[PATCH v2] scsi: target: fix demo-mode dynamic ACL dev-entry UAF vs LUN teardown

[Yu Zhang <yuz08559@gmail.com>]

A demo-mode (generate_node_acls=1) login that allocates a dynamic
NodeACL populates that ACL's per-LUN dev entries in
core_tpg_add_node_to_devs() *before* the ACL is linked onto
tpg->acl_node_list by target_add_node_acl():

	core_tpg_check_initiator_node_acl():
		...
		core_tpg_add_node_to_devs(acl, tpg, NULL); /* builds deves */
		target_add_node_acl(acl);                  /* only now listed */

The LUN teardown sweep, core_clear_lun_from_tpg(), enumerates only
tpg->acl_node_list, so a dev entry whose ACL is still in the window
above is invisible to the sweep and is never disabled.  It is left on
the se_lun->lun_deve_list after core_tpg_remove_lun() frees the se_lun;
when the same unpacked_lun is later re-added and the se_lun is
reallocated at the same address, the stale dev entry is rediscovered by
target_nacl_find_deve() and core_enable_device_list_for_node() corrupts
lun_deve_list:

  BUG: KASAN: slab-use-after-free ...
   core_enable_device_list_for_node+0x5ce/0x880
   core_tpg_add_node_to_devs+0x169/0x190
   core_dev_add_lun+0x146/0x190
   target_fabric_port_link+0xe9/0x1b0
   configfs_symlink+0x381/0xae0
   __x64_sys_symlink+0x55/0x80

The demo-mode login path that drives the ACL/dev-entry creation is
reachable by an unauthenticated initiator; the colliding LUN add/remove
is a local configfs operation.

Commit 49cb77e297dc ("target: Avoid mappedlun symlink creation during
lun shutdown") introduced se_lun->lun_shutdown and guarded the
*explicit* ACL MappedLUN path (target_fabric_mappedlun_link()), but the
demo-mode automatic path, core_tpg_add_node_to_devs() ->
core_enable_device_list_for_node(), was never given the equivalent
guard, and the publish-after-populate ordering in
core_tpg_check_initiator_node_acl() was left unchanged.

Fix it in three cooperating parts:

 1) Publish the dynamic NodeACL onto acl_node_list *before* creating its
    dev entries, so the teardown sweep can observe them.

 2) Refuse to attach a dev entry to an se_lun that is shutting down,
    checked under nacl->lun_entry_mutex -- the same lock the sweep takes
    -- so once the sweep has passed an ACL any later enable for that LUN
    observes lun_shutdown and skips it.

 3) In core_enable_device_list_for_node(), insert the new dev entry onto
    lun->lun_deve_list while still holding nacl->lun_entry_mutex, in both
    the replace and the new-entry branch.  Previously the entry was added
    to nacl->lun_entry_hlist under the mutex but to lun->lun_deve_list
    only after dropping it; with the ACL now published early, the sweep
    could take the mutex in that gap, find the entry on the hlist,
    core_disable_device_list_for_node() it (list_del() on the not-yet-
    linked lun_link succeeds and the entry is freed), and then the enable
    would list_add_tail() the freed entry back onto lun_deve_list.
    Linking both lists under the mutex makes enable and the sweep
    mutually exclusive, closing that window.

core_tpg_add_node_to_devs() already ignores the return value of
core_enable_device_list_for_node(), so a shutting-down LUN is simply
skipped.

