CC'ing x86 folks because this patch touches x86/mm which I am no expert of. [Copied from Patch 1] Ideally, after kernel assumes control of the platform firmware shouldn't access EFI Boot Services Code/Data regions. But, it's noticed that this is not so true in many x86 platforms. Hence, during boot, kernel reserves efi boot services code/data regions [1] and maps [2] them to efi_pgd so that call to set_virtual_address_map() doesn't fail. After returning from set_virtual_address_map(), kernel frees the reserved regions [3] but they still remain mapped. This means that any code that's running in efi_pgd address space (e.g: any efi runtime service) would still be able to access efi boot services code/data regions but the contents of these regions would have long been over written by someone else as they are freed by efi_free_boot_services(). So, it's important to unmap these regions. After unmapping boot services code/data regions, any illegal access by buggy firmware to these regions would result in page fault which will be handled by efi specific fault handler. [1] Please see efi_reserve_boot_services() [2] Please see efi_map_region() -> __map_region() [3] Please see efi_free_boot_services() Testing the patch set: ---------------------- 1. Download buggy firmware (which accesses boot regions even after kernel has booted) from here [1]. 2. Without the patch set, you shouldn't see any kernel warning/error messages (i.e. kernel allows accesses to efi boot services code/data regions even after call to set_virtual_address_map()). 3. With the patch set, you should see a kernel warning about buggy firmware, efi_rts_wq beeing freezed and disabling runtime services forever. Please note that this patch will change kernel's existing behavior for some efi runtime services but I think it's OK because kernel should have never allowed those accesses in the first place. Also please note that this patch set needs lot of real time trashing as I just tested it out with OVMF. Note: ----- Patch set based on "next" branch in efi tree. [1] https://drive.google.com/drive/folders/1VozKTms92ifyVHAT0ZDQe55ZYL1UE5wt Sai Praneeth (2): x86/efi: Unmap efi boot services code/data regions from efi_pgd x86/efi: Move efi_<reserve/free>_boot_services() to arch/x86 arch/x86/include/asm/efi.h | 2 ++ arch/x86/include/asm/pgtable_types.h | 2 ++ arch/x86/mm/pageattr.c | 21 +++++++++++++++++++++ arch/x86/platform/efi/efi.c | 2 ++ arch/x86/platform/efi/quirks.c | 26 ++++++++++++++++++++++++++ include/linux/efi.h | 3 --- init/main.c | 4 ---- 7 files changed, 53 insertions(+), 7 deletions(-) Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@xxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Dave Hansen <dave.hansen@xxxxxxxxx> Cc: Bhupesh Sharma <bhsharma@xxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> -- 2.7.4
