x86/boot: Defer initialization of VM space related global variables

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 <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Linus Torvalds <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: ardbiesheuvel

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;
 }

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();
 
 	/*