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Commit bd4e2d29 authored by Nicholas Bellinger's avatar Nicholas Bellinger
Browse files

target: Fix NULL dereference during LUN lookup + active I/O shutdown 



When transport_clear_lun_ref() is shutting down a se_lun via
configfs with new I/O in-flight, it's possible to trigger a
NULL pointer dereference in transport_lookup_cmd_lun() due
to the fact percpu_ref_get() doesn't do any __PERCPU_REF_DEAD
checking before incrementing lun->lun_ref.count after
lun->lun_ref has switched to atomic_t mode.

This results in a NULL pointer dereference as LUN shutdown
code in core_tpg_remove_lun() continues running after the
existing ->release() -> core_tpg_lun_ref_release() callback
completes, and clears the RCU protected se_lun->lun_se_dev
pointer.

During the OOPs, the state of lun->lun_ref in the process
which triggered the NULL pointer dereference looks like
the following on v4.1.y stable code:

struct se_lun {
  lun_link_magic = 4294932337,
  lun_status = TRANSPORT_LUN_STATUS_FREE,

  .....

  lun_se_dev = 0x0,
  lun_sep = 0x0,

  .....

  lun_ref = {
    count = {
      counter = 1
    },
    percpu_count_ptr = 3,
    release = 0xffffffffa02fa1e0 <core_tpg_lun_ref_release>,
    confirm_switch = 0x0,
    force_atomic = false,
    rcu = {
      next = 0xffff88154fa1a5d0,
      func = 0xffffffff8137c4c0 <percpu_ref_switch_to_atomic_rcu>
    }
  }
}

To address this bug, use percpu_ref_tryget_live() to ensure
once __PERCPU_REF_DEAD is visable on all CPUs and ->lun_ref
has switched to atomic_t, all new I/Os will fail to obtain
a new lun->lun_ref reference.

Also use an explicit percpu_ref_kill_and_confirm() callback
to block on ->lun_ref_comp to allow the first stage and
associated RCU grace period to complete, and then block on
->lun_ref_shutdown waiting for the final percpu_ref_put()
to drop the last reference via transport_lun_remove_cmd()
before continuing with core_tpg_remove_lun() shutdown.

Reported-by: default avatarRob Millner <rlm@daterainc.com>
Tested-by: default avatarRob Millner <rlm@daterainc.com>
Cc: Rob Millner <rlm@daterainc.com>
Tested-by: default avatarVaibhav Tandon <vst@datera.io>
Cc: Vaibhav Tandon <vst@datera.io>
Tested-by: default avatarBryant G. Ly <bryantly@linux.vnet.ibm.com>
Cc: <stable@vger.kernel.org> # v3.14+
Signed-off-by: default avatarNicholas Bellinger <nab@linux-iscsi.org>
parent 51ec502a

drivers/target/target_core_device.c

+8 −2
Original line number Diff line number Diff line
@@ -78,12 +78,16 @@ transport_lookup_cmd_lun(struct se_cmd *se_cmd, u64 unpacked_lun) 
					&deve->read_bytes);



		se_lun = rcu_dereference(deve->se_lun);



		if (!percpu_ref_tryget_live(&se_lun->lun_ref)) {

			se_lun = NULL;

			goto out_unlock;

		}



		se_cmd->se_lun = rcu_dereference(deve->se_lun);

		se_cmd->pr_res_key = deve->pr_res_key;

		se_cmd->orig_fe_lun = unpacked_lun;

		se_cmd->se_cmd_flags |= SCF_SE_LUN_CMD;



		percpu_ref_get(&se_lun->lun_ref);

		se_cmd->lun_ref_active = true;



		if ((se_cmd->data_direction == DMA_TO_DEVICE) &&
@@ -97,6 +101,7 @@ transport_lookup_cmd_lun(struct se_cmd *se_cmd, u64 unpacked_lun) 
			goto ref_dev;

