linux-kernel - Re: [PATCH 2/2] proc: fix PAGE_SIZE limit of /proc/$PID/cmdline

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]

Message-ID: <20150527231207.GA29641@p183.telecom.by>
Date:	Thu, 28 May 2015 02:12:07 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	jarod@...hat.com, jstancek@...hat.com
Subject: Re: [PATCH 2/2] proc: fix PAGE_SIZE limit of /proc/$PID/cmdline

On Thu, May 28, 2015 at 01:48:25AM +0300, Cyrill Gorcunov wrote:
> On Thu, May 28, 2015 at 01:29:42AM +0300, Alexey Dobriyan wrote:
> > > > +
> > > > +	page = (char *)__get_free_page(GFP_TEMPORARY);
> > > > +	if (!page) {
> > > > +		rv = -ENOMEM;
> > > > +		goto out_mmput;
> > > > +	}
> > > > +
> > > > +	down_read(&mm->mmap_sem);
> > > > +	arg_start = mm->arg_start;
> > > > +	arg_end = mm->arg_end;
> > > > +	env_start = mm->env_start;
> > > > +	env_end = mm->env_end;
> > > > +	up_read(&mm->mmap_sem);
> > > 
> > > Could you please explain why this down/up is needed?
> > 
> > Code is written this way to get constistent snapshot of data.
> 
> it does not. you fetch data into local variables which is the
> same as simply read them locklessly in general (because later
> you refer to local vars).

It is snapshot w.r.t getting both pairs not snapshot w.r.t atomicity or
something (unsigned long access is atomic after all). Once down_write()
is used in the other place, it even becomes obviously correct code!

> > If you look at PR_SET_MM_* code, you'll notice down_read(&mm->mmap_sem)
> > as well which is a separate bug because you're _writing_ those fields
> > eventually in prctl_set_mm(), yuck!
> 
> yes, there members are modified under read-lock and initially
> i didn't see any problem with that except one can have inconsistent
> statistics output because another process modified these fields
> (we validate that new members are having sane values at least
>  in new interface and after your first patch). But now I think
> that down_write may be more suitable here.
> 
> 	Cyrill
--
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/