Re: [PATCH v17 11/26] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW

[Borislav Petkov]

On Tue, Dec 29, 2020 at 01:30:38PM -0800, 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, therefore we don't need a TLB flush here.

Who's "we"?

> The second case is that when the software, without atomic, tests & replaces

"... when _PAGE_DIRTY is replaced with _PAGE_COW non-atomically, a transient
shadow stack PTE can be created, as a result."

> _PAGE_DIRTY with _PAGE_COW, a transient shadow stack PTE can exist.
> This is prevented 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>
> ---
>  arch/x86/include/asm/pgtable.h | 52 ++++++++++++++++++++++++++++++++++
>  1 file changed, 52 insertions(+)
> 
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index 666c25ab9564..1c84f1ba32b9 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -1226,6 +1226,32 @@ 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)
>  {
> +	/*
> +	 * Some processors can start a write, but end up seeing a read-only
> +	 * PTE by the time they get to the Dirty bit.  In this case, they
> +	 * will set the Dirty bit, leaving a read-only, Dirty PTE which
> +	 * looks like a shadow stack PTE.
> +	 *
> +	 * However, this behavior has been improved

Improved how?

> and will not occur on
> +	 * processors supporting Shadow Stack.  Without this guarantee, a

Which guarantee? That it won't happen on CPUs which support SHSTK?

> +	 * transition to a non-present PTE and flush the TLB would be

s/flush the TLB/TLB flush/

> +	 * needed.
> +	 *
> +	 * When changing a writable PTE to read-only and if the PTE has
> +	 * _PAGE_DIRTY set, move that bit to _PAGE_COW so that the PTE is
> +	 * not a shadow stack PTE.
> +	 */

This sentence doesn't belong here as it refers to what pte_wrprotect()
does. You could expand the comment in pte_wrprotect() with this here as
it is better.

> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> +		pte_t old_pte, new_pte;
> +
> +		do {
> +			old_pte = READ_ONCE(*ptep);
> +			new_pte = pte_wrprotect(old_pte);

Maybe I'm missing something but those two can happen outside of the
loop, no? Or is *ptep somehow changing concurrently while the loop is
doing the CMPXCHG and you need to recreate it each time?

IOW, you can generate upfront and do the empty loop...

> +
> +		} while (!try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte));
> +
> +		return;
> +	}
>  	clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
>  }
>  
> @@ -1282,6 +1308,32 @@ 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)
>  {
> +	/*
> +	 * Some processors can start a write, but end up seeing a read-only
> +	 * PMD by the time they get to the Dirty bit.  In this case, they
> +	 * will set the Dirty bit, leaving a read-only, Dirty PMD which
> +	 * looks like a Shadow Stack PMD.
> +	 *
> +	 * However, this behavior has been improved and will not occur on
> +	 * processors supporting Shadow Stack.  Without this guarantee, a
> +	 * transition to a non-present PMD and flush the TLB would be
> +	 * needed.
> +	 *
> +	 * When changing a writable PMD to read-only and if the PMD has
> +	 * _PAGE_DIRTY set, move that bit to _PAGE_COW so that the PMD is
> +	 * not a shadow stack PMD.
> +	 */

Same comments as above.

> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> +		pmd_t old_pmd, new_pmd;
> +
> +		do {
> +			old_pmd = READ_ONCE(*pmdp);
> +			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
> 

-- 
Regards/Gruss,
    Boris.

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