[PATCH RFC 1/5] efi/x86: Disable paging when booting via efistub

[Baskov Evgeniy <baskov@ispras.ru>]

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 <baskov@ispras.ru>
---
 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