x86/mm: Use WRITE_ONCE() when setting PTEs

BugLink: https://bugs.launchpad.net/bugs/1830433
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.

Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180902181451.80520-1-namit@vmware.com
(backported from commit 9bc4f28af75a91aea0ae383f50b0a430c4509303)
Tested-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index c9f9d2334866c6c4e5662d09be23f258ea16745f..6b7267cd94997af351f61c440c4ec00070ff7ffb 100644 (file)
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1186,7 +1186,7 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
                return xchg(pmdp, pmd);
        } else {
                pmd_t old = *pmdp;
-               *pmdp = pmd;
+               WRITE_ONCE(*pmdp, pmd);
                return old;
        }
 }

diff --git a/arch/x86/include/asm/pgtable_64.h b/arch/x86/include/asm/pgtable_64.h
index 4a30e60837cade25cf88600115c532ab9cda95c3..a9d9de0cfdf818a7a2df2bb69c3b02239e0edf17 100644 (file)
--- 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)
@@ -135,9 +135,9 @@ static inline pud_t native_pudp_get_and_clear(pud_t *xp)
 static inline void native_set_p4d(p4d_t *p4dp, p4d_t p4d)
 {
 #if defined(CONFIG_PAGE_TABLE_ISOLATION) && !defined(CONFIG_X86_5LEVEL)
-       p4dp->pgd = pti_set_user_pgtbl(&p4dp->pgd, p4d.pgd);
+       WRITE_ONCE(p4dp->pgd, pti_set_user_pgtbl(&p4dp->pgd, p4d.pgd));
 #else
-       *p4dp = p4d;
+       WRITE_ONCE(*p4dp, p4d);
 #endif
 }
 
@@ -153,9 +153,9 @@ static inline void native_p4d_clear(p4d_t *p4d)
 static inline void native_set_pgd(pgd_t *pgdp, pgd_t pgd)
 {
 #ifdef CONFIG_PAGE_TABLE_ISOLATION
-       *pgdp = pti_set_user_pgtbl(pgdp, pgd);
+       WRITE_ONCE(*pgdp, pti_set_user_pgtbl(pgdp, pgd));
 #else
-       *pgdp = pgd;
+       WRITE_ONCE(*pgdp, pgd);
 #endif
 }
 

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index 5ae734ca4408fd9f2d72b97584345c386b908e55..d4da0f766e919884fe88c48fbfecc966ac407744 100644 (file)
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -265,7 +265,7 @@ static void mop_up_one_pmd(struct mm_struct *mm, pgd_t *pgdp)
        if (pgd_val(pgd) != 0) {
                pmd_t *pmd = (pmd_t *)pgd_page_vaddr(pgd);
 
-               *pgdp = native_make_pgd(0);
+               pgd_clear(pgdp);
 
                paravirt_release_pmd(pgd_val(pgd) >> PAGE_SHIFT);
                pmd_free(mm, pmd);
@@ -493,7 +493,7 @@ int ptep_set_access_flags(struct vm_area_struct *vma,
        int changed = !pte_same(*ptep, entry);
 
        if (changed && dirty)
-               *ptep = entry;
+               set_pte(ptep, entry);
 
        return changed;
 }
@@ -508,7 +508,7 @@ int pmdp_set_access_flags(struct vm_area_struct *vma,
        VM_BUG_ON(address & ~HPAGE_PMD_MASK);
 
        if (changed && dirty) {
-               *pmdp = entry;
+               set_pmd(pmdp, entry);
                /*
                 * We had a write-protection fault here and changed the pmd
                 * to to more permissive. No need to flush the TLB for that,
@@ -528,7 +528,7 @@ int pudp_set_access_flags(struct vm_area_struct *vma, unsigned long address,
        VM_BUG_ON(address & ~HPAGE_PUD_MASK);
 
        if (changed && dirty) {
-               *pudp = entry;
+               set_pud(pudp, entry);
                /*
                 * We had a write-protection fault here and changed the pud
                 * to to more permissive. No need to flush the TLB for that,