Messages in this thread Patch in this message | | | Subject | Re: [PATCH v17 08/26] x86/mm: Introduce _PAGE_COW | From | "Yu, Yu-cheng" <> | Date | Thu, 21 Jan 2021 12:16:23 -0800 |
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On 1/21/2021 10:44 AM, Borislav Petkov wrote: > On Tue, Dec 29, 2020 at 01:30:35PM -0800, Yu-cheng Yu wrote: [...] >> @@ -343,6 +349,16 @@ static inline pte_t pte_mkold(pte_t pte) >> >> static inline pte_t pte_wrprotect(pte_t pte) >> { >> + /* >> + * Blindly clearing _PAGE_RW might accidentally create >> + * a shadow stack PTE (RW=0, Dirty=1). Move the hardware >> + * dirty value to the software bit. >> + */ >> + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) { >> + pte.pte |= (pte.pte & _PAGE_DIRTY) >> _PAGE_BIT_DIRTY << _PAGE_BIT_COW; > > Why the unreadable shifting when you can simply do: > > if (pte.pte & _PAGE_DIRTY) > pte.pte |= _PAGE_COW; > > ?
It clears _PAGE_DIRTY and sets _PAGE_COW. That is,
if (pte.pte & _PAGE_DIRTY) { pte.pte &= ~_PAGE_DIRTY; pte.pte |= _PAGE_COW; }
So, shifting makes resulting code more efficient.
>> @@ -434,16 +469,40 @@ static inline pmd_t pmd_mkold(pmd_t pmd) >> >> static inline pmd_t pmd_mkclean(pmd_t pmd) >> { >> - return pmd_clear_flags(pmd, _PAGE_DIRTY); >> + return pmd_clear_flags(pmd, _PAGE_DIRTY_BITS); >> } >> >> static inline pmd_t pmd_wrprotect(pmd_t pmd) >> { >> + /* >> + * Blindly clearing _PAGE_RW might accidentally create >> + * a shadow stack PMD (RW=0, Dirty=1). Move the hardware >> + * dirty value to the software bit. >> + */ >> + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) { >> + pmdval_t v = native_pmd_val(pmd); >> + >> + v |= (v & _PAGE_DIRTY) >> _PAGE_BIT_DIRTY << _PAGE_BIT_COW; > > As above. > >> @@ -488,17 +554,35 @@ static inline pud_t pud_mkold(pud_t pud) >> >> static inline pud_t pud_mkclean(pud_t pud) >> { >> - return pud_clear_flags(pud, _PAGE_DIRTY); >> + return pud_clear_flags(pud, _PAGE_DIRTY_BITS); >> } >> >> static inline pud_t pud_wrprotect(pud_t pud) >> { >> + /* >> + * Blindly clearing _PAGE_RW might accidentally create >> + * a shadow stack PUD (RW=0, Dirty=1). Move the hardware >> + * dirty value to the software bit. >> + */ >> + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) { >> + pudval_t v = native_pud_val(pud); >> + >> + v |= (v & _PAGE_DIRTY) >> _PAGE_BIT_DIRTY << _PAGE_BIT_COW; > > Ditto. > >> @@ -1131,6 +1222,12 @@ extern int pmdp_clear_flush_young(struct vm_area_struct *vma, >> #define pmd_write pmd_write >> static inline int pmd_write(pmd_t pmd) >> { >> + /* >> + * If _PAGE_DIRTY is set, then the PMD must either have _PAGE_RW or >> + * be a shadow stack PMD, which is logically writable. >> + */ >> + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) >> + return pmd_flags(pmd) & (_PAGE_RW | _PAGE_DIRTY); > > else > > >> return pmd_flags(pmd) & _PAGE_RW; >> } >>
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