Re: [PATCH v28 12/32] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW

["Yu, Yu-cheng" <yu-cheng.yu@xxxxxxxxx>]

On 8/16/2021 9:01 AM, Borislav Petkov wrote:
> On Thu, Jul 22, 2021 at 01:51:59PM -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@xxxxxxxxx>
> > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > ---
> >   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 cf7316e968df..df4ce715560a 100644
> > --- a/arch/x86/include/asm/pgtable.h
> > +++ b/arch/x86/include/asm/pgtable.h
> > @@ -1278,6 +1278,24 @@ 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);
> >   }
> >   
> > @@ -1322,6 +1340,24 @@ 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)));
>
> Why is that try_cmpxchg() call doing casting to its operands instead of
> like the pte one above?
>
> I.e., why aren't you doing here the same thing as above:
>
> 		...
> 		} while (!try_cmpxchg(&pmdp->pmd, &old_pmd.pmd, new_pmd.pmd));
>
> ?

If !(CONFIG_PGTABLE_LEVELS > 2), we don't have pmd_t.pmd.

> Thx.