linux-kernel - Re: [RFC v3 PATCH 0/2] Introduce Security Version to EFI Stub

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Message-ID: <20171207081816.jy2rw5y5iyxeqw6n@gmail.com>
Date:   Thu, 7 Dec 2017 09:18:16 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Gary Lin <glin@...e.com>
Cc:     Josh Boyer <jwboyer@...oraproject.org>, x86 <x86@...nel.org>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>, Joey Lee <jlee@...e.com>
Subject: Re: [RFC v3 PATCH 0/2] Introduce Security Version to EFI Stub


* Gary Lin <glin@...e.com> wrote:

> On Thu, Dec 07, 2017 at 07:09:27AM +0100, Ingo Molnar wrote:
> > 
> > * Gary Lin <glin@...e.com> wrote:
> > 
> > > On Wed, Dec 06, 2017 at 07:37:34PM +0100, Ingo Molnar wrote:
> > > > 
> > > > * Gary Lin <glin@...e.com> wrote:
> > > > 
> > > > > On Tue, Dec 05, 2017 at 04:14:26PM -0500, Josh Boyer wrote:
> > > > > > On Tue, Dec 5, 2017 at 5:01 AM, Gary Lin <glin@...e.com> wrote:
> > > > > > > The series of patches introduce Security Version to EFI stub.
> > > > > > >
> > > > > > > Security Version is a monotonically increasing number and designed to
> > > > > > > prevent the user from loading an insecure kernel accidentally. The
> > > > > > > bootloader maintains a list of security versions corresponding to
> > > > > > > different distributions. After fixing a critical vulnerability, the
> > > > > > > distribution kernel maintainer bumps the "version", and the bootloader
> > > > > > > updates the list automatically. When the user tries to load a kernel
> > > > > > > with a lower security version, the bootloader shows a warning prompt
> > > > > > > to notify the user the potential risk.
> > > > > > 
> > > > > > If a distribution releases a kernel with a higher security version and
> > > > > > that it automatically updated on boot, what happens if that kernel
> > > > > > contains a different bug that causes it to fail to boot or break
> > > > > > critical functionality?  At that point, the user's machine would be in
> > > > > > a state where the higher security version is enforced but the only
> > > > > > kernel that provides that is broken.  Wouldn't that make a bad
> > > > > > situation even worse by now requiring manual acceptance of the older
> > > > > > SV kernel boot physically at the machine?
> > > > > > 
> > > > > > I feel like I'm missing a detail here or something.
> > > > > > 
> > > > > If the new kernel fails to boot, then the user has to choose the kernel
> > > > > manually anyway, and there will be an option in the warning prompt to
> > > > > lower SV.
> > > > 
> > > > And what if the firmware does not support a lowering of the SV?
> > > > 
> > > The SV list is manipulated by the bootloader, and the firmware only
> > > provides the interface to the storage, i.e. non-volatile flash.
> > 
> > What about systems where the bootloader is part of the system and users only have 
> > the ability to provide kernel images, but no ability to change the boot loader?
> 
> It depends on how the bootloader works. If the system uses my
> implementation of shim loader, it surely has the ability to lower SV,
> but it requires physical access on purpose.

And that's my problem: if in practice the bootloader is 'part of the system', is 
signed and is updated like the firmware, then putting a "Security Version" into 
the kernel image and architecting a boot protocol for a monotonic method for the 
bootloader to restrict the loading of kernel images is an obviously bad idea.

Thanks,

	Ingo