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Date: Mon, 07 Nov 2011 13:35:01 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
CC: Vasiliy Kulikov <segoon@...nwall.com>, Eric Paris <eparis@...isplace.org>,
kernel-hardening@...ts.openwall.com, Valdis.Kletnieks@...edu,
linux-kernel@...r.kernel.org, Alexey Dobriyan <adobriyan@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-security-module@...r.kernel.org
Subject: Re: Re: [PATCH] proc: restrict access to /proc/interrupts
On 11/07/2011 01:23 PM, Linus Torvalds wrote:
> On Mon, Nov 7, 2011 at 12:47 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> You didn't really get my point. There are global nodes which are
>> dynamic, and more importantly the *set* changes across the system life.
>> A global policy option is a lot easier to deal with for the vast
>> majority of users who don't need fine grain control.
>
> I want *one* global policy that the kernel would actually know about:
> is the user physically at the machine right now.
>
> Sadly, I don't think the kernel has any good way to figure that out
> automatically.
>
> Because quite frankly, a lot of the /proc files should be "root or
> desktop user". If you control the hardware, you should damn well be
> able to see the interrupt counts in order to do bug reports etc
> without having to 'sudo' or similar.
>
> I realize that pam & co could give us this info, or we could just add
> a new capability flag, but I think this is something where the kernel
> really could just do the RightThing(tm) automatically, and screw the
> crazy login managers, odd policies (I really don't believe that adding
> magic selinux rules actually improves security all that much, because
> it's too painful and too hard to know for any normal user).
>
> The person in front of the hardware really *is* fundamentally special.
>
> Right now all the distros do magic things with the audio device
> because they know the person in front of the machine is special. But
> all those things are ad-hoc per device, and never cover things like
> random /proc files etc.
>
I was going to say "let's just have the login manager add a group to the
desktop user's permission set" but then I realized that this would be
really bad because of setgid files.
Which exposes a real problem with chgrp and setgid files overall.
The way setgid works effectively means that any user can become a member
of any group that they have at any time been a member of, simply by
"stashing" a copy of the group as a setgid file:
cp /bin/sh my-saved-group
chgrp mygroup my-saved-group
chmod g+s my-saved-group
This is rather messy, because gids are otherwise a very nice capability
mechanism.
-hpa
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