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? If the bootloader loads a kernel with lower SV, by default, the warning prompt lasts for 30 seconds, and the system continues booting if the user doesn't intervene. For the paranoid user, a variable can be set to make the bootloader wait forever. Is it acceptable? Thanks, Gary Lin -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
