On Thu, Dec 07, 2017 at 11:35:52AM +0100, Ingo Molnar wrote: > > > * Gary Lin <g...@suse.com> wrote: > > > On Thu, Dec 07, 2017 at 09:18:16AM +0100, Ingo Molnar wrote: > > > > > > * Gary Lin <g...@suse.com> wrote: > > > > > > > On Thu, Dec 07, 2017 at 07:09:27AM +0100, Ingo Molnar wrote: > > > > > > > > > > * Gary Lin <g...@suse.com> wrote: > > > > > > > > > > > On Wed, Dec 06, 2017 at 07:37:34PM +0100, Ingo Molnar wrote: > > > > > > > > > > > > > > * Gary Lin <g...@suse.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 <g...@suse.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. > > > > > 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
