[PATCH] sd: sd_open: prevent device removal during sd_open

[Christian Löhle]

sd and parent devices must not be removed as sd_open checks for events

sd_need_revalidate and sd_revalidate_disk traverse the device path
to check for event changes. If during this, e.g. the scsi host is being
removed and its resources freed, this traversal crashes.
Locking with scan_mutex for just a scsi disk open may seem blunt, but there
does not seem to be a more granular option. Also opening /dev/sdX directly
happens rarely enough that this shouldn't cause any issues.

The issue occurred on an older kernel with the following trace:
stack segment: 0000 [#1] PREEMPT SMP PTI
CPU: 1 PID: 121457 Comm: python3 Not tainted 4.14.238hyLinux #1
Hardware name: ASUS All Series/H81M-D, BIOS 0601 02/20/2014
task: ffff888213dbb700 task.stack: ffffc90008c14000
RIP: 0010:kobject_get_path+0x2a/0xe0
...
Call Trace:
kobject_uevent_env+0xe6/0x550
disk_check_events+0x101/0x160
disk_clear_events+0x75/0x100
check_disk_change+0x22/0x60
sd_open+0x70/0x170 [sd_mod]
__blkdev_get+0x3fd/0x4b0
? get_empty_filp+0x57/0x1b0
blkdev_get+0x11b/0x330
? bd_acquire+0xc0/0xc0
do_dentry_open+0x1ef/0x320
? __inode_permission+0x85/0xc0
path_openat+0x5cb/0x1500
? terminate_walk+0xeb/0x100
do_filp_open+0x9b/0x110
? __check_object_size+0xb4/0x190
? do_sys_open+0x1bd/0x250
do_sys_open+0x1bd/0x250
do_syscall_64+0x67/0x120
entry_SYSCALL_64_after_hwframe+0x41/0xa6

and this commit fixed that issue, as there has been no other such
synchronization in place since then, the issue should still be present in
recent kernels.

Signed-off-by: Christian Loehle <cloehle@xxxxxxxxxxxxxx>
---
 drivers/scsi/sd.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index 610ebba0d66e..ad4da985a473 100644
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -1436,6 +1436,16 @@ static int sd_open(struct block_device *bdev, fmode_t mode)
 	if (!scsi_block_when_processing_errors(sdev))
 		goto error_out;
 
+	/*
+	 * Checking for changes to the device must not race with the device
+	 * or its parent host being removed, so lock until sd_open returns.
+	 */
+	mutex_lock(&sdev->host->scan_mutex);
+	if (sdev->sdev_state != SDEV_RUNNING) {
+		retval = -ERESTARTSYS;
+		goto unlock_scan_error_out;
+	}
+
 	if (sd_need_revalidate(bdev, sdkp))
 		sd_revalidate_disk(bdev->bd_disk);
 
@@ -1444,7 +1454,7 @@ static int sd_open(struct block_device *bdev, fmode_t mode)
 	 */
 	retval = -ENOMEDIUM;
 	if (sdev->removable && !sdkp->media_present && !(mode & FMODE_NDELAY))
-		goto error_out;
+		goto unlock_scan_error_out;
 
 	/*
 	 * If the device has the write protect tab set, have the open fail
@@ -1452,7 +1462,7 @@ static int sd_open(struct block_device *bdev, fmode_t mode)
 	 */
 	retval = -EROFS;
 	if (sdkp->write_prot && (mode & FMODE_WRITE))
-		goto error_out;
+		goto unlock_scan_error_out;
 
 	/*
 	 * It is possible that the disk changing stuff resulted in
@@ -1462,15 +1472,19 @@ static int sd_open(struct block_device *bdev, fmode_t mode)
 	 */
 	retval = -ENXIO;
 	if (!scsi_device_online(sdev))
-		goto error_out;
+		goto unlock_scan_error_out;
 
 	if ((atomic_inc_return(&sdkp->openers) == 1) && sdev->removable) {
 		if (scsi_block_when_processing_errors(sdev))
 			scsi_set_medium_removal(sdev, SCSI_REMOVAL_PREVENT);
 	}
 
+	mutex_unlock(&sdev->host->scan_mutex);
 	return 0;
 
+unlock_scan_error_out:
+	mutex_unlock(&sdev->host->scan_mutex);
+
 error_out:
 	scsi_disk_put(sdkp);
 	return retval;	
-- 
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