x86/KASLR: Fix detection 32/64 bit bootloaders for 5-level paging

KASLR uses hack to detect whether we booted via startup_32() or
startup_64(): it checks what is loaded into cr3 and compares it to
_pgtables. _pgtables is the array of page tables where early code
allocates page table from.

KASLR expects cr3 to point to _pgtables if we booted via startup_32(), but
that's not true if we booted with 5-level paging enabled. In this case top
level page table is allocated separately and only the first p4d page table
is allocated from the array.

Let's modify the check to cover both 4- and 5-level paging cases.

The patch also renames 'level4p' to 'top_level_pgt' as it now can hold
page table for 4th or 5th level, depending on configuration.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arch@vger.kernel.org
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/20170628121730.43079-1-kirill.shutemov@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/boot/compressed/pagetable.c

@@ -63,7 +63,7 @@ static void *alloc_pgt_page(void *context)
 static struct alloc_pgt_data pgt_data;
 
 /* The top level page table entry pointer. */
-static unsigned long level4p;
+static unsigned long top_level_pgt;
 
 /*
  * Mapping information structure passed to kernel_ident_mapping_init().
@@ -91,9 +91,15 @@ void initialize_identity_maps(void)
 	 * If we came here via startup_32(), cr3 will be _pgtable already
 	 * and we must append to the existing area instead of entirely
 	 * overwriting it.
+	 *
+	 * With 5-level paging, we use '_pgtable' to allocate the p4d page table,
+	 * the top-level page table is allocated separately.
+	 *
+	 * p4d_offset(top_level_pgt, 0) would cover both the 4- and 5-level
+	 * cases. On 4-level paging it's equal to 'top_level_pgt'.
 	 */
-	level4p = read_cr3_pa();
-	if (level4p == (unsigned long)_pgtable) {
+	top_level_pgt = read_cr3_pa();
+	if (p4d_offset((pgd_t *)top_level_pgt, 0) == (p4d_t *)_pgtable) {
 		debug_putstr("booted via startup_32()\n");
 		pgt_data.pgt_buf = _pgtable + BOOT_INIT_PGT_SIZE;
 		pgt_data.pgt_buf_size = BOOT_PGT_SIZE - BOOT_INIT_PGT_SIZE;
@@ -103,7 +109,7 @@ void initialize_identity_maps(void)
 		pgt_data.pgt_buf = _pgtable;
 		pgt_data.pgt_buf_size = BOOT_PGT_SIZE;
 		memset(pgt_data.pgt_buf, 0, pgt_data.pgt_buf_size);
-		level4p = (unsigned long)alloc_pgt_page(&pgt_data);
+		top_level_pgt = (unsigned long)alloc_pgt_page(&pgt_data);
 	}
 }
 
@@ -123,7 +129,7 @@ void add_identity_map(unsigned long start, unsigned long size)
 		return;
 
 	/* Build the mapping. */
-	kernel_ident_mapping_init(&mapping_info, (pgd_t *)level4p,
+	kernel_ident_mapping_init(&mapping_info, (pgd_t *)top_level_pgt,
 				  start, end);
 }
 
@@ -134,5 +140,5 @@ void add_identity_map(unsigned long start, unsigned long size)
  */
 void finalize_identity_maps(void)
 {
-	write_cr3(level4p);
+	write_cr3(top_level_pgt);
 }