[RFC PATCH v2 1/6] x86/boot: Defer initialization of VM space related global variables

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

The global pseudo-constants page_offset_base, vmalloc_base and
vmemmap_base are not used extremely early during the boot, and cannot be
used safely until after the KASLR memory randomization code in
kernel_randomize_memory() executes, which may update their values.

So there is no point in setting these variables extremely early, and it
can wait until after the kernel itself is mapped and running from its
permanent virtual mapping.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/boot/startup/map_kernel.c | 3 ---
 arch/x86/kernel/head64.c           | 9 ++++++---
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/x86/boot/startup/map_kernel.c b/arch/x86/boot/startup/map_kernel.c
index 099ae2559336..905e8734b5a3 100644
--- a/arch/x86/boot/startup/map_kernel.c
+++ b/arch/x86/boot/startup/map_kernel.c
@@ -29,9 +29,6 @@ static inline bool check_la57_support(void)
 	__pgtable_l5_enabled	= 1;
 	pgdir_shift		= 48;
 	ptrs_per_p4d		= 512;
-	page_offset_base	= __PAGE_OFFSET_BASE_L5;
-	vmalloc_base		= __VMALLOC_BASE_L5;
-	vmemmap_base		= __VMEMMAP_BASE_L5;
 
 	return true;
 }
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 510fb41f55fc..14f7dda20954 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -62,13 +62,10 @@ EXPORT_SYMBOL(ptrs_per_p4d);
 #ifdef CONFIG_DYNAMIC_MEMORY_LAYOUT
 unsigned long page_offset_base __ro_after_init = __PAGE_OFFSET_BASE_L4;
 EXPORT_SYMBOL(page_offset_base);
-SYM_PIC_ALIAS(page_offset_base);
 unsigned long vmalloc_base __ro_after_init = __VMALLOC_BASE_L4;
 EXPORT_SYMBOL(vmalloc_base);
-SYM_PIC_ALIAS(vmalloc_base);
 unsigned long vmemmap_base __ro_after_init = __VMEMMAP_BASE_L4;
 EXPORT_SYMBOL(vmemmap_base);
-SYM_PIC_ALIAS(vmemmap_base);
 #endif
 
 /* Wipe all early page tables except for the kernel symbol map */
@@ -244,6 +241,12 @@ asmlinkage __visible void __init __noreturn x86_64_start_kernel(char * real_mode
 	/* Kill off the identity-map trampoline */
 	reset_early_page_tables();
 
+	if (pgtable_l5_enabled()) {
+		page_offset_base	= __PAGE_OFFSET_BASE_L5;
+		vmalloc_base		= __VMALLOC_BASE_L5;
+		vmemmap_base		= __VMEMMAP_BASE_L5;
+	}
+
 	clear_bss();
 
 	/*
-- 
2.49.0.1045.g170613ef41-goog