linux-kernel - [PATCH v3 19/19] x86/startup_64: Don't bother setting up GS before the kernel is mapped

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Message-ID: <20240129180502.4069817-40-ardb+git@google.com>
Date: Mon, 29 Jan 2024 19:05:22 +0100
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-kernel@...r.kernel.org
Cc: Ard Biesheuvel <ardb@...nel.org>, Kevin Loughlin <kevinloughlin@...gle.com>, 
	Tom Lendacky <thomas.lendacky@....com>, Dionna Glaze <dionnaglaze@...gle.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>, 
	Arnd Bergmann <arnd@...db.de>, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
	Kees Cook <keescook@...omium.org>, Brian Gerst <brgerst@...il.com>, linux-arch@...r.kernel.org, 
	llvm@...ts.linux.dev
Subject: [PATCH v3 19/19] x86/startup_64: Don't bother setting up GS before
 the kernel is mapped

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

The code that executes from the early 1:1 mapping of the kernel should
set up the kernel page tables and nothing else. C code that is linked
into this code path is severely restricted in what it can do, and is
therefore required to remain uninstrumented. It also built with -fPIC
and without stack protector support.

This makes it unnecessary to enable per-CPU variable access this early,
and for the boot CPU, the initialization that occurs in the common CPU
startup path is sufficient.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/kernel/head_64.S | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index e671caafd932..ae211cb62a1e 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -76,13 +76,6 @@ SYM_CODE_START_LOCAL(primary_startup_64)
 	/* Set up the stack for verify_cpu() */
 	leaq	(__end_init_task - PTREGS_SIZE)(%rip), %rsp
 
-	/* Setup GSBASE to allow stack canary access for C code */
-	movl	$MSR_GS_BASE, %ecx
-	leaq	INIT_PER_CPU_VAR(fixed_percpu_data)(%rip), %rdx
-	movl	%edx, %eax
-	shrq	$32,  %rdx
-	wrmsr
-
 	call	startup_64_setup_env
 
 	/* Now switch to __KERNEL_CS so IRET works reliably */
-- 
2.43.0.429.g432eaa2c6b-goog