[PATCH 3.16 140/245] x86/efistub: Disable paging at mixed mode entry

From: Ben Hutchings
Date: Thu Apr 23 2020 - 19:07:38 EST


3.16.83-rc1 review patch. If anyone has any objections, please let me know.

------------------

From: Ard Biesheuvel <ardb@xxxxxxxxxx>

commit 4911ee401b7ceff8f38e0ac597cbf503d71e690c upstream.

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: 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>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
arch/x86/boot/compressed/head_64.S | 5 +++++
1 file changed, 5 insertions(+)

--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -216,6 +216,11 @@ ENTRY(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
ENDPROC(efi32_stub_entry)
#endif