| Message ID | 20210316151054.5405-13-yu-cheng.yu@intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Control-flow Enforcement: Shadow Stack | expand |
On Tue, Mar 16, 2021 at 08:10:38AM -0700, Yu-cheng Yu wrote: > When Shadow Stack is introduced, [R/O + _PAGE_DIRTY] PTE is reserved for > shadow stack. Copy-on-write PTEs have [R/O + _PAGE_COW]. > > When a PTE goes from [R/W + _PAGE_DIRTY] to [R/O + _PAGE_COW], it could > become a transient shadow stack PTE in two cases: > > The first case is that some processors can start a write but end up seeing > a read-only PTE by the time they get to the Dirty bit, creating a transient > shadow stack PTE. However, this will not occur on processors supporting > Shadow Stack, and a TLB flush is not necessary. > > The second case is that when _PAGE_DIRTY is replaced with _PAGE_COW non- > atomically, a transient shadow stack PTE can be created as a result. > Thus, prevent that with cmpxchg. > > Dave Hansen, Jann Horn, Andy Lutomirski, and Peter Zijlstra provided many > insights to the issue. Jann Horn provided the cmpxchg solution. > > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com> > Reviewed-by: Kees Cook <keescook@chromium.org> > --- > arch/x86/include/asm/pgtable.h | 36 ++++++++++++++++++++++++++++++++++ > 1 file changed, 36 insertions(+) > > diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h > index e1739f590ca6..46d9394b884f 100644 > --- a/arch/x86/include/asm/pgtable.h > +++ b/arch/x86/include/asm/pgtable.h > @@ -1306,6 +1306,24 @@ static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm, > static inline void ptep_set_wrprotect(struct mm_struct *mm, > unsigned long addr, pte_t *ptep) > { > + /* > + * If Shadow Stack is enabled, pte_wrprotect() moves _PAGE_DIRTY > + * to _PAGE_COW (see comments at pte_wrprotect()). > + * When a thread reads a RW=1, Dirty=0 PTE and before changing it > + * to RW=0, Dirty=0, another thread could have written to the page > + * and the PTE is RW=1, Dirty=1 now. Use try_cmpxchg() to detect > + * PTE changes and update old_pte, then try again. > + */ > + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) { > + pte_t old_pte, new_pte; > + > + old_pte = READ_ONCE(*ptep); > + do { > + new_pte = pte_wrprotect(old_pte); > + } while (!try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte)); I think this is wrong. You need to update old_pte on every loop iteration, otherwise you can get in to endless loop. The same issue for pmdp_set_wrprotect(). > + > + return; > + } > clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte); > } > > @@ -1350,6 +1368,24 @@ static inline pud_t pudp_huge_get_and_clear(struct mm_struct *mm, > static inline void pmdp_set_wrprotect(struct mm_struct *mm, > unsigned long addr, pmd_t *pmdp) > { > + /* > + * If Shadow Stack is enabled, pmd_wrprotect() moves _PAGE_DIRTY > + * to _PAGE_COW (see comments at pmd_wrprotect()). > + * When a thread reads a RW=1, Dirty=0 PMD and before changing it > + * to RW=0, Dirty=0, another thread could have written to the page > + * and the PMD is RW=1, Dirty=1 now. Use try_cmpxchg() to detect > + * PMD changes and update old_pmd, then try again. > + */ > + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) { > + pmd_t old_pmd, new_pmd; > + > + old_pmd = READ_ONCE(*pmdp); > + do { > + new_pmd = pmd_wrprotect(old_pmd); > + } while (!try_cmpxchg((pmdval_t *)pmdp, (pmdval_t *)&old_pmd, pmd_val(new_pmd))); > + > + return; > + } > clear_bit(_PAGE_BIT_RW, (unsigned long *)pmdp); > } > > -- > 2.21.0 >
On Mon, Mar 22, 2021 at 01:15:02PM +0300, Kirill A. Shutemov wrote: > On Tue, Mar 16, 2021 at 08:10:38AM -0700, Yu-cheng Yu wrote: > > + pte_t old_pte, new_pte; > > + > > + old_pte = READ_ONCE(*ptep); > > + do { > > + new_pte = pte_wrprotect(old_pte); > > + } while (!try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte)); > > I think this is wrong. You need to update old_pte on every loop iteration, > otherwise you can get in to endless loop. It is correct, please consider why the old argument is a pointer.
On Mon, Mar 22, 2021 at 11:46:21AM +0100, Peter Zijlstra wrote: > On Mon, Mar 22, 2021 at 01:15:02PM +0300, Kirill A. Shutemov wrote: > > On Tue, Mar 16, 2021 at 08:10:38AM -0700, Yu-cheng Yu wrote: > > > > + pte_t old_pte, new_pte; > > > + > > > + old_pte = READ_ONCE(*ptep); > > > + do { > > > + new_pte = pte_wrprotect(old_pte); > > > + } while (!try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte)); > > > > I think this is wrong. You need to update old_pte on every loop iteration, > > otherwise you can get in to endless loop. > > It is correct, please consider why the old argument is a pointer. Ah, right. Sorry for the noise.
diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h index e1739f590ca6..46d9394b884f 100644 --- a/arch/x86/include/asm/pgtable.h +++ b/arch/x86/include/asm/pgtable.h @@ -1306,6 +1306,24 @@ static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm, static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addr, pte_t *ptep) { + /* + * If Shadow Stack is enabled, pte_wrprotect() moves _PAGE_DIRTY + * to _PAGE_COW (see comments at pte_wrprotect()). + * When a thread reads a RW=1, Dirty=0 PTE and before changing it + * to RW=0, Dirty=0, another thread could have written to the page + * and the PTE is RW=1, Dirty=1 now. Use try_cmpxchg() to detect + * PTE changes and update old_pte, then try again. + */ + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) { + pte_t old_pte, new_pte; + + old_pte = READ_ONCE(*ptep); + do { + new_pte = pte_wrprotect(old_pte); + } while (!try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte)); + + return; + } clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte); } @@ -1350,6 +1368,24 @@ static inline pud_t pudp_huge_get_and_clear(struct mm_struct *mm, static inline void pmdp_set_wrprotect(struct mm_struct *mm, unsigned long addr, pmd_t *pmdp) { + /* + * If Shadow Stack is enabled, pmd_wrprotect() moves _PAGE_DIRTY + * to _PAGE_COW (see comments at pmd_wrprotect()). + * When a thread reads a RW=1, Dirty=0 PMD and before changing it + * to RW=0, Dirty=0, another thread could have written to the page + * and the PMD is RW=1, Dirty=1 now. Use try_cmpxchg() to detect + * PMD changes and update old_pmd, then try again. + */ + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) { + pmd_t old_pmd, new_pmd; + + old_pmd = READ_ONCE(*pmdp); + do { + new_pmd = pmd_wrprotect(old_pmd); + } while (!try_cmpxchg((pmdval_t *)pmdp, (pmdval_t *)&old_pmd, pmd_val(new_pmd))); + + return; + } clear_bit(_PAGE_BIT_RW, (unsigned long *)pmdp); }