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SubjectRe: [PATCH v17 08/26] x86/mm: Introduce _PAGE_COW
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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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