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

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Sep 02, 2021 at 01:57:13PM +0000, Christian Löhle wrote:
> 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@hyperstone.com>
> ---
>  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;	
> -- 
> 2.32.0=
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> Managing Directors: Dr. Jan Peter Berns.
> Commercial register of local courts: Freiburg HRB381782
> 


<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>