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

[Gary Lin]

On Thu, Dec 07, 2017 at 11:35:52AM +0100, Ingo Molnar wrote:
> 
> 
> * Gary Lin <glin@xxxxxxxx> wrote:
> 
> > On Thu, Dec 07, 2017 at 09:18:16AM +0100, Ingo Molnar wrote:
> > > 
> > > * Gary Lin <glin@xxxxxxxx> wrote:
> > > 
> > > > On Thu, Dec 07, 2017 at 07:09:27AM +0100, Ingo Molnar wrote:
> > > > > 
> > > > > * Gary Lin <glin@xxxxxxxx> wrote:
> > > > > 
> > > > > > On Wed, Dec 06, 2017 at 07:37:34PM +0100, Ingo Molnar wrote:
> > > > > > > 
> > > > > > > * Gary Lin <glin@xxxxxxxx> 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@xxxxxxxx> 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.
> > > 
> > Even though the bootloader doesn't actually block the booting?
> 
> We don't know that for sure, in that scenario *how* the bootloader interprets the 
> SV is not under the user's control...
> 
OK, it seems the implementation in shim brings up some concern. I'll
discuss with my colleagues for other possible solutions.

Cheers,

Gary Lin