linux-kernel - Re: [PATCH 2/2] block: fix accounting bug on cross partition merges

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 5 Jan 2011 08:00:34 -0800
From:	Greg KH <greg@...ah.com>
To:	Jerome Marchand <jmarchan@...hat.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>, Jens Axboe <jaxboe@...ionio.com>,
	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] block: fix accounting bug on cross partition merges

On Wed, Jan 05, 2011 at 02:51:28PM +0100, Jerome Marchand wrote:
> On 01/04/2011 10:00 PM, Greg KH wrote:
> > On Tue, Jan 04, 2011 at 04:55:13PM +0100, Jerome Marchand wrote:
> >> Also add a refcount to struct hd_struct to keep the partition in
> >> memory as long as users exist. We use kref_test_and_get() to ensure
> >> we don't add a reference to a partition which is going away.
> > 
> > No, don't do this, use a kref correctly and no such function should be
> > needed.
> > 
> >> +	} else {
> >> +		part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
> > 
> > That is the function that should properly increment the reference count
> > on the object.
> 
> Agreed.
> 
> >  If the object is "being removed", then it will return
> > NULL and you need to check that.  Do that and you do not need to add:
> 
> The object is actually removed in a rcu callback function. We could
> certainly add a flag to hd_struct, set by the release function, to
> indicate disk_map_sector_rcu() that the partition is being removed, but
> why not use the refcount instead?

Because you have to properly serialize the grabbing of a kref if you
don't have a valid pointer in the first place, otherwise it will not
work properly at all.  Your new function still does not properly handle
the race condition of dropping the last reference and then having the
kref be cleaned up.  You are giving false hope to the user of the api
that what they are doing is correct.

thanks,

greg k-h
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/