[PATCH] wifi: mac80211: Fix a race when expiring a mesh path

[Ginger Li <ginger.jzllee@gmail.com>]

My static analyzer identified a potential issue in 'net/mac80211/mesh_pathtbl.c':
mpath_expired() reads mpath->flags and mpath->exp_time without holding
mpath->state_lock, while mesh_path_fix_nexthop() and the other writers update
both fields under that lock.  In mpath_lookup() and
__mesh_path_lookup_by_idx() the unlocked check is followed by taking the lock
and clearing MESH_PATH_ACTIVE, so the check and the update are not atomic:

CPU0 (mpath_lookup)                    CPU1 (mesh_path_fix_nexthop)
  mpath_expired() reads the stale
  flags/exp_time and returns true
                                        spin_lock_bh(&mpath->state_lock)
                                        mpath->exp_time = 0;
                                        mpath->flags = FIXED|SN_VALID;
                                        mesh_path_activate(mpath);
                                        spin_unlock_bh(&mpath->state_lock)
  spin_lock_bh(&mpath->state_lock)
  mpath->flags &= ~MESH_PATH_ACTIVE;
  spin_unlock_bh(&mpath->state_lock)

A path which mesh_path_fix_nexthop() has just set up as MESH_PATH_FIXED and
MESH_PATH_ACTIVE then has MESH_PATH_ACTIVE cleared again, and forwarding to
that destination silently stops.

Take mpath->state_lock before evaluating mpath_expired() so that the check and
the clearing of MESH_PATH_ACTIVE are atomic with respect to the writers.

Fixes: 60854fd94573 ("mac80211: mesh: convert path table to rhashtable")
Signed-off-by: Ginger Li <ginger.jzllee@gmail.com>
---
 net/mac80211/mesh_pathtbl.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/net/mac80211/mesh_pathtbl.c b/net/mac80211/mesh_pathtbl.c
--- a/net/mac80211/mesh_pathtbl.c
+++ b/net/mac80211/mesh_pathtbl.c
@@ -247,9 +247,10 @@ static struct mesh_path *mpath_lookup(struct mesh_tabl
 
 	mpath = rhashtable_lookup(&tbl->rhead, dst, mesh_rht_params);
 
-	if (mpath && mpath_expired(mpath)) {
+	if (mpath) {
 		spin_lock_bh(&mpath->state_lock);
-		mpath->flags &= ~MESH_PATH_ACTIVE;
+		if (mpath_expired(mpath))
+			mpath->flags &= ~MESH_PATH_ACTIVE;
 		spin_unlock_bh(&mpath->state_lock);
 	}
 	return mpath;
@@ -290,11 +291,11 @@ __mesh_path_lookup_by_idx(struct mesh_table *tbl, int 
 	if (!mpath)
 		return NULL;
 
-	if (mpath_expired(mpath)) {
-		spin_lock_bh(&mpath->state_lock);
+	spin_lock_bh(&mpath->state_lock);
+	if (mpath_expired(mpath))
 		mpath->flags &= ~MESH_PATH_ACTIVE;
-		spin_unlock_bh(&mpath->state_lock);
-	}
+	spin_unlock_bh(&mpath->state_lock);
+
 	return mpath;
 }
 
-- 
2.43.0