[tip: efi/urgent] x86/efistub: Disable paging at mixed mode entry

[tip-bot2 for Ard Biesheuvel]

The following commit has been merged into the efi/urgent branch of tip:

Commit-ID:     4911ee401b7ceff8f38e0ac597cbf503d71e690c
Gitweb:        https://git.kernel.org/tip/4911ee401b7ceff8f38e0ac597cbf503d71e690c
Author:        Ard Biesheuvel <ardb@xxxxxxxxxx>
AuthorDate:    Tue, 24 Dec 2019 14:29:09 +01:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Wed, 25 Dec 2019 10:46:07 +01:00

x86/efistub: Disable paging at mixed mode entry

The EFI mixed mode entry code goes through the ordinary startup_32()
routine before jumping into the kernel's EFI boot code in 64-bit
mode. The 32-bit startup code must be entered with paging disabled,
but this is not documented as a requirement for the EFI handover
protocol, and so we should disable paging explicitly when entering
the kernel from 32-bit EFI firmware.

Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Cc: Arvind Sankar <nivedita@xxxxxxxxxxxx>
Cc: Hans de Goede <hdegoede@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-efi@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20191224132909.102540-4-ardb@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/boot/compressed/head_64.S | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 58a512e..ee60b81 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -244,6 +244,11 @@ SYM_FUNC_START(efi32_stub_entry)
 	leal	efi32_config(%ebp), %eax
 	movl	%eax, efi_config(%ebp)
 
+	/* Disable paging */
+	movl	%cr0, %eax
+	btrl	$X86_CR0_PG_BIT, %eax
+	movl	%eax, %cr0
+
 	jmp	startup_32
 SYM_FUNC_END(efi32_stub_entry)
 #endif