4.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx> commit bd4e2d2907fa23a11d46217064ecf80470ddae10 upstream. 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: Rob Millner <rlm@xxxxxxxxxxxxx> Tested-by: Rob Millner <rlm@xxxxxxxxxxxxx> Cc: Rob Millner <rlm@xxxxxxxxxxxxx> Tested-by: Vaibhav Tandon <vst@xxxxxxxxx> Cc: Vaibhav Tandon <vst@xxxxxxxxx> Tested-by: Bryant G. Ly <bryantly@xxxxxxxxxxxxxxxxxx> Signed-off-by: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- drivers/target/target_core_device.c | 10 ++++++++-- drivers/target/target_core_tpg.c | 3 ++- drivers/target/target_core_transport.c | 31 ++++++++++++++++++++++++++++++- include/target/target_core_base.h | 1 + 4 files changed, 41 insertions(+), 4 deletions(-) --- a/drivers/target/target_core_device.c +++ b/drivers/target/target_core_device.c @@ -77,12 +77,16 @@ transport_lookup_cmd_lun(struct se_cmd * &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) && @@ -96,6 +100,7 @@ transport_lookup_cmd_lun(struct se_cmd * goto ref_dev; } } +out_unlock: rcu_read_unlock(); if (!se_lun) { @@ -826,6 +831,7 @@ struct se_device *target_alloc_device(st 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); --- a/drivers/target/target_core_tpg.c +++ b/drivers/target/target_core_tpg.c @@ -539,7 +539,7 @@ static void core_tpg_lun_ref_release(str { 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( @@ -666,6 +666,7 @@ 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); --- a/drivers/target/target_core_transport.c +++ b/drivers/target/target_core_transport.c @@ -2680,10 +2680,39 @@ void target_wait_for_sess_cmds(struct se } 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 --- a/include/target/target_core_base.h +++ b/include/target/target_core_base.h @@ -740,6 +740,7 @@ 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;