Cc: stable@vger.kernel.org
Fixes: 49cb77e297dc ("target: Avoid mappedlun symlink creation during lun shutdown")
Signed-off-by: Yu Zhang <yuz08559@gmail.com>
Tested-by: Yu Zhang <yuz08559@gmail.com>
---
v2: core_enable_device_list_for_node() now links the new dev entry onto
    lun->lun_deve_list while still holding nacl->lun_entry_mutex (part 3
    above), closing a TOCTOU window the v1 reorder opened where the
    teardown sweep could free a dev entry between its hlist insert and
    its lun_deve_list insert.  Caught by automated review.  No functional
    change to the v1 tpg.c reorder or the lun_shutdown guard.
 drivers/target/target_core_device.c | 18 +++++++++++-------
 drivers/target/target_core_tpg.c    | 10 +++++++---
 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/drivers/target/target_core_device.c b/drivers/target/target_core_device.c
index 1111111..2222222 100644
@@ -349,6 +349,19 @@
 	new->attach_count++;
 
 	mutex_lock(&nacl->lun_entry_mutex);
+	/*
+	 * Do not attach a new dev entry to an se_lun that is being torn down.
+	 * lun_shutdown is set by core_tpg_remove_lun() before its
+	 * core_clear_lun_from_tpg() sweep; checking it here under the same
+	 * nacl->lun_entry_mutex the sweep takes guarantees that once the sweep
+	 * has passed this ACL no dev entry can be left dangling on the
+	 * se_lun->lun_deve_list that is about to be freed.
+	 */
+	if (lun->lun_shutdown) {
+		mutex_unlock(&nacl->lun_entry_mutex);
+		ret = -ENODEV;
+		goto free_stats;
+	}
 	orig = target_nacl_find_deve(nacl, mapped_lun);
 	if (orig && orig->se_lun) {
 		struct se_lun *orig_lun = orig->se_lun;
@@ -375,12 +388,12 @@
 		new->se_lun_acl = lun_acl;
 		hlist_del_rcu(&orig->link);
 		hlist_add_head_rcu(&new->link, &nacl->lun_entry_hlist);
-		mutex_unlock(&nacl->lun_entry_mutex);
 
 		spin_lock(&lun->lun_deve_lock);
 		list_del(&orig->lun_link);
 		list_add_tail(&new->lun_link, &lun->lun_deve_list);
 		spin_unlock(&lun->lun_deve_lock);
+		mutex_unlock(&nacl->lun_entry_mutex);
 
 		kref_put(&orig->pr_kref, target_pr_kref_release);
 		wait_for_completion(&orig->pr_comp);
@@ -393,11 +406,10 @@
 	new->se_lun = lun;
 	new->se_lun_acl = lun_acl;
 	hlist_add_head_rcu(&new->link, &nacl->lun_entry_hlist);
-	mutex_unlock(&nacl->lun_entry_mutex);
-
 	spin_lock(&lun->lun_deve_lock);
 	list_add_tail(&new->lun_link, &lun->lun_deve_list);
 	spin_unlock(&lun->lun_deve_lock);
+	mutex_unlock(&nacl->lun_entry_mutex);
 
 	target_luns_data_has_changed(nacl, new, true);
 	return 0;
diff --git a/drivers/target/target_core_tpg.c b/drivers/target/target_core_tpg.c
index 3333333..4444444 100644
@@ -267,6 +267,15 @@
 	acl->dynamic_node_acl = 1;
 
 	/*
+	 * Publish the dynamic NodeACL on tpg->acl_node_list before creating
+	 * its demo-mode MappedLUN dev entries below, so that a concurrent LUN
+	 * teardown sweep (core_clear_lun_from_tpg(), which walks acl_node_list)
+	 * can observe and disable those dev entries.  Otherwise a dev entry can
+	 * be left on an se_lun->lun_deve_list that is about to be freed.
+	 */
+	target_add_node_acl(acl);
+
+	/*
 	 * Here we only create demo-mode MappedLUNs from the active
 	 * TPG LUNs if the fabric is not explicitly asking for
 	 * tpg_check_demo_mode_login_only() == 1.
@@ -275,7 +284,6 @@
 	    (tpg->se_tpg_tfo->tpg_check_demo_mode_login_only(tpg) != 1))
 		core_tpg_add_node_to_devs(acl, tpg, NULL);
 
-	target_add_node_acl(acl);
 	return acl;
 }
 EXPORT_SYMBOL(core_tpg_check_initiator_node_acl);
-- 
2.43.0