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

[Andy Lutomirski]

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 <l...@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