[PATCH] x86: avoid VLA in pgd_alloc()

* [PATCH] x86: avoid VLA in pgd_alloc()
@ 2018-10-05 16:13 Arnd Bergmann
  2018-10-05 16:24 ` [tip:x86/urgent] x86/mm: Avoid " tip-bot for Arnd Bergmann
  0 siblings, 1 reply; 7+ messages in thread
From: Arnd Bergmann @ 2018-10-05 16:13 UTC (permalink / raw)
  To: Dave Hansen, Andy Lutomirski, Peter Zijlstra, Thomas Gleixner,
	Ingo Molnar, Borislav Petkov, x86
  Cc: Kees Cook, Arnd Bergmann, H. Peter Anvin, Toshi Kani,
	Andrew Morton, Joerg Roedel, linux-kernel

Turning on -Wvla found a new VLA usage:

In file included from include/linux/kernel.h:15,
                 from include/asm-generic/bug.h:18,
                 from arch/x86/include/asm/bug.h:83,
                 from include/linux/bug.h:5,
                 from include/linux/mmdebug.h:5,
                 from include/linux/mm.h:9,
                 from arch/x86/mm/pgtable.c:2:
arch/x86/mm/pgtable.c: In function 'pgd_alloc':
include/linux/build_bug.h:29:45: error: ISO C90 forbids variable length array 'u_pmds' [-Werror=vla]
arch/x86/mm/pgtable.c:190:34: note: in expansion of macro 'static_cpu_has'
 #define PREALLOCATED_USER_PMDS  (static_cpu_has(X86_FEATURE_PTI) ? \
                                  ^~~~~~~~~~~~~~
arch/x86/mm/pgtable.c:431:16: note: in expansion of macro 'PREALLOCATED_USER_PMDS'
  pmd_t *u_pmds[PREALLOCATED_USER_PMDS];
                ^~~~~~~~~~~~~~~~~~~~~~

Use the actual size of the array that is used for X86_FEATURE_PTI
instead of the variable size.

Fixes: f59dbe9ca670 ("x86/pgtable/pae: Use separate kernel PMDs for user page-table")
Fixes: 68664695ae57 ("Makefile: Globally enable VLA warning")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 arch/x86/mm/pgtable.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index 089e78c4effd..386b43e3e0ac 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -189,6 +189,7 @@ static void pgd_dtor(pgd_t *pgd)
  */
 #define PREALLOCATED_USER_PMDS	 (static_cpu_has(X86_FEATURE_PTI) ? \
 					KERNEL_PGD_PTRS : 0)
+#define MAX_PREALLOCATED_USER_PMDS KERNEL_PGD_PTRS
 
 void pud_populate(struct mm_struct *mm, pud_t *pudp, pmd_t *pmd)
 {
@@ -211,6 +212,7 @@ void pud_populate(struct mm_struct *mm, pud_t *pudp, pmd_t *pmd)
 /* No need to prepopulate any pagetable entries in non-PAE modes. */
 #define PREALLOCATED_PMDS	0
 #define PREALLOCATED_USER_PMDS	 0
+#define MAX_PREALLOCATED_USER_PMDS 0
 #endif	/* CONFIG_X86_PAE */
 
 static void free_pmds(struct mm_struct *mm, pmd_t *pmds[], int count)
@@ -428,8 +430,8 @@ static inline void _pgd_free(pgd_t *pgd)
 pgd_t *pgd_alloc(struct mm_struct *mm)
 {
 	pgd_t *pgd;
-	pmd_t *u_pmds[PREALLOCATED_USER_PMDS];
-	pmd_t *pmds[PREALLOCATED_PMDS];
+	pmd_t *u_pmds[MAX_PREALLOCATED_USER_PMDS];
+	pmd_t *pmds[MAX_PREALLOCATED_USER_PMDS];
 
 	pgd = _pgd_alloc();
 
-- 
2.18.0


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