linux-hardening - [PATCH 03/16] x86/boot: Set cr0 to known state in trampoline

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Message-Id: <a155c20eec6343d34b3bdf40c024ce8b35a90e02.1662459668.git.baskov@ispras.ru>
Date:   Tue,  6 Sep 2022 13:41:07 +0300
From:   Evgeniy Baskov <baskov@...ras.ru>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Evgeniy Baskov <baskov@...ras.ru>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        lvc-project@...uxtesting.org, x86@...nel.org,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: [PATCH 03/16] x86/boot: Set cr0 to known state in trampoline

Ensure WP bit to be set to prevent boot code from writing to
non-writable memory pages.

Signed-off-by: Evgeniy Baskov <baskov@...ras.ru>
---
 arch/x86/boot/compressed/head_64.S | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index d33f060900d2..5273367283b7 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -619,9 +619,8 @@ SYM_CODE_START(trampoline_32bit_src)
 	/* Set up new stack */
 	leal	TRAMPOLINE_32BIT_STACK_END(%ecx), %esp
 
-	/* Disable paging */
-	movl	%cr0, %eax
-	btrl	$X86_CR0_PG_BIT, %eax
+	/* Disable paging and setup CR0 */
+	movl	$(CR0_STATE & ~X86_CR0_PG), %eax
 	movl	%eax, %cr0
 
 	/* Check what paging mode we want to be in after the trampoline */
-- 
2.35.1