scsi: sg: Avoid sg device teardown race

sg_remove_sfp_usercontext() must not use sg_device_destroy() after calling
scsi_device_put().

sg_device_destroy() is accessing the parent scsi_device request_queue which
will already be set to NULL when the preceding call to scsi_device_put()
removed the last reference to the parent scsi_device.

The resulting NULL pointer exception will then crash the kernel.

Link: https://lore.kernel.org/r/20240305150509.23896-1-Alexander@wetzel-home.de
Fixes: db59133e ("scsi: sg: fix blktrace debugfs entries leakage")
Cc: stable@vger.kernel.org
Signed-off-by: Alexander Wetzel <Alexander@wetzel-home.de>
Link: https://lore.kernel.org/r/20240320213032.18221-1-Alexander@wetzel-home.deReviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/sg.c

@@ -2207,6 +2207,7 @@ sg_remove_sfp_usercontext(struct work_struct *work)
 {
 	struct sg_fd *sfp = container_of(work, struct sg_fd, ew.work);
 	struct sg_device *sdp = sfp->parentdp;
+	struct scsi_device *device = sdp->device;
 	Sg_request *srp;
 	unsigned long iflags;
 
@@ -2232,8 +2233,9 @@ sg_remove_sfp_usercontext(struct work_struct *work)
 			"sg_remove_sfp: sfp=0x%p\n", sfp));
 	kfree(sfp);
 
-	scsi_device_put(sdp->device);
+	WARN_ON_ONCE(kref_read(&sdp->d_ref) != 1);
 	kref_put(&sdp->d_ref, sg_device_destroy);
+	scsi_device_put(device);
 	module_put(THIS_MODULE);
 }