[PATCH v2 2/2] x86/mm/64: Tighten up vmalloc_fault() sanity checks on 5-level kernels

[Andy Lutomirski <luto@kernel.org>]

On a 5-level kernel, if a non-init mm has a top-level entry, it needs
to match init_mm's, but the vmalloc_fault() code skipped over the
BUG_ON() that would have checked it.

While we're at it, get rid of the rather confusing 4-level folded
"pgd" logic.

Cleans-up: b50858ce3e2a ("x86/mm/vmalloc: Add 5-level paging support")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/mm/fault.c | 22 +++++++++-------------
 1 file changed, 9 insertions(+), 13 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 06fe3d51d385..aaeb3862a5b4 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -438,18 +438,13 @@ static noinline int vmalloc_fault(unsigned long address)
 	if (pgd_none(*pgd_ref))
 		return -1;
 
-	if (pgd_none(*pgd)) {
-		set_pgd(pgd, *pgd_ref);
-		arch_flush_lazy_mmu_mode();
-	} else if (CONFIG_PGTABLE_LEVELS > 4) {
-		/*
-		 * With folded p4d, pgd_none() is always false, so the pgd may
-		 * point to an empty page table entry and pgd_page_vaddr()
-		 * will return garbage.
-		 *
-		 * We will do the correct sanity check on the p4d level.
-		 */
-		BUG_ON(pgd_page_vaddr(*pgd) != pgd_page_vaddr(*pgd_ref));
+	if (CONFIG_PGTABLE_LEVELS > 4) {
+		if (pgd_none(*pgd)) {
+			set_pgd(pgd, *pgd_ref);
+			arch_flush_lazy_mmu_mode();
+		} else {
+			BUG_ON(pgd_page_vaddr(*pgd) != pgd_page_vaddr(*pgd_ref));
+		}
 	}
 
 	/* With 4-level paging, copying happens on the p4d level. */
@@ -458,7 +453,7 @@ static noinline int vmalloc_fault(unsigned long address)
 	if (p4d_none(*p4d_ref))
 		return -1;
 
-	if (p4d_none(*p4d)) {
+	if (p4d_none(*p4d) && CONFIG_PGTABLE_LEVELS == 4) {
 		set_p4d(p4d, *p4d_ref);
 		arch_flush_lazy_mmu_mode();
 	} else {
@@ -469,6 +464,7 @@ static noinline int vmalloc_fault(unsigned long address)
 	 * Below here mismatches are bugs because these lower tables
 	 * are shared:
 	 */
+	BUILD_BUG_ON(CONFIG_PGTABLE_LEVELS < 4);
 
 	pud = pud_offset(p4d, address);
 	pud_ref = pud_offset(p4d_ref, address);
-- 
2.14.3