[PATCH net] ipv4: Fix in_device refcount resurrection in in_dev_get()

in_dev_get() reads dev->ip_ptr under RCU and then unconditionally
increments its refcount. inetdev_destroy() clears the pointer and drops
the last reference under RTNL, with no grace period in between, so a
reader that fetched the pointer before the store can increment a
refcount that has already reached zero. That resurrects an object whose
RCU free is queued: dropping the resurrected reference re-enters
in_dev_finish_destroy() for a second netdev_put() and a second
call_rcu() on the same rcu_head, and if the grace period elapses first
the drop itself is a use-after-free.

inet_netconf_get_devconf() is registered RTNL_FLAG_DOIT_UNLOCKED, and
rtnetlink_rcv_msg() exempts RTNL_KIND_GET from the CAP_NET_ADMIN check,
so an unprivileged user can drive the reader side. Reproduced as UID
65534 on v7.2-rc7:

  refcount_t: addition on 0; use-after-free.
  WARNING: lib/refcount.c:25 at refcount_warn_saturate+0x14c/0x180
  CPU: 0 UID: 65534 PID: 655 Comm: j1_poc
   refcount_warn_saturate+0x14c/0x180 (P)
   inet_netconf_get_devconf+0x4b0/0x4c4
   rtnetlink_rcv_msg+0x434/0x4d0

followed by the matching underflow when the reference is dropped.

Use refcount_inc_not_zero() and return NULL for an in_device that has
already reached zero. All callers already handle a NULL return, which
in_dev_get() gives today whenever dev->ip_ptr is NULL. Callers under
RTNL see no change: ip_ptr is cleared before the last put, so a non-NULL
ip_ptr there implies a non-zero refcount.

Discovered by XBOW, triaged by Baul Lee <[email protected]>

Fixes: bbcf91053bb6 ("inet: do not use RTNL in inet_netconf_get_devconf()")
Signed-off-by: Baul Lee <[email protected]>
---
 include/linux/inetdevice.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/inetdevice.h b/include/linux/inetdevice.h
index 6032eea2539a..a1446da64200 100644
--- a/include/linux/inetdevice.h
+++ b/include/linux/inetdevice.h
@@ -245,8 +245,8 @@ static inline struct in_device *in_dev_get(const struct net_device *dev)
 
 	rcu_read_lock();
 	in_dev = __in_dev_get_rcu(dev);
-	if (in_dev)
-		refcount_inc(&in_dev->refcnt);
+	if (in_dev && !refcount_inc_not_zero(&in_dev->refcnt))
+		in_dev = NULL;
 	rcu_read_unlock();
 	return in_dev;
 }
-- 
2.50.1