netdev - [PATCH net] fib: fib_dump_info can no longer use __in_dev_get

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Date:   Thu,  2 Nov 2017 16:02:20 +0100
From:   Florian Westphal <fw@...len.de>
To:     <netdev@...r.kernel.org>
Cc:     syzkaller-bugs@...glegroups.com,
        bot+e52a2ae091b628f72765583c9faedc961c83b7e7@...kaller.appspotmail.com,
        Florian Westphal <fw@...len.de>
Subject: [PATCH net] fib: fib_dump_info can no longer use __in_dev_get_rtnl

syzbot reported yet another regression added with DOIT_UNLOCKED.
When nexthop is marked as dead, fib_dump_info uses __in_dev_get_rtnl():

./include/linux/inetdevice.h:230 suspicious rcu_dereference_protected() usage!
rcu_scheduler_active = 2, debug_locks = 1
1 lock held by syz-executor2/23859:
 #0:  (rcu_read_lock){....}, at: [<ffffffff840283f0>]
inet_rtm_getroute+0xaa0/0x2d70 net/ipv4/route.c:2738
[..]
  lockdep_rcu_suspicious+0x123/0x170 kernel/locking/lockdep.c:4665
  __in_dev_get_rtnl include/linux/inetdevice.h:230 [inline]
  fib_dump_info+0x1136/0x13d0 net/ipv4/fib_semantics.c:1377
  inet_rtm_getroute+0xf97/0x2d70 net/ipv4/route.c:2785
..

This isn't safe anymore, callers either hold RTNL mutex or rcu read lock,
so these spots must use rcu_dereference_rtnl() or plain rcu_derefence()
(plus unconditional rcu read lock).

This does the latter.

Fixes: 394f51abb3d04f ("ipv4: route: set ipv4 RTM_GETROUTE to not use rtnl")
Reported-by: syzbot <syzkaller@...glegroups.com>
Signed-off-by: Florian Westphal <fw@...len.de>
---
 net/ipv4/fib_semantics.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index 57a5d48acee8..01ed22139ac2 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -1365,8 +1365,6 @@ int fib_dump_info(struct sk_buff *skb, u32 portid, u32 seq, int event,
 	    nla_put_in_addr(skb, RTA_PREFSRC, fi->fib_prefsrc))
 		goto nla_put_failure;
 	if (fi->fib_nhs == 1) {
-		struct in_device *in_dev;
-
 		if (fi->fib_nh->nh_gw &&
 		    nla_put_in_addr(skb, RTA_GATEWAY, fi->fib_nh->nh_gw))
 			goto nla_put_failure;
@@ -1374,10 +1372,14 @@ int fib_dump_info(struct sk_buff *skb, u32 portid, u32 seq, int event,
 		    nla_put_u32(skb, RTA_OIF, fi->fib_nh->nh_oif))
 			goto nla_put_failure;
 		if (fi->fib_nh->nh_flags & RTNH_F_LINKDOWN) {
-			in_dev = __in_dev_get_rtnl(fi->fib_nh->nh_dev);
+			struct in_device *in_dev;
+
+			rcu_read_lock();
+			in_dev = __in_dev_get_rcu(fi->fib_nh->nh_dev);
 			if (in_dev &&
 			    IN_DEV_IGNORE_ROUTES_WITH_LINKDOWN(in_dev))
 				rtm->rtm_flags |= RTNH_F_DEAD;
+			rcu_read_unlock();
 		}
 		if (fi->fib_nh->nh_flags & RTNH_F_OFFLOAD)
 			rtm->rtm_flags |= RTNH_F_OFFLOAD;
@@ -1400,18 +1402,20 @@ int fib_dump_info(struct sk_buff *skb, u32 portid, u32 seq, int event,
 			goto nla_put_failure;
 
 		for_nexthops(fi) {
-			struct in_device *in_dev;
-
 			rtnh = nla_reserve_nohdr(skb, sizeof(*rtnh));
 			if (!rtnh)
 				goto nla_put_failure;
 
 			rtnh->rtnh_flags = nh->nh_flags & 0xFF;
 			if (nh->nh_flags & RTNH_F_LINKDOWN) {
-				in_dev = __in_dev_get_rtnl(nh->nh_dev);
+				struct in_device *in_dev;
+
+				rcu_read_lock();
+				in_dev = __in_dev_get_rcu(nh->nh_dev);
 				if (in_dev &&
 				    IN_DEV_IGNORE_ROUTES_WITH_LINKDOWN(in_dev))
 					rtnh->rtnh_flags |= RTNH_F_DEAD;
+				rcu_read_unlock();
 			}
 			rtnh->rtnh_hops = nh->nh_weight - 1;
 			rtnh->rtnh_ifindex = nh->nh_oif;
-- 
2.13.6