Greetings Matthew, First thank you to you and James for taking time out of your busy schedules to sit down with us and work through all of this. Hey Ard, On 3/30/22 03:02, Ard Biesheuvel wrote:>> 1) From an EFI maintainer perspective, is making the contract between >> the boot stub and the kernel explicit viable? >> > > No. The direction of travel has been to define EFI boot only in terms > of the handover from the loader to the stub. What happens next is up > to the architecture, and is deliberately not specified, because it is > considered to be internal Linux ABI. We've deviated from this once for > Xen on ARM, but this means we have already painted ourselves into a > corner when it comes the way we use DT internally at the handover > point between stub and kernel proper, and I am not eager to repeat > that. Locking down the stub-to-kernel protocol for all architectures > is not the way to go. To help provide some visual context, for EFI today there is, bzImage [EFI boot manager] -> [[efi-stub] -> [setup kernel] -> [main kernel]] Where the efi-stub is responsible for interacting with firmware to configure the system, store that configuration for the setup kernel and the main kernel, and then call EBS before entering the setup kernel. For Secure Launch the flow (on Intel) is, CPU instruction bzImage [preamble] -> [ACM] -> [[sl-stub] -> [setup kernel] -> [main kernel]] In order to make the CPU instruction call to enter the ACM the system must be in a very specific quiescent state. This includes but not exhaustively, * EBS must have been called * TPM should have all localities closed * IOMMU PMRs must be programmed appropriately * TXT heap space allocated * TXT heap space populated with config structures * All APs must be in a specific idle state * Execution is on the BSP Carrying all this out is what is considered the DRTM preamble. This is the wrinkle because the setup kernel and main kernel are both predicated on the efi-stub and the efi-stub is predicated on running before EBS. So how can this wrinkle be addressed? The TrenchBoot project proposed that the information collected by the efi-stub be formally documented for two reasons, 1. to allow the sl-stub to be aware of what and where all external data is being injected into the kernel so any data that may be security critical could be measured, and 2. it would allow third parties, e.g. GRUB, could correctly configure the system, pass all EFI related information correctly to the setup kernel and the main kernel before executing the preamble. Where the former is more of a concern than enabling the latter. Relating to what information is security critical, this can be a bit subjective. For example Dave Weston has a twitter thread[1][2][3] over what state Azure Attestation can validate for a DRTM Windows system. This reflects what Microsoft believes it's customers will want to validate about a Windows system before accessing services and data stored in Azure. For Linux Secure Launch measuring everything would obviously provide the finest grain of assertions regarding how the kernel was configured to run but that comes at the expense that is incurred for taking the measurements. To date the selection of measurements have been an attempt of balancing the most meaningful measurements with how many measurements should be taken. For instance it would be possible to hash the SRTM (firmware) TPM event log to provide an assertion of what its contents were at dynamic launch. Is that a meaningful measurement? Not necessarily since there are other means to validate the log but there is likely someone who is super cautious and would like to see the extra layer of validation. >> 2) If so, is it desirable? >> >> 3) If either (1) or (2) is "no", is it reasonable to ensure that all >> potentially security-critical state or configuration changes to the >> boot stub are reviewed by DRTM people to verify whether the secure >> launch code needs to be updated to match? > > Wouldn't it be better for the secure launch kernel to boot the EFI > entrypoint directly? As it happens, I just completed a PoC last week > for a minimal implementation of EFI (in Rust) that only carries the > pieces that the EFI stub needs to boot Linux. It is currently just a > proof of concept that only works on QEMU/arm64, but it should not be > too hard to adapt it for x86 and for booting a kernel that has already > been loaded to memory. IIUC, as you can see above, entering the efi-stub from the dynamic launch will fail because EBS will have been called. v/r, dps [1] https://twitter.com/dwizzzlemsft/status/1508517758635446273 [2] https://twitter.com/dwizzzlemsft/status/1508602527482146816 [3] https://twitter.com/dwizzzlemsft/status/1508852935261794312