linux-kernel - [PATCH v3 2/6] x86/boot/64: Determine VA/PA offset before entering C code

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Message-ID: <20241125170411.1898410-10-ardb+git@google.com>
Date: Mon, 25 Nov 2024 18:04:13 +0100
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-kernel@...r.kernel.org
Cc: x86@...nel.org, Ard Biesheuvel <ardb@...nel.org>, Tom Lendacky <thomas.lendacky@....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>, Kees Cook <keescook@...omium.org>, Brian Gerst <brgerst@...il.com>, 
	Kevin Loughlin <kevinloughlin@...gle.com>
Subject: [PATCH v3 2/6] x86/boot/64: Determine VA/PA offset before entering C code

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

Implicit absolute symbol references (e.g., taking the address of a
global variable) must be avoided in the C code that runs from the early
1:1 mapping of the kernel, given that this is a practice that violates
assumptions on the part of the toolchain. I.e., RIP-relative and
absolute references are expected to produce the same values, and so the
compiler is free to choose either. However, the code currently assumes
that RIP-relative references are never emitted here.

So an explicit virtual-to-physical offset needs to be used instead to
derive the kernel virtual addresses of _text and _end, instead of simply
taking the addresses and assuming that the compiler will not choose to
use a RIP-relative references in this particular case.

Currently, phys_base is already used to perform such calculations, but
it is derived from the kernel virtual address of _text, which is taken
using an implicit absolute symbol reference. So instead, derive this
VA-to-PA offset in asm code, and pass it to the C startup code.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/include/asm/setup.h |  2 +-
 arch/x86/kernel/head64.c     |  8 +++++---
 arch/x86/kernel/head_64.S    | 12 +++++++++---
 3 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
index 0667b2a88614..85f4fde3515c 100644
--- a/arch/x86/include/asm/setup.h
+++ b/arch/x86/include/asm/setup.h
@@ -49,7 +49,7 @@ extern unsigned long saved_video_mode;
 
 extern void reserve_standard_io_resources(void);
 extern void i386_reserve_resources(void);
-extern unsigned long __startup_64(unsigned long physaddr, struct boot_params *bp);
+extern unsigned long __startup_64(unsigned long p2v_offset, struct boot_params *bp);
 extern void startup_64_setup_gdt_idt(void);
 extern void early_setup_idt(void);
 extern void __init do_early_exception(struct pt_regs *regs, int trapnr);
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 4b9d4557fc94..a7cd4053eeb3 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -138,12 +138,14 @@ static unsigned long __head sme_postprocess_startup(struct boot_params *bp, pmdv
  * doesn't have to generate PC-relative relocations when accessing globals from
  * that function. Clang actually does not generate them, which leads to
  * boot-time crashes. To work around this problem, every global pointer must
- * be accessed using RIP_REL_REF().
+ * be accessed using RIP_REL_REF(). Kernel virtual addresses can be determined
+ * by subtracting p2v_offset from the RIP-relative address.
  */
-unsigned long __head __startup_64(unsigned long physaddr,
+unsigned long __head __startup_64(unsigned long p2v_offset,
 				  struct boot_params *bp)
 {
 	pmd_t (*early_pgts)[PTRS_PER_PMD] = RIP_REL_REF(early_dynamic_pgts);
+	unsigned long physaddr = (unsigned long)&RIP_REL_REF(_text);
 	unsigned long pgtable_flags;
 	unsigned long load_delta;
 	pgdval_t *pgd;
@@ -163,7 +165,7 @@ unsigned long __head __startup_64(unsigned long physaddr,
 	 * Compute the delta between the address I am compiled to run at
 	 * and the address I am actually running at.
 	 */
-	load_delta = physaddr - (unsigned long)(_text - __START_KERNEL_map);
+	load_delta = __START_KERNEL_map + p2v_offset;
 	RIP_REL_REF(phys_base) = load_delta;
 
 	/* Is the address not 2M aligned? */
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 56163e2124cf..31345e0ba006 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -94,13 +94,19 @@ SYM_CODE_START_NOALIGN(startup_64)
 	/* Sanitize CPU configuration */
 	call verify_cpu
 
+	/*
+	 * Derive the kernel's physical-to-virtual offset from the physical and
+	 * virtual addresses of common_startup_64().
+	 */
+	leaq	common_startup_64(%rip), %rdi
+	subq	.Lcommon_startup_64(%rip), %rdi
+
 	/*
 	 * Perform pagetable fixups. Additionally, if SME is active, encrypt
 	 * the kernel and retrieve the modifier (SME encryption mask if SME
 	 * is active) to be added to the initial pgdir entry that will be
 	 * programmed into CR3.
 	 */
-	leaq	_text(%rip), %rdi
 	movq	%r15, %rsi
 	call	__startup_64
 
@@ -128,11 +134,11 @@ SYM_CODE_START_NOALIGN(startup_64)
 
 	/* Branch to the common startup code at its kernel virtual address */
 	ANNOTATE_RETPOLINE_SAFE
-	jmp	*0f(%rip)
+	jmp	*.Lcommon_startup_64(%rip)
 SYM_CODE_END(startup_64)
 
 	__INITRODATA
-0:	.quad	common_startup_64
+SYM_DATA_LOCAL(.Lcommon_startup_64, .quad common_startup_64)
 
 	.text
 SYM_CODE_START(secondary_startup_64)
-- 
2.47.0.371.ga323438b13-goog