Re: [PATCH v2 12/39] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW

Nadav Amit
2022-10-03T18:11:50Z

On Sep 29, 2022, at 3:29 PM, Rick Edgecombe <rick.p.edgecombe@intel.com> wrote:

> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
>
> When Shadow Stack is in use, Write=0,Dirty=1 PTE are reserved for shadow
> stack. Copy-on-write PTes then have Write=0,Cow=1.
>
> When a PTE goes from Write=1,Dirty=1 to Write=0,Cow=1, 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 Write=0 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>
> Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
>
> ---
>
> v2:
> - Compile out some code due to clang build error
> - Clarify commit log (dhansen)
> - Normalize PTE bit descriptions between patches (dhansen)
> - Update comment with text from (dhansen)
>
> Yu-cheng v30:
> - Replace (pmdval_t) cast with CONFIG_PGTABLE_LEVELES > 2 (Borislav Petkov).
>
> 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 2f2963429f48..58c7bf9d7392 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -1287,6 +1287,23 @@ 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)
> {
> +#ifdef CONFIG_X86_SHADOW_STACK
> + /*
> + * Avoid accidentally creating shadow stack PTEs
> + * (Write=0,Dirty=1). Use cmpxchg() to prevent races with
> + * the hardware setting Dirty=1.
> + */
> + 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;
> + }
> +#endif

There is no way of using IS_ENABLED() here instead of these ifdefs?

Did you have a look at ptep_set_access_flags() and friends and checked they
do not need to be changed too? Perhaps you should at least add some
assertion just to ensure nothing breaks.