Re: [PATCH] ses: Fix racy cleanup of /sys in remove_dev()

[Calvin Owens <calvinowens@fb.com>]

On 05/13/2016 01:28 PM, Calvin Owens wrote:
> Currently we free the resources backing the enclosure device before we
> call device_unregister(). This is racy: during rmmod of low-level SCSI
> drivers that hook into enclosure, we end up with a small window of time
> during which writing to /sys can OOPS. Example trace with mpt3sas:

Ping?

>    general protection fault: 0000 [#1] SMP KASAN
>    Modules linked in: mpt3sas(-) <...>
>    RIP: [<ffffffffa0388a98>] ses_get_page2_descriptor.isra.6+0x38/0x220 [ses]
>    Call Trace:
>     [<ffffffffa0389d14>] ses_set_fault+0xf4/0x400 [ses]
>     [<ffffffffa0361069>] set_component_fault+0xa9/0xf0 [enclosure]
>     [<ffffffff8205bffc>] dev_attr_store+0x3c/0x70
>     [<ffffffff81677df5>] sysfs_kf_write+0x115/0x180
>     [<ffffffff81675725>] kernfs_fop_write+0x275/0x3a0
>     [<ffffffff8151f810>] __vfs_write+0xe0/0x3e0
>     [<ffffffff8152281f>] vfs_write+0x13f/0x4a0
>     [<ffffffff81526731>] SyS_write+0x111/0x230
>     [<ffffffff828b401b>] entry_SYSCALL_64_fastpath+0x13/0x94
>
> Fortunately the solution is extremely simple: call device_unregister()
> before we free the resources, and the race no longer exists. The driver
> core holds a reference over ->remove_dev(), so AFAICT this is safe.
>
> Signed-off-by: Calvin Owens <calvinowens@fb.com>
> ---
>   drivers/scsi/ses.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/scsi/ses.c b/drivers/scsi/ses.c
> index 53ef1cb..0e8601a 100644
> --- a/drivers/scsi/ses.c
> +++ b/drivers/scsi/ses.c
> @@ -778,6 +778,8 @@ static void ses_intf_remove_enclosure(struct scsi_device *sdev)
>   	if (!edev)
>   		return;
>
> +	enclosure_unregister(edev);
> +
>   	ses_dev = edev->scratch;
>   	edev->scratch = NULL;
>
> @@ -789,7 +791,6 @@ static void ses_intf_remove_enclosure(struct scsi_device *sdev)
>   	kfree(edev->component[0].scratch);
>
>   	put_device(&edev->edev);
> -	enclosure_unregister(edev);
>   }
>
>   static void ses_intf_remove(struct device *cdev,
>