linux-kernel - Re: [PATCH 0/2] efivars: reading variables can generate SMIs

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Date:   Sat, 17 Feb 2018 18:12:18 +0000
From:   Andy Lutomirski <luto@...nel.org>
To:     Matthew Garrett <mjg59@...gle.com>
Cc:     Tony Luck <tony.luck@...el.com>,
        Andrew Lutomirski <luto@...nel.org>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Joe Konno <joe.konno@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jeremy Kerr <jk@...abs.org>, Andi Kleen <ak@...ux.intel.com>,
        Benjamin Drung <benjamin.drung@...fitbricks.com>,
        Peter Jones <pjones@...hat.com>
Subject: Re: [PATCH 0/2] efivars: reading variables can generate SMIs

On Fri, Feb 16, 2018 at 10:03 PM, Matthew Garrett <mjg59@...gle.com> wrote:
> On Fri, Feb 16, 2018 at 2:02 PM Luck, Tony <tony.luck@...el.com> wrote:
>
>> > If the default is 600 then it makes sense to allow a privileged service
> to
>> > selectively make certain variables world readable at runtime.
>
>> As soon as you make one variable world readable you are vulnerable to
>> a local user launching a DoS attack by reading that variable over and over
>> generating a flood of SMIs.
>
> I'm not terribly worried about untrusted users on my laptop, but I would
> prefer to run as little code as root as possible.

I think that, for the most part, systemwide configuration should not
be accessible to non-root.  Unprivileged users, in general, have no
legitimate reason to know that my default boot is Boot0000* Fedora
HD(1,GPT,ee...,0x800,0x64000)/File(\EFI\fedora\shim.efi).  Even more
so if I'm network booting.

Alternatively, we could call this a distro issue.  Distros could
easily change the permissions on /sys/firmware/efi/efivars to disallow
unprivileged access.