On Tue, Apr 3, 2018 at 11:45 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > On Tue, Apr 3, 2018 at 9:45 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: >> On Tue, Apr 3, 2018 at 9:29 AM, Matthew Garrett <mjg59@xxxxxxxxxx> wrote: >>> On Tue, Apr 3, 2018 at 8:11 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote: >>>> Can you explain that much more clearly? I'm asking why booting via >>>> UEFI Secure Boot should enable lockdown, and I don't see what this has >>>> to do with kexec. And "someone blacklist[ing] your key in the >>>> bootloader" sounds like a political issue, not a technical issue. >>> >>> A kernel that allows users arbitrary access to ring 0 is just an >>> overfeatured bootloader. Why would you want secure boot in that case? >> >> To get a chain of trust. I can provision a system with some public >> keys, stored in UEFI authenticated variables, such that the system >> will only boot a signed image. That signed image, can, in turn, load >> a signed (or hashed or otherwise verfified) kernel and a verified >> initramfs. The initramfs can run a full system from a verified (using >> dm-verity or similar) filesystem, for example. Now it's very hard to >> persistently attack this system. Chromium OS does something very much >> like this, except that it doesn't use UEFI as far as I know. So does >> iOS, and so do some Android versions. > > Correct, Chrome OS does not use UEFI, and we still want this patch > series, as it plugs all the known "intentional" escalation paths from > uid-0 to ring-0. Happily, that means all the politics around the UEFI > and Secure Boot case can be ignored, because those issues are specific > to Secure Boot, not the lockdown series. (They are _related_, sure, > but lockdown isn't only about Secure Boot -- it's just that SB is one > of the widely deployed implementations of this kind of > trust-chain-booting-thing. Chrome OS and Android's Verified Boot do > similar things and have the same expectations about the uid-0/ring-0 > separation.) > > The goal for that bright line on Chrome OS and Android is to stop > attack persistence. We want to know that a reboot onto a new kernel > and OS image will actually result in getting the desired system state, > and that any attack on persistent system data (even for things running > with full root privileges) can't result in using kernel interfaces to > gain kernel control. This isn't expected to be _perfect_, since > nothing is. But it creates a place to work from. The idea that uid-0 > is NOT ring-0 is still relatively new, so the existing designs in the > kernel aren't well suited to building that distinction. I view this > series as a solid first step to getting there, though. > But wouldn't Chrome OS possibly want to lock down kernel memory write vectors but not read vectors? After all, debugging is useful even on Chrome OS. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
