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Subject[PATCH 3.16 176/233] target: Fix kref->refcount underflow in transport_cmd_finish_abort
3.16.48-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Nicholas Bellinger <nab@linux-iscsi.org>

commit 73d4e580ccc5c3e05cea002f18111f66c9c07034 upstream.

This patch fixes a se_cmd->cmd_kref underflow during CMD_T_ABORTED
when a fabric driver drops it's second reference from below the
target_core_tmr.c based callers of transport_cmd_finish_abort().

Recently with the conversion of kref to refcount_t, this bug was
manifesting itself as:

[705519.601034] refcount_t: underflow; use-after-free.
[705519.604034] INFO: NMI handler (kgdb_nmi_handler) took too long to run: 20116.512 msecs
[705539.719111] ------------[ cut here ]------------
[705539.719117] WARNING: CPU: 3 PID: 26510 at lib/refcount.c:184 refcount_sub_and_test+0x33/0x51

Since the original kref atomic_t based kref_put() didn't check for
underflow and only invoked the final callback when zero was reached,
this bug did not manifest in practice since all se_cmd memory is
using preallocated tags.

To address this, go ahead and propigate the existing return from
transport_put_cmd() up via transport_cmd_finish_abort(), and
change transport_cmd_finish_abort() + core_tmr_handle_tas_abort()
callers to only do their local target_put_sess_cmd() if necessary.

Reported-by: Bart Van Assche <bart.vanassche@sandisk.com>
Tested-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Mike Christie <mchristi@redhat.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Himanshu Madhani <himanshu.madhani@qlogic.com>
Cc: Sagi Grimberg <sagig@mellanox.com>
Tested-by: Gary Guo <ghg@datera.io>
Tested-by: Chu Yuan Lin <cyl@datera.io>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/target/target_core_internal.h | 2 +-
drivers/target/target_core_tmr.c | 16 ++++++++--------
drivers/target/target_core_transport.c | 9 ++++++---
3 files changed, 15 insertions(+), 12 deletions(-)

--- a/drivers/target/target_core_internal.h
+++ b/drivers/target/target_core_internal.h
@@ -92,7 +92,7 @@ int init_se_kmem_caches(void);
void release_se_kmem_caches(void);
u32 scsi_get_new_index(scsi_index_t);
void transport_subsystem_check_init(void);
-void transport_cmd_finish_abort(struct se_cmd *, int);
+int transport_cmd_finish_abort(struct se_cmd *, int);
unsigned char *transport_dump_cmd_direction(struct se_cmd *);
void transport_dump_dev_state(struct se_device *, char *, int *);
void transport_dump_dev_info(struct se_device *, struct se_lun *,
--- a/drivers/target/target_core_tmr.c
+++ b/drivers/target/target_core_tmr.c
@@ -82,7 +82,7 @@ void core_tmr_release_req(
kfree(tmr);
}

-static void core_tmr_handle_tas_abort(struct se_cmd *cmd, int tas)
+static int core_tmr_handle_tas_abort(struct se_cmd *cmd, int tas)
{
unsigned long flags;
bool remove = true, send_tas;
@@ -98,7 +98,7 @@ static void core_tmr_handle_tas_abort(st
transport_send_task_abort(cmd);
}

- transport_cmd_finish_abort(cmd, remove);
+ return transport_cmd_finish_abort(cmd, remove);
}

static int target_check_cdb_and_preempt(struct list_head *list,
@@ -195,8 +195,8 @@ void core_tmr_abort_task(
cancel_work_sync(&se_cmd->work);
transport_wait_for_tasks(se_cmd);

- transport_cmd_finish_abort(se_cmd, true);
- target_put_sess_cmd(se_sess, se_cmd);
+ if (!transport_cmd_finish_abort(se_cmd, true))
+ target_put_sess_cmd(se_sess, se_cmd);

printk("ABORT_TASK: Sending TMR_FUNCTION_COMPLETE for"
" ref_tag: %d\n", ref_tag);
@@ -296,8 +296,8 @@ static void core_tmr_drain_tmr_list(
cancel_work_sync(&cmd->work);
transport_wait_for_tasks(cmd);

- transport_cmd_finish_abort(cmd, 1);
- target_put_sess_cmd(cmd->se_sess, cmd);
+ if (!transport_cmd_finish_abort(cmd, 1))
+ target_put_sess_cmd(cmd->se_sess, cmd);
}
}

@@ -395,8 +395,8 @@ static void core_tmr_drain_state_list(
cancel_work_sync(&cmd->work);
transport_wait_for_tasks(cmd);

- core_tmr_handle_tas_abort(cmd, tas);
- target_put_sess_cmd(cmd->se_sess, cmd);
+ if (!core_tmr_handle_tas_abort(cmd, tas))
+ target_put_sess_cmd(cmd->se_sess, cmd);
}
}

--- a/drivers/target/target_core_transport.c
+++ b/drivers/target/target_core_transport.c
@@ -600,9 +600,10 @@ static void transport_lun_remove_cmd(str
percpu_ref_put(&lun->lun_ref);
}

-void transport_cmd_finish_abort(struct se_cmd *cmd, int remove)
+int transport_cmd_finish_abort(struct se_cmd *cmd, int remove)
{
bool ack_kref = (cmd->se_cmd_flags & SCF_ACK_KREF);
+ int ret = 0;

if (cmd->se_cmd_flags & SCF_SE_LUN_CMD)
transport_lun_remove_cmd(cmd);
@@ -614,9 +615,11 @@ void transport_cmd_finish_abort(struct s
cmd->se_tfo->aborted_task(cmd);

if (transport_cmd_check_stop_to_fabric(cmd))
- return;
+ return 1;
if (remove && ack_kref)
- transport_put_cmd(cmd);
+ ret = transport_put_cmd(cmd);
+
+ return ret;
}

static void target_complete_failure_work(struct work_struct *work)
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