On Thu, 12 Sept 2024 at 12:23, Usama Arif <usamaarif642@xxxxxxxxx> wrote: > > > > On 11/09/2024 12:51, Ard Biesheuvel wrote: > > On Wed, 11 Sept 2024 at 12:41, Usama Arif <usamaarif642@xxxxxxxxx> wrote: > >> > >> Looking at the TPM spec [1] > >> > >> If the ACPI TPM2 table contains the address and size of the Platform > >> Firmware TCG log, firmware “pins” the memory associated with the > >> Platform FirmwareTCG log, and reports this memory as “Reserved” memory > >> via the INT 15h/E820 interface. > >> > >> It looks like the firmware should pass this as reserved in e820 memory > >> map. However, it doesn't seem to. The firmware being tested on is: > >> dmidecode -s bios-version > >> edk2-20240214-2.el9 > >> > >> When this area is not reserved, it comes up as usable in > >> /sys/firmware/memmap. This means that kexec, which uses that memmap > >> to find usable memory regions, can select the region where efi.tpm_log > >> is and overwrite it and relocate_kernel. > >> > >> Having a fix in firmware can be difficult to get through. As a secondary > >> fix, this patch marks that region as reserved in e820_table_firmware if it > >> is currently E820_TYPE_RAM so that kexec doesn't use it for kernel segments. > >> > >> [1] https://trustedcomputinggroup.org/wp-content/uploads/PC-ClientPlatform_Profile_for_TPM_2p0_Systems_v49_161114_public-review.pdf > >> > >> Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx> > > > > I would expect the EFI memory map to E820 conversion implemented in > > the EFI stub to take care of this. > > > > So I have been making a prototype with EFI stub, and the unfinished version is looking like a > horrible hack. > > The only way to do this in libstub is to pass log_tbl all the way from efi_retrieve_tcg2_eventlog > to efi_stub_entry and from there to setup_e820. > While going through the efi memory map and converting it to e820 table in setup_e820, you have to check > if log_tbl falls in any of the ranges and if the range is E820_TYPE_RAM. If that condition is satisfied, > then you have to split that range into 3. i.e. the E820_TYPE_RAM range before tpm_log, the tpm_log > E820_TYPE_RESERVED range, and the E820_TYPE_RAM range after. There are no helper functions, so this > splitting involves playing with a lot of pointers, and it looks quite ugly. I believe doing this > way is more likely to introduce bugs. > > If we are having to compensate for an EFI bug, would it make sense to do it in the way done > in RFC and do it in kernel rather than libstub? It is simple and very likely to be bug free. > I don't see how this could be an EFI bug, given that it does not deal with E820 tables at all.
