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

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Date:   Fri, 24 Apr 2020 00:06:07 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Arvind Sankar" <nivedita@...m.mit.edu>,
        "Ingo Molnar" <mingo@...nel.org>,
        "Ard Biesheuvel" <ardb@...nel.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "Hans de Goede" <hdegoede@...hat.com>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        linux-efi@...r.kernel.org
Subject: [PATCH 3.16 140/245] x86/efistub: Disable paging at mixed mode entry

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

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

From: Ard Biesheuvel <ardb@...nel.org>

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@...nel.org>
Cc: Arvind Sankar <nivedita@...m.mit.edu>
Cc: Hans de Goede <hdegoede@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-efi@...r.kernel.org
Link: https://lkml.kernel.org/r/20191224132909.102540-4-ardb@kernel.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 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