[PATCH 4.19 02/23] ipv6: A few fixes on dereferencing rt->from

Greg Kroah-Hartman
2019-05-04T10:28:46Z

From: Martin KaFai Lau <kafai@fb.com>

[ Upstream commit 886b7a50100a50f1cbd08a6f8ec5884dfbe082dc ]

It is a followup after the fix in
commit 9c69a1320515 ("route: Avoid crash from dereferencing NULL rt->from")

rt6_do_redirect():
1. NULL checking is needed on rt->from because a parallel
fib6_info delete could happen that sets rt->from to NULL.
(e.g. rt6_remove_exception() and fib6_drop_pcpu_from()).

2. fib6_info_hold() is not enough. Same reason as (1).
Meaning, holding dst->__refcnt cannot ensure
rt->from is not NULL or rt->from->fib6_ref is not 0.

Instead of using fib6_info_hold_safe() which ip6_rt_cache_alloc()
is already doing, this patch chooses to extend the rcu section
to keep "from" dereference-able after checking for NULL.

inet6_rtm_getroute():
1. NULL checking is also needed on rt->from for a similar reason.
Note that inet6_rtm_getroute() is using RTNL_FLAG_DOIT_UNLOCKED.

Fixes: a68886a69180 ("net/ipv6: Make from in rt6_info rcu protected")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Wei Wang <weiwan@google.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/ipv6/route.c | 38 ++++++++++++++++++--------------------
1 file changed, 18 insertions(+), 20 deletions(-)

--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -3454,11 +3454,8 @@ static void rt6_do_redirect(struct dst_e

rcu_read_lock();
from = rcu_dereference(rt->from);
- /* This fib6_info_hold() is safe here because we hold reference to rt
- * and rt already holds reference to fib6_info.
- */
- fib6_info_hold(from);
- rcu_read_unlock();
+ if (!from)
+ goto out;

nrt = ip6_rt_cache_alloc(from, &msg->dest, NULL);
if (!nrt)
@@ -3470,10 +3467,7 @@ static void rt6_do_redirect(struct dst_e

nrt->rt6i_gateway = *(struct in6_addr *)neigh->primary_key;

- /* No need to remove rt from the exception table if rt is
- * a cached route because rt6_insert_exception() will
- * takes care of it
- */
+ /* rt6_insert_exception() will take care of duplicated exceptions */
if (rt6_insert_exception(nrt, from)) {
dst_release_immediate(&nrt->dst);
goto out;
@@ -3486,7 +3480,7 @@ static void rt6_do_redirect(struct dst_e
call_netevent_notifiers(NETEVENT_REDIRECT, &netevent);

out:
- fib6_info_release(from);
+ rcu_read_unlock();
neigh_release(neigh);
}

@@ -4991,16 +4985,20 @@ static int inet6_rtm_getroute(struct sk_

rcu_read_lock();
from = rcu_dereference(rt->from);
-
- if (fibmatch)
- err = rt6_fill_node(net, skb, from, NULL, NULL, NULL, iif,
- RTM_NEWROUTE, NETLINK_CB(in_skb).portid,
- nlh->nlmsg_seq, 0);
- else
- err = rt6_fill_node(net, skb, from, dst, &fl6.daddr,
- &fl6.saddr, iif, RTM_NEWROUTE,
- NETLINK_CB(in_skb).portid, nlh->nlmsg_seq,
- 0);
+ if (from) {
+ if (fibmatch)
+ err = rt6_fill_node(net, skb, from, NULL, NULL, NULL,
+ iif, RTM_NEWROUTE,
+ NETLINK_CB(in_skb).portid,
+ nlh->nlmsg_seq, 0);
+ else
+ err = rt6_fill_node(net, skb, from, dst, &fl6.daddr,
+ &fl6.saddr, iif, RTM_NEWROUTE,
+ NETLINK_CB(in_skb).portid,
+ nlh->nlmsg_seq, 0);
+ } else {
+ err = -ENETUNREACH;
+ }
rcu_read_unlock();

if (err < 0) {