linux-kernel - Re: [PATCH] power: disable hibernation if module loading is disabled

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Date:	Sat, 5 Mar 2011 00:46:32 +0300
From:	Vasiliy Kulikov <segoon@...nwall.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	linux-kernel@...r.kernel.org, security@...nel.org,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	linux-pm@...ts.linux-foundation.org
Subject: Re: [PATCH] power: disable hibernation if module loading is
 disabled

On Fri, Mar 04, 2011 at 22:21 +0100, Rafael J. Wysocki wrote:
> On Friday, March 04, 2011, Vasiliy Kulikov wrote:
> > If modules_disabled is set to 1, then nobody, even full root may not write
> > to the kernel, right?  So, if something permits to indirectly pass
> > modules_disabled restriction, this is a bug.  Otherwise,
> > modules_disabled is confusing as it gives false sense of security.
> > 
> > -OR-
> > 
> > modules_disabled's documentation should be changed to note that it
> > doesn't prevent rootkit uploading, but only forbids modprob'ing modules
> > via the "official" init_module(2) gate, disallowing e.g. module autoloading.
> 
> Why not to change that documentation, then?

Because it's better to fix something (if it is possible, of course) than
simply documenting the bug.

> Also, please note that in order to "write" into memory using the hibernation
> interface you need to have write access to swap,

No, you may just "write the kernel" via write() /dev/snapshot, this is
the way uswsusp works.  I didn't check whether it really needs
temporary file to change the kernel memory or it may be done entirely
without disk iteraction.  This is irrelevant to modules_disabled policy
violation, though.

> which you can use to corrupt
> memory regardless of the modules_disabled setting AFAICS.

Please correct me if I'm wrong, but kernel memory is not swappable at
all and only userspace memory is written to the swap.  Root with
CAP_SYS_ADMIN already may do everything with all processes, so this is
not a threat.

If one may change kernel memory via swap then it is another problem with
modules_disabled.

Thanks,

-- 
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments
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