[PATCH] scsi: target: fix se_lun use-after-free in SET TARGET PORT GROUPS secondary path

[Yu Zhang <yuz08559@gmail.com>]

target_emulate_set_target_port_groups() handles a SECONDARY target-port
asymmetric access state (OFFLINE) by walking dev->dev_sep_list under
dev->se_port_lock to find the se_lun whose relative target port identifier
matches the request, then dropping se_port_lock and calling
core_alua_set_tg_pt_secondary_state() on that lun:

	list_for_each_entry(lun, &dev->dev_sep_list, lun_dev_link) {
		if (lun->lun_tpg->tpg_rtpi != rtpi)
			continue;
		// XXX: racy unlock
		spin_unlock(&dev->se_port_lock);
		if (!core_alua_set_tg_pt_secondary_state(lun, 1, 1))
			found = true;
		spin_lock(&dev->se_port_lock);
		break;
	}

core_alua_set_tg_pt_secondary_state() sleeps while using the lun --
msleep(trans_delay_msecs) and, when alua_write_metadata is set, a
kernel_write() of the ALUA metadata. The matched lun is a different target
port than the one the command arrived on, so it is not pinned by the
command's own lun_ref. A concurrent core_tpg_remove_lun() of that port
(configfs LUN removal) drains its lun_ref, removes it from dev_sep_list and
frees the se_lun via call_rcu(); since the command holds no reference and
is not in an RCU read-side section across the sleep, the grace period can
elapse and the lun is freed under it:

	BUG: KASAN: slab-use-after-free in core_alua_set_tg_pt_secondary_state
	 core_alua_set_tg_pt_secondary_state
	 target_emulate_set_target_port_groups
	 __target_execute_cmd
	Allocated by task 3486:
	 configfs_mkdir
	Freed by task 0:
	 __call_rcu_common

Take the matched lun's lun_ref with percpu_ref_tryget_live() before
dropping se_port_lock and release it after the transition, mirroring
core_alua_queue_state_change_ua(). This pins the se_lun across the sleep,
so a concurrent LUN removal waits in transport_clear_lun_ref() instead of
freeing it.

Reproduced with two tcm_loop target ports (distinct RTPIs) on one
backstore, explicit ALUA, large trans_delay_msecs: a SET TARGET PORT
GROUPS (secondary OFFLINE, targeting the second port's RTPI) on the
first port races a configfs removal of the second port's LUN -> the
KASAN splat above; with this patch the LUN removal blocks until the
transition completes and there is no use-after-free.

Fixes: adf653f92f38e8 ("target: Subsume se_port + t10_alua_tg_pt_gp_member into se_lun")
Cc: stable@vger.kernel.org
Signed-off-by: Yu Zhang <yuz08559@gmail.com>
---
 drivers/target/target_core_alua.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/target/target_core_alua.c b/drivers/target/target_core_alua.c
index 1111111..2222222 100644
--- a/drivers/target/target_core_alua.c
+++ b/drivers/target/target_core_alua.c
@@ -402,13 +402,16 @@
 				if (lun->lun_tpg->tpg_rtpi != rtpi)
 					continue;
 
-				// XXX: racy unlock
+				if (!percpu_ref_tryget_live(&lun->lun_ref))
+					continue;
+
 				spin_unlock(&dev->se_port_lock);
 
 				if (!core_alua_set_tg_pt_secondary_state(
 						lun, 1, 1))
 					found = true;
 
+				percpu_ref_put(&lun->lun_ref);
 				spin_lock(&dev->se_port_lock);
 				break;
 			}