linux-kernel - Re: [PATCH 2.6.32 19/38] [PATCH 19/38] pagemap: hide physical addresses from non-privileged users

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Date:	Mon, 30 Nov 2015 16:14:21 +0100
From:	Willy Tarreau <w@....eu>
To:	Ben Hutchings <ben@...adent.org.uk>
Cc:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Mark Williamson <mwilliamson@...o-software.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 2.6.32 19/38] [PATCH 19/38] pagemap: hide physical addresses from non-privileged users

On Mon, Nov 30, 2015 at 02:55:20PM +0000, Ben Hutchings wrote:
> On Mon, 2015-11-30 at 12:30 +0100, Willy Tarreau wrote:
> > On Mon, Nov 30, 2015 at 08:01:36AM +0100, Willy Tarreau wrote:
> > > On Mon, Nov 30, 2015 at 01:54:22AM +0000, Ben Hutchings wrote:
> > > > On Sun, 2015-11-29 at 22:47 +0100, Willy Tarreau wrote:
> > > > This is wrong; see
> > > > <https://marc.info/?l=linux-api&m=143144321020852&w=2>.
> > > 
> > > Damned, and I now remember this discussion. The worst thing is that
> > > I purposely booted a machine to test the fix and was happy with it,
> > > I forgot this point :-(
> > > 
> > > > For 2.6.32 perhaps you could retain the capability check at open time
> > > > but store the result in private state for use at read time.
> > > 
> > > I'll see if it is possible to opencode security_capable() with 2.6.32's
> > > infrastructure, and how far this brings us. Or maybe we should even drop
> > > this one completely and leave pagemap readable only for superuser on
> > > 2.6.32, it doesn't seem to be that big of a deal either.
> > 
> > It was easy enough to open-code security_capable() in the end. I've
> > tested this version which works fine for me here. If that's OK for you
> > I'll emit an -rc2 with the last two patches.
> [...]
> > +	/* do not disclose physical addresses: attack vector */
> > +	pm.show_pfn = !cap_capable(current, file->f_cred, CAP_SYS_ADMIN, SECURITY_CAP_AUDIT);
> [...]
> 
> But this bypasses SELinux's additional restrictions on capabilities.

Got it, I didn't think about it.

> I think it would be better to cherry-pick this first:
> 
> commit 6037b715d6fab139742c3df8851db4c823081561
> Author: Chris Wright <chrisw@...s-sol.org>
> Date:   Wed Feb 9 22:11:51 2011 -0800
> 
>     security: add cred argument to security_capable()
> 
> and then you can pass file->f_cred to security_capable().

That makes sense indeed, the patch should fit nicely. Thanks for the
pointer.

Willy

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