[PATCH v3] scsi: sg: Avoid race in error handling & drop bogus warn

[Alexander Wetzel <Alexander@wetzel-home.de>]

commit 27f58c04a8f4 ("scsi: sg: Avoid sg device teardown race")
introduced an incorrect WARN_ON_ONCE() and missed a sequence where
sg_device_destroy() was used after 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.

Drop the incorrect WARN_ON_ONCE() - allowing more than one concurrent
access to the sg device - and make sure sg_device_destroy() is not used
after scsi_device_put() in the error handling.

Link: https://lore.kernel.org/all/5375B275-D137-4D5F-BE25-6AF8ACAE41EF@linux.ibm.com
Fixes: 27f58c04a8f4 ("scsi: sg: Avoid sg device teardown race")
Cc: stable@vger.kernel.org
Signed-off-by: Alexander Wetzel <Alexander@wetzel-home.de>
---

Changes compared to V1: fixed commit message
Changes compared to V2: Fix use-after free
---
 drivers/scsi/sg.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index 386981c6976a..baf870a03ecf 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -285,6 +285,7 @@ sg_open(struct inode *inode, struct file *filp)
 	int dev = iminor(inode);
 	int flags = filp->f_flags;
 	struct request_queue *q;
+	struct scsi_device *device;
 	Sg_device *sdp;
 	Sg_fd *sfp;
 	int retval;
@@ -301,11 +302,12 @@ sg_open(struct inode *inode, struct file *filp)
 
 	/* This driver's module count bumped by fops_get in <linux/fs.h> */
 	/* Prevent the device driver from vanishing while we sleep */
-	retval = scsi_device_get(sdp->device);
+	device = sdp->device;
+	retval = scsi_device_get(device);
 	if (retval)
 		goto sg_put;
 
-	retval = scsi_autopm_get_device(sdp->device);
+	retval = scsi_autopm_get_device(device);
 	if (retval)
 		goto sdp_put;
 
@@ -313,7 +315,7 @@ sg_open(struct inode *inode, struct file *filp)
 	 * check if O_NONBLOCK. Permits SCSI commands to be issued
 	 * during error recovery. Tread carefully. */
 	if (!((flags & O_NONBLOCK) ||
-	      scsi_block_when_processing_errors(sdp->device))) {
+	      scsi_block_when_processing_errors(device))) {
 		retval = -ENXIO;
 		/* we are in error recovery for this device */
 		goto error_out;
@@ -344,7 +346,7 @@ sg_open(struct inode *inode, struct file *filp)
 
 	if (sdp->open_cnt < 1) {  /* no existing opens */
 		sdp->sgdebug = 0;
-		q = sdp->device->request_queue;
+		q = device->request_queue;
 		sdp->sg_tablesize = queue_max_segments(q);
 	}
 	sfp = sg_add_sfp(sdp);
@@ -370,10 +372,11 @@ sg_open(struct inode *inode, struct file *filp)
 error_mutex_locked:
 	mutex_unlock(&sdp->open_rel_lock);
 error_out:
-	scsi_autopm_put_device(sdp->device);
+	scsi_autopm_put_device(device);
 sdp_put:
-	scsi_device_put(sdp->device);
-	goto sg_put;
+	kref_put(&sdp->d_ref, sg_device_destroy);
+	scsi_device_put(device);
+	return retval;
 }
 
 /* Release resources associated with a successful sg_open()
@@ -2233,7 +2236,6 @@ sg_remove_sfp_usercontext(struct work_struct *work)
 			"sg_remove_sfp: sfp=0x%p\n", sfp));
 	kfree(sfp);
 
-	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);
-- 
2.44.0