RCU protection in ip6_route_check_nh_onlink()" is included in the Linux 5.0 release. This change is authored by Paolo Abeni <pabeni [at] redhat.com> on Thu Feb 21 11:19:42 2019 +0100. The commit for this change in Linux stable tree is bf1dc8b (patch).

ipv6: route: enforce RCU protection in ip6_route_check_nh_onlink()

We need a RCU critical section around rt6_info->from deference, and
proper annotation.

Fixes: 4ed591c8ab44 ("net/ipv6: Allow onlink routes to have a device mismatch if it is the default route")
Signed-off-by: Paolo Abeni <[email protected]>
Reviewed-by: David Ahern <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

There are 6 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 net/ipv6/route.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 2b1ed8c..74b9b6f 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -2743,20 +2743,24 @@ static int ip6_route_check_nh_onlink(struct net *net,
    u32 tbid = l3mdev_fib_table(dev) ? : RT_TABLE_MAIN;
    const struct in6_addr *gw_addr = &cfg->fc_gateway;
    u32 flags = RTF_LOCAL | RTF_ANYCAST | RTF_REJECT;
+   struct fib6_info *from;
    struct rt6_info *grt;
    int err;

    err = 0;
    grt = ip6_nh_lookup_table(net, cfg, gw_addr, tbid, 0);
    if (grt) {
+       rcu_read_lock();
+       from = rcu_dereference(grt->from);
        if (!grt->dst.error &&
            /* ignore match if it is the default route */
-           grt->from && !ipv6_addr_any(&grt->from->fib6_dst.addr) &&
+           from && !ipv6_addr_any(&from->fib6_dst.addr) &&
            (grt->rt6i_flags & flags || dev != grt->dst.dev)) {
            NL_SET_ERR_MSG(extack,
                       "Nexthop has invalid gateway or device mismatch");
            err = -EINVAL;
        }
+       rcu_read_unlock();

        ip6_rt_put(grt);
    }

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