From: Sai Praneeth <sai.praneeth.prakhya@xxxxxxxxx> There may exist some buggy UEFI firmware implementations that access efi memory regions other than EFI_RUNTIME_SERVICES_<CODE/DATA> even after the kernel has assumed control of the platform. This violates UEFI specification. Hence, provide a debug config option which when enabled detects and recovers from page faults caused by buggy firmware. The above said illegal accesses trigger page fault in ring 0 because firmware executes at ring 0 and if unhandled it hangs the kernel. Provide an efi specific page fault handler to: 1. Avoid panics/hangs caused by buggy firmware. 2. Shout loud that the firmware is buggy and hence is not a kernel bug. Upon detetcing that the illegally accessed region is any region other than EFI_RUNTIME_SERVICES_<CODE/DATA>, the efi page fault handler will check if the access is by efi_reset_system(). 1. If so, then the efi page fault handler will reboot the machine through BIOS and not through efi_reset_system(). 2. If not, then the efi page fault handler will freeze efi_rts_wq and schedules a new process. This issue was reported by Al Stone when he saw that reboot via EFI hangs the machine. Upon debugging, I found that it's efi_reset_system() that's touching memory regions which it shouldn't. To reproduce the same behavior, I have hacked OVMF and made efi_reset_system() buggy. Along with efi_reset_system(), I have also modified get_next_high_mono_count() and set_virtual_address_map(). They illegally access both boot time and other efi regions. Testing the patch set: ---------------------- 1. Download buggy firmware from here [1]. 2. Run a qemu instance with this buggy BIOS and boot mainline kernel. Add reboot=efi to the kernel command line arguments and after the kernel is up and running, type "reboot". The kernel should hang while rebooting. 3. With the same setup, boot kernel after applying patches and the reboot should work fine. Also please notice warning/error messages printed by kernel. Changes from RFC to V1: ----------------------- 1. Drop "long jump" technique of dealing with illegal access and instead use scheduling away from efi_rts_wq. Changes from V1 to V2: ---------------------- 1. Shortened config name to CONFIG_EFI_WARN_ON_ILLEGAL_ACCESS from CONFIG_EFI_WARN_ON_ILLEGAL_ACCESSES. 2. Made the config option available only to expert users. 3. efi_free_boot_services() should be called only when CONFIG_EFI_WARN_ON_ILLEGAL_ACCESS is not enabled. Previously, this was part of init/main.c file. As it is an architecture agnostic code, moved the change to arch/x86/platform/efi/quirks.c file. Changes from V2 to V3: ---------------------- 1. Drop treating illegal access to EFI_BOOT_SERVICES_<CODE/DATA> regions separatley from illegal accesses to other regions like EFI_CONVENTIONAL_MEMORY or EFI_LOADER_<CODE/DATA>. In previous versions, illegal access to EFI_BOOT_SERVICES_<CODE/DATA> regions were handled by mapping requested region to efi_pgd but from V3 they are handled similar to illegal access to other regions i.e by freezing efi_rts_wq and scheduling new process. 2. Change __efi_init_fixup attribute to __efi_init. Note: ----- Patch set based on "next" branch in efi tree. [1] https://drive.google.com/drive/folders/1VozKTms92ifyVHAT0ZDQe55ZYL1UE5wt Sai Praneeth (5): efi: Make efi_rts_work accessible to efi page fault handler efi: Introduce __efi_init attribute x86/efi: Permanently save the EFI_MEMORY_MAP passed by the firmware x86/efi: Add efi page fault handler to recover from the page faults caused by firmware x86/efi: Introduce EFI_WARN_ON_ILLEGAL_ACCESS arch/x86/Kconfig | 17 +++ arch/x86/include/asm/efi.h | 11 ++ arch/x86/mm/fault.c | 9 ++ arch/x86/platform/efi/efi.c | 2 + arch/x86/platform/efi/quirks.c | 188 ++++++++++++++++++++++++++++++++ drivers/firmware/efi/efi.c | 4 +- drivers/firmware/efi/runtime-wrappers.c | 60 +++------- include/linux/efi.h | 51 ++++++++- 8 files changed, 295 insertions(+), 47 deletions(-) Suggested-by: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> Based-on-code-from: Ricardo Neri <ricardo.neri@xxxxxxxxx> Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@xxxxxxxxx> Cc: Al Stone <astone@xxxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Bhupesh Sharma <bhsharma@xxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> -- 2.7.4
