linux-hardening - [PATCH v5 03/27] x86/boot: Set cr0 to known state in trampoline

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Message-Id: <63368ff665956a64f07aee9bc863b70c86b8b0c8.1678785672.git.baskov@ispras.ru>
Date:   Tue, 14 Mar 2023 13:13:30 +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>,
        Peter Jones <pjones@...hat.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        "Limonciello, Mario" <mario.limonciello@....com>,
        joeyli <jlee@...e.com>, lvc-project@...uxtesting.org,
        x86@...nel.org, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH v5 03/27] 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.

Tested-by: Mario Limonciello <mario.limonciello@....com>
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 03c4328a88cb..01fa42d31648 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -660,9 +660,8 @@ SYM_CODE_START(trampoline_32bit_src)
 	pushl	$__KERNEL_CS
 	pushl	%eax
 
-	/* Enable paging again. */
-	movl	%cr0, %eax
-	btsl	$X86_CR0_PG_BIT, %eax
+	/* Enable paging and set CR0 to known state (this also sets WP flag) */
+	movl	$CR0_STATE, %eax
 	movl	%eax, %cr0
 
 	lret
-- 
2.39.2