On Fri, Aug 18, 2017 at 11:52 AM, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote: > On 4 August 2017 at 22:20, Matthew Garrett <mjg59@xxxxxxxxxx> wrote: >> + * Enable reboot attack mitigation. This requests that the firmware clear the >> + * RAM on next reboot before proceeding with boot, ensuring that any secrets >> + * are cleared. If userland has ensured that all secrets have bene removed > > s/bene/been/ Thanks. >> + set_efi_var(efi_MemoryOverWriteRequest_name, &var_guid, >> + EFI_VARIABLE_NON_VOLATILE | >> + EFI_VARIABLE_BOOTSERVICE_ACCESS | >> + EFI_VARIABLE_RUNTIME_ACCESS, sizeof(val), val); > > Shouldn't this be &val? Ooh, good catch - not sure how that got eaten. >> What's the threat model? If there's no way for userland to ask the >> kernel to drop any secrets it still holds, that seems like a problem >> in any case. If the concern is that someone may be able to clear the >> flag and then reboot in order to deliberately attempt to obtain kernel >> secrets then there's no hugely easy way around this without special >> casing the variable and preventing userland from being able to modify >> it. There's a MemoryOverwriteRequestLock spec from Microsoft that >> provides a mechanism for this (the firmware and the OS configure a >> shared secret that controls access to MemoryOverwriteRequestControl, >> so we'd keep that in the kernel and clear it on reboot), but that's >> not implemented everywhere and we'd still want to base on top of this. > > So how would that work with, e.g., the keys for your encrypted file > system? Surely, you can't expect the kernel to drop that secret when > userland asks it to, so there will always be a window where userland > has set the variable but the kernel is not ready to drop its secrets > yet. If the kernel doesn't synchronously zero the key when dm-crypt is torn down, that feels like a bug? -- 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
