[SRU][Xenial][CVE-2018-20836][PATCH] scsi: libsas: fix a race condition when smp task timeout

[Connor Kuehl]

From: Jason Yan <yanaijie at huawei.com>

CVE-2018-20836

BugLink: https://bugs.launchpad.net/bugs/1808912

When the lldd is processing the complete sas task in interrupt and set the
task stat as SAS_TASK_STATE_DONE, the smp timeout timer is able to be
triggered at the same time. And smp_task_timedout() will complete the task
wheter the SAS_TASK_STATE_DONE is set or not. Then the sas task may freed
before lldd end the interrupt process. Thus a use-after-free will happen.

Fix this by calling the complete() only when SAS_TASK_STATE_DONE is not
set. And remove the check of the return value of the del_timer(). Once the
LLDD sets DONE, it must call task->done(), which will call
smp_task_done()->complete() and the task will be completed and freed
correctly.

Reported-by: chenxiang <chenxiang66 at hisilicon.com>
Signed-off-by: Jason Yan <yanaijie at huawei.com>
CC: John Garry <john.garry at huawei.com>
CC: Johannes Thumshirn <jthumshirn at suse.de>
CC: Ewan Milne <emilne at redhat.com>
CC: Christoph Hellwig <hch at lst.de>
CC: Tomas Henzl <thenzl at redhat.com>
CC: Dan Williams <dan.j.williams at intel.com>
CC: Hannes Reinecke <hare at suse.com>
Reviewed-by: Hannes Reinecke <hare at suse.com>
Reviewed-by: John Garry <john.garry at huawei.com>
Reviewed-by: Johannes Thumshirn <jthumshirn at suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen at oracle.com>
(cherry picked from commit b90cd6f2b905905fb42671009dc0e27c310a16ae)
Signed-off-by: Connor Kuehl <connor.kuehl at canonical.com>
---
 drivers/scsi/libsas/sas_expander.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/drivers/scsi/libsas/sas_expander.c b/drivers/scsi/libsas/sas_expander.c
index 7be581f7c35d..1a6f65db615e 100644
--- a/drivers/scsi/libsas/sas_expander.c
+++ b/drivers/scsi/libsas/sas_expander.c
@@ -47,17 +47,16 @@ static void smp_task_timedout(unsigned long _task)
 	unsigned long flags;
 
 	spin_lock_irqsave(&task->task_state_lock, flags);
-	if (!(task->task_state_flags & SAS_TASK_STATE_DONE))
+	if (!(task->task_state_flags & SAS_TASK_STATE_DONE)) {
 		task->task_state_flags |= SAS_TASK_STATE_ABORTED;
+		complete(&task->slow_task->completion);
+	}
 	spin_unlock_irqrestore(&task->task_state_lock, flags);
-
-	complete(&task->slow_task->completion);
 }
 
 static void smp_task_done(struct sas_task *task)
 {
-	if (!del_timer(&task->slow_task->timer))
-		return;
+	del_timer(&task->slow_task->timer);
 	complete(&task->slow_task->completion);
 }
 
-- 
2.20.1