linux-kernel - [PATCH v5 06/16] x86/startup_64: Use RIP_REL_REF() to access early_top

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Date: Wed, 21 Feb 2024 12:35:13 +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 v5 06/16] x86/startup_64: Use RIP_REL_REF() to access early_top_pgt[]

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

early_top_pgt[] is assigned from code that executes from a 1:1 mapping
so it cannot use a plain access from C. Replace the use of
fixup_pointer() with RIP_REL_REF(), which is better and simpler.

For legibility and to align with the code that populates the lower page
table levels, statically initialize the root level page table with an
entry pointing to level3_kernel_pgt[], and overwrite it when needed to
enable 5-level paging.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/kernel/head64.c  | 21 +++++++++-----------
 arch/x86/kernel/head_64.S |  3 ++-
 2 files changed, 11 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 7e2c9b581d58..72351c3121a6 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -77,6 +77,7 @@ static struct desc_struct startup_gdt[GDT_ENTRIES] __initdata = {
 	[GDT_ENTRY_KERNEL_DS]           = GDT_ENTRY_INIT(DESC_DATA64, 0, 0xfffff),
 };
 
+#ifdef CONFIG_X86_5LEVEL
 static void __head *fixup_pointer(void *ptr, unsigned long physaddr)
 {
 	return ptr - (void *)_text + (void *)physaddr;
@@ -87,7 +88,6 @@ static unsigned long __head *fixup_long(void *ptr, unsigned long physaddr)
 	return fixup_pointer(ptr, physaddr);
 }
 
-#ifdef CONFIG_X86_5LEVEL
 static unsigned int __head *fixup_int(void *ptr, unsigned long physaddr)
 {
 	return fixup_pointer(ptr, physaddr);
@@ -165,14 +165,14 @@ 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 adjusted using fixup_pointer().
+ * be accessed using RIP_REL_REF().
  */
 unsigned long __head __startup_64(unsigned long physaddr,
 				  struct boot_params *bp)
 {
 	pmd_t (*early_pgts)[PTRS_PER_PMD] = RIP_REL_REF(early_dynamic_pgts);
-	unsigned long load_delta, *p;
 	unsigned long pgtable_flags;
+	unsigned long load_delta;
 	pgdval_t *pgd;
 	p4dval_t *p4d;
 	pudval_t *pud;
@@ -202,17 +202,14 @@ unsigned long __head __startup_64(unsigned long physaddr,
 
 	/* Fixup the physical addresses in the page table */
 
-	pgd = fixup_pointer(early_top_pgt, physaddr);
-	p = pgd + pgd_index(__START_KERNEL_map);
-	if (la57)
-		*p = (unsigned long)level4_kernel_pgt;
-	else
-		*p = (unsigned long)level3_kernel_pgt;
-	*p += _PAGE_TABLE_NOENC - __START_KERNEL_map + load_delta;
+	pgd = &RIP_REL_REF(early_top_pgt)->pgd;
+	pgd[pgd_index(__START_KERNEL_map)] += load_delta;
 
 	if (la57) {
-		p4d = fixup_pointer(level4_kernel_pgt, physaddr);
-		p4d[511] += load_delta;
+		p4d = (p4dval_t *)&RIP_REL_REF(level4_kernel_pgt);
+		p4d[MAX_PTRS_PER_P4D - 1] += load_delta;
+
+		pgd[pgd_index(__START_KERNEL_map)] = (pgdval_t)p4d | _PAGE_TABLE_NOENC;
 	}
 
 	RIP_REL_REF(level3_kernel_pgt)[PTRS_PER_PUD - 2].pud += load_delta;
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 3cac98c61066..fb2a98c29094 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -653,7 +653,8 @@ SYM_CODE_END(vc_no_ghcb)
 	.balign 4
 
 SYM_DATA_START_PTI_ALIGNED(early_top_pgt)
-	.fill	512,8,0
+	.fill	511,8,0
+	.quad	level3_kernel_pgt - __START_KERNEL_map + _PAGE_TABLE_NOENC
 	.fill	PTI_USER_PGD_FILL,8,0
 SYM_DATA_END(early_top_pgt)
 
-- 
2.44.0.rc0.258.g7320e95886-goog