Re: [PATCH v6 27/41] x86/mm: Warn if create Write=0,Dirty=1 with raw prot

[Kees Cook <keescook@chromium.org>]

On Sat, Feb 18, 2023 at 01:14:19PM -0800, Rick Edgecombe wrote:
> When user shadow stack is use, Write=0,Dirty=1 is treated by the CPU as
> shadow stack memory. So for shadow stack memory this bit combination is
> valid, but when Dirty=1,Write=1 (conventionally writable) memory is being
> write protected, the kernel has been taught to transition the Dirty=1
> bit to SavedDirty=1, to avoid inadvertently creating shadow stack
> memory. It does this inside pte_wrprotect() because it knows the PTE is
> not intended to be a writable shadow stack entry, it is supposed to be
> write protected.
> 
> However, when a PTE is created by a raw prot using mk_pte(), mk_pte()
> can't know whether to adjust Dirty=1 to SavedDirty=1. It can't
> distinguish between the caller intending to create a shadow stack PTE or
> needing the SavedDirty shift.
> 
> The kernel has been updated to not do this, and so Write=0,Dirty=1
> memory should only be created by the pte_mkfoo() helpers. Add a warning
> to make sure no new mk_pte() start doing this.
> 
> Tested-by: Pengfei Xu <pengfei.xu@intel.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> 
> ---
> v6:
>  - New patch (Note, this has already been a useful warning, it caught the
>    newly added set_memory_rox() doing this)
> ---
>  arch/x86/include/asm/pgtable.h | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index f3dc16fc4389..db8fe5511c74 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -1032,7 +1032,15 @@ static inline unsigned long pmd_page_vaddr(pmd_t pmd)
>   * (Currently stuck as a macro because of indirect forward reference
>   * to linux/mm.h:page_to_nid())
>   */
> -#define mk_pte(page, pgprot)   pfn_pte(page_to_pfn(page), (pgprot))
> +#define mk_pte(page, pgprot)						 \
> +({									 \
> +	pgprot_t __pgprot = pgprot;					 \
> +									 \
> +	WARN_ON_ONCE(cpu_feature_enabled(X86_FEATURE_USER_SHSTK) &&	 \
> +		    (pgprot_val(__pgprot) & (_PAGE_DIRTY | _PAGE_RW)) == \
> +		    _PAGE_DIRTY);					 \
> +	pfn_pte(page_to_pfn(page), __pgprot);				 \
> +})

This only warns? Should it also enforce the state?

-- 
Kees Cook