libata: fix NULL sdev dereference race in atapi_qc_complete()

SCSI commands may be issued between __scsi_add_device() and dev->sdev
assignment, so it's unsafe for ata_qc_complete() to dereference
dev->sdev->locked without checking whether it's NULL or not.  Fix it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@kernel.org
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
index d050e073e570..3f91c01c217f 100644
--- a/drivers/ata/libata-scsi.c
+++ b/drivers/ata/libata-scsi.c
@@ -2552,8 +2552,11 @@ static void atapi_qc_complete(struct ata_queued_cmd *qc)
                  *
                  * If door lock fails, always clear sdev->locked to
                  * avoid this infinite loop.
+                 *
+                 * This may happen before SCSI scan is complete.  Make
+                 * sure qc->dev->sdev isn't NULL before dereferencing.
                  */
-                if (qc->cdb[0] == ALLOW_MEDIUM_REMOVAL)
+                if (qc->cdb[0] == ALLOW_MEDIUM_REMOVAL && qc->dev->sdev)
                         qc->dev->sdev->locked = 0;
 
                 qc->scsicmd->result = SAM_STAT_CHECK_CONDITION;