[PATCH RFC 1/5] efi/x86: Disable paging when booting via efistub
From: Baskov Evgeniy <hidden>
Date: 2021-11-10 10:57:02
Also in:
linux-efi, lkml
Subsystem:
the rest, x86 architecture (32-bit and 64-bit) · Maintainers:
Linus Torvalds, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen
Some UEFI implementations protect lower 1M memory regions and memory regions allocated by libstub from being executable, which prevents Linux kernel from booting. Disable paging after returning from efi_main() before jumping to potentially relocated code to prevent page fault from happening. Signed-off-by: Baskov Evgeniy <redacted> --- arch/x86/boot/compressed/head_32.S | 12 ++++++++++++ 1 file changed, 12 insertions(+)
diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index 659fad53ca82..c66fccaa90a2 100644
--- a/arch/x86/boot/compressed/head_32.S
+++ b/arch/x86/boot/compressed/head_32.S@@ -156,6 +156,18 @@ SYM_FUNC_START_ALIAS(efi_stub_entry) add $0x4, %esp movl 8(%esp), %esi /* save boot_params pointer */ call efi_main + +#ifdef CONFIG_EFI_STRICT_PGTABLE + /* + * Disable paging before jumping to relocated address to prevent + * page faulting on EFI firmware versions that enforces restricted + * permissions on identity page tables + */ + movl %cr0, %ecx + btrl $31, %ecx + movl %ecx, %cr0 +#endif + /* efi_main returns the possibly relocated address of startup_32 */ jmp *%eax SYM_FUNC_END(efi32_stub_entry)
--
2.33.1