		}

	}

out_unlock:

	rcu_read_unlock();



	if (!se_lun) {
@@ -815,6 +820,7 @@ struct se_device *target_alloc_device(struct se_hba *hba, const char *name) 
	xcopy_lun = &dev->xcopy_lun;

	rcu_assign_pointer(xcopy_lun->lun_se_dev, dev);

	init_completion(&xcopy_lun->lun_ref_comp);

	init_completion(&xcopy_lun->lun_shutdown_comp);

	INIT_LIST_HEAD(&xcopy_lun->lun_deve_list);

	INIT_LIST_HEAD(&xcopy_lun->lun_dev_link);

	mutex_init(&xcopy_lun->lun_tg_pt_md_mutex);

drivers/target/target_core_tpg.c

+2 −1
Original line number Diff line number Diff line
@@ -445,7 +445,7 @@ static void core_tpg_lun_ref_release(struct percpu_ref *ref) 
{

	struct se_lun *lun = container_of(ref, struct se_lun, lun_ref);



	complete(&lun->lun_ref_comp);

	complete(&lun->lun_shutdown_comp);

}



int core_tpg_register(
@@ -571,6 +571,7 @@ struct se_lun *core_tpg_alloc_lun( 
	lun->lun_link_magic = SE_LUN_LINK_MAGIC;

	atomic_set(&lun->lun_acl_count, 0);

	init_completion(&lun->lun_ref_comp);

	init_completion(&lun->lun_shutdown_comp);

	INIT_LIST_HEAD(&lun->lun_deve_list);

	INIT_LIST_HEAD(&lun->lun_dev_link);

	atomic_set(&lun->lun_tg_pt_secondary_offline, 0);

drivers/target/target_core_transport.c

+30 −1
Original line number Diff line number Diff line
@@ -2700,10 +2700,39 @@ void target_wait_for_sess_cmds(struct se_session *se_sess) 
}

EXPORT_SYMBOL(target_wait_for_sess_cmds);



static void target_lun_confirm(struct percpu_ref *ref)

{

	struct se_lun *lun = container_of(ref, struct se_lun, lun_ref);



	complete(&lun->lun_ref_comp);

}



void transport_clear_lun_ref(struct se_lun *lun)

{

	percpu_ref_kill(&lun->lun_ref);

	/*

	 * Mark the percpu-ref as DEAD, switch to atomic_t mode, drop

	 * the initial reference and schedule confirm kill to be

	 * executed after one full RCU grace period has completed.

	 */

	percpu_ref_kill_and_confirm(&lun->lun_ref, target_lun_confirm);

	/*

	 * The first completion waits for percpu_ref_switch_to_atomic_rcu()

	 * to call target_lun_confirm after lun->lun_ref has been marked

	 * as __PERCPU_REF_DEAD on all CPUs, and switches to atomic_t

	 * mode so that percpu_ref_tryget_live() lookup of lun->lun_ref

	 * fails for all new incoming I/O.

	 */

	wait_for_completion(&lun->lun_ref_comp);

	/*

	 * The second completion waits for percpu_ref_put_many() to

	 * invoke ->release() after lun->lun_ref has switched to

	 * atomic_t mode, and lun->lun_ref.count has reached zero.

	 *

	 * At this point all target-core lun->lun_ref references have

	 * been dropped via transport_lun_remove_cmd(), and it's safe

	 * to proceed with the remaining LUN shutdown.

	 */

	wait_for_completion(&lun->lun_shutdown_comp);

}



static bool

include/target/target_core_base.h

+1 −0
Original line number Diff line number Diff line
@@ -730,6 +730,7 @@ struct se_lun { 
	struct config_group	lun_group;

	struct se_port_stat_grps port_stat_grps;

	struct completion	lun_ref_comp;

	struct completion	lun_shutdown_comp;

	struct percpu_ref	lun_ref;

	struct list_head	lun_dev_link;

	struct hlist_node	link;