linux-kernel - Re: [PATCH] ses: Fix racy cleanup of /sys in remove

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Message-ID: <20160615202416.GA52103@calvinowens-mba.dhcp.thefacebook.com>
Date:	Wed, 15 Jun 2016 16:24:16 -0400
From:	Calvin Owens <calvinowens@...com>
To:	"James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>
CC:	<linux-scsi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<calvinowens@...com>
Subject: Re: [PATCH] ses: Fix racy cleanup of /sys in remove_dev()

On Thursday 06/02 at 15:50 -0700, Calvin Owens wrote:
> 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?

Any thoughts? Squinting at this more it still seems racy, but a narrow race
is surely better than just blatantly freeing everything while the file is
still exposed in /sys? Is there a better way you'd prefer I accomplish this?

(I have boxes that OOPS all the time from monitoring code reading the /sys
files, with this patch I haven't seen a single one.)

Thanks,
Calvin

> >    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@...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,
> > 
>