[RFC PATCH] x86: use WRITE_ONCE() when setting PTEs

[Nadav Amit]

When page-table entries are set, the compiler might optimize their
assignment by using multiple instructions to set the PTE. This might
turn into a security hazard if the user somehow manages to use the
interim PTE. L1TF does not make our lives easier, making even an interim
non-present PTE a security hazard.

Using WRITE_ONCE() to set PTEs and friends should prevent this potential
security hazard.

I skimmed the differences in the binary with and without this patch. The
differences are (obviously) greater when CONFIG_PARAVIRT=n as more
code optimizations are possible. For better and worse, the impact on the
binary with this patch is pretty small. Skimming the code did not cause
anything to jump out as a security hazard, but it seems that at least
move_soft_dirty_pte() caused set_pte_at() to use multiple writes.

Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
---
 arch/x86/include/asm/pgtable_64.h | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/arch/x86/include/asm/pgtable_64.h b/arch/x86/include/asm/pgtable_64.h
index f773d5e6c8cc..ce2b59047cb8 100644
--- a/arch/x86/include/asm/pgtable_64.h
+++ b/arch/x86/include/asm/pgtable_64.h
@@ -55,15 +55,15 @@ struct mm_struct;
 void set_pte_vaddr_p4d(p4d_t *p4d_page, unsigned long vaddr, pte_t new_pte);
 void set_pte_vaddr_pud(pud_t *pud_page, unsigned long vaddr, pte_t new_pte);
 
-static inline void native_pte_clear(struct mm_struct *mm, unsigned long addr,
-				    pte_t *ptep)
+static inline void native_set_pte(pte_t *ptep, pte_t pte)
 {
-	*ptep = native_make_pte(0);
+	WRITE_ONCE(*ptep, pte);
 }
 
-static inline void native_set_pte(pte_t *ptep, pte_t pte)
+static inline void native_pte_clear(struct mm_struct *mm, unsigned long addr,
+				    pte_t *ptep)
 {
-	*ptep = pte;
+	native_set_pte(ptep, native_make_pte(0));
 }
 
 static inline void native_set_pte_atomic(pte_t *ptep, pte_t pte)
@@ -73,7 +73,7 @@ static inline void native_set_pte_atomic(pte_t *ptep, pte_t pte)
 
 static inline void native_set_pmd(pmd_t *pmdp, pmd_t pmd)
 {
-	*pmdp = pmd;
+	WRITE_ONCE(*pmdp, pmd);
 }
 
 static inline void native_pmd_clear(pmd_t *pmd)
@@ -109,7 +109,7 @@ static inline pmd_t native_pmdp_get_and_clear(pmd_t *xp)
 
 static inline void native_set_pud(pud_t *pudp, pud_t pud)
 {
-	*pudp = pud;
+	WRITE_ONCE(*pudp, pud);
 }
 
 static inline void native_pud_clear(pud_t *pud)
@@ -137,13 +137,13 @@ static inline void native_set_p4d(p4d_t *p4dp, p4d_t p4d)
 	pgd_t pgd;
 
 	if (pgtable_l5_enabled() || !IS_ENABLED(CONFIG_PAGE_TABLE_ISOLATION)) {
-		*p4dp = p4d;
+		WRITE_ONCE(*p4dp, p4d);
 		return;
 	}
 
 	pgd = native_make_pgd(native_p4d_val(p4d));
 	pgd = pti_set_user_pgtbl((pgd_t *)p4dp, pgd);
-	*p4dp = native_make_p4d(native_pgd_val(pgd));
+	WRITE_ONCE(*p4dp, native_make_p4d(native_pgd_val(pgd)));
 }
 
 static inline void native_p4d_clear(p4d_t *p4d)
@@ -153,7 +153,7 @@ static inline void native_p4d_clear(p4d_t *p4d)
 
 static inline void native_set_pgd(pgd_t *pgdp, pgd_t pgd)
 {
-	*pgdp = pti_set_user_pgtbl(pgdp, pgd);
+	WRITE_ONCE(*pgdp, pti_set_user_pgtbl(pgdp, pgd));
 }
 
 static inline void native_pgd_clear(pgd_t *pgd)
-- 
2.17.1