From 9bc4f28af75a91aea0ae383f50b0a430c4509303 Mon Sep 17 00:00:00 2001 From: Nadav Amit Date: Sun, 2 Sep 2018 11:14:50 -0700 Subject: x86/mm: Use WRITE_ONCE() when setting PTEs 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 Signed-off-by: Thomas Gleixner Acked-by: Peter Zijlstra (Intel) Cc: Dave Hansen Cc: Andi Kleen Cc: Josh Poimboeuf Cc: Michal Hocko Cc: Vlastimil Babka Cc: Sean Christopherson Cc: Andy Lutomirski Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/20180902181451.80520-1-namit@vmware.com --- arch/x86/mm/pgtable.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) (limited to 'arch/x86/mm/pgtable.c') diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c index e848a4811785..ae394552fb94 100644 --- a/arch/x86/mm/pgtable.c +++ b/arch/x86/mm/pgtable.c @@ -269,7 +269,7 @@ 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); @@ -494,7 +494,7 @@ 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; } @@ -509,7 +509,7 @@ 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, @@ -529,7 +529,7 @@ 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, -- cgit v1.2.3