linux-hardening - Re: Fwd: Waking up from resume locks up on sr device

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Date:   Thu, 15 Jun 2023 13:57:37 +0900
From:   Damien Le Moal <dlemoal@...nel.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Hannes Reinecke <hare@...e.de>,
        Joe Breuer <linux-kernel@...reuer.net>,
        Bart Van Assche <bvanassche@....org>,
        Bagas Sanjaya <bagasdotme@...il.com>,
        Pavel Machek <pavel@....cz>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <len.brown@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        Tony Luck <tony.luck@...el.com>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        Thorsten Leemhuis <linux@...mhuis.info>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Phillip Potter <phil@...lpotter.co.uk>,
        Linux Power Management <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Hardening <linux-hardening@...r.kernel.org>,
        Linux Regressions <regressions@...ts.linux.dev>,
        Linux SCSI <linux-scsi@...r.kernel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Hannes Reinecke <hare@...e.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Martin Kepplinger <martin.kepplinger@...i.sm>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>
Subject: Re: Fwd: Waking up from resume locks up on sr device

On 6/15/23 13:40, Christoph Hellwig wrote:
> On Thu, Jun 15, 2023 at 09:10:28AM +0900, Damien Le Moal wrote:
>>> Here's what commit e27829dc92e5 ("scsi: serialize ->rescan against 
>>> ->remove", written by Christoph Hellwig) says:
>>>
>>>     Lock the device embedded in the scsi_device to protect against
>>>     concurrent calls to ->remove.
>>>
>>> That's the commit which added the device_lock() call.
>>
>> Thanks for the information.
>>
>> +Christoph
>>
>> Why is adding the device_lock() needed ? We could just do a
>> scsi_device_get()+scsi_device_put() to serialize against remove. No ?
> 
> No.  scsi_device_get just increments a reference count, and thus
> prevents ->release from beeing called.  ->remove is not in any way
> affected by the refcount.

What ->remove cb are you talking about ? The gendev one ?
I am trying to understand why the use of device_lock() helps in any way given
that this is not used by any other functions in scsi. And given that
scsi_rescan_device() should always be called with a ref on the scsi device (and
so on the gendev as well) held, why would this function be racy with device remove ?

Note that I did find a couple of places where scsi_rescan_device() seems to not
be called with a reference to the scsi dev held, e.g.  store_rescan_field() and
store_state_field().

-- 
Damien Le Moal
Western Digital Research