Fix rare double free in vdev_geom_attrchanged

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c
	Don't drop the g_topology_lock before freeing old_physpath. That
	opens up a race where one thread can call vdev_geom_attrchanged,
	set old_physpath, drop the g_topology_lock, then block trying to
	acquire the SCL_STATE lock. Then another thread can come into
	vdev_geom_attrchanged, set old_physpath to the same value, and
	proceed to free it. When the first thread resumes, it will free
	the same location.

	It turns out that the SCL_STATE lock isn't needed. It was
	originally added by gibbs to protect vd->vdev_physpath while
	updating the same. However, the update process subsequently was
	switched to an atomic operation (a pointer swap). Now, there is
	no need for the SCL_STATE lock, and hence no need to drop the
	g_topology_lock.

Reviewed by:	delphij
MFC after:	4 weeks
Sponsored by:	Spectra Logic Corp
Differential Revision:	https://reviews.freebsd.org/D5413

1 files changed, 3 insertions, 16 deletions

diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c
index f459655..1777fba 100644
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c
@@ -115,27 +115,14 @@ vdev_geom_attrchanged(struct g_consumer *cp, const char *attr)
 	if (error == 0) {
 		char *old_physpath;
 
+		/* g_topology lock ensures that vdev has not been closed */
+		g_topology_assert();
 		old_physpath = vd->vdev_physpath;
 		vd->vdev_physpath = spa_strdup(physpath);
 		spa_async_request(spa, SPA_ASYNC_CONFIG_UPDATE);
 
-		if (old_physpath != NULL) {
-			int held_lock;
-
-			held_lock = spa_config_held(spa, SCL_STATE, RW_WRITER);
-			if (held_lock == 0) {
-				g_topology_unlock();
-				spa_config_enter(spa, SCL_STATE, FTAG,
-				    RW_WRITER);
-			}
-
+		if (old_physpath != NULL)
 			spa_strfree(old_physpath);
-
-			if (held_lock == 0) {
-				spa_config_exit(spa, SCL_STATE, FTAG);
-				g_topology_lock();
-			}
-		}
 	}
 	g_free(physpath);
 }