Re: [PATCH v23 18/28] mm/mmap: Add shadow stack pages to memory accounting

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, Mar 26, 2021 at 08:46:30AM -0700, Yu, Yu-cheng wrote:
> On 3/22/2021 3:57 AM, Kirill A. Shutemov wrote:
> > On Tue, Mar 16, 2021 at 08:10:44AM -0700, Yu-cheng Yu wrote:
> > > Account shadow stack pages to stack memory.
> > > 
> > > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> > > Reviewed-by: Kees Cook <keescook@chromium.org>
> > > ---
> > >   arch/x86/mm/pgtable.c   |  7 +++++++
> > >   include/linux/pgtable.h | 11 +++++++++++
> > >   mm/mmap.c               |  5 +++++
> > >   3 files changed, 23 insertions(+)
> > > 
> > > diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> > > index 0f4fbf51a9fc..948d28c29964 100644
> > > --- a/arch/x86/mm/pgtable.c
> > > +++ b/arch/x86/mm/pgtable.c
> > > @@ -895,3 +895,10 @@ int pmd_free_pte_page(pmd_t *pmd, unsigned long addr)
> > >   #endif /* CONFIG_X86_64 */
> > >   #endif	/* CONFIG_HAVE_ARCH_HUGE_VMAP */
> > > +
> > > +#ifdef CONFIG_ARCH_HAS_SHADOW_STACK
> > > +bool arch_shadow_stack_mapping(vm_flags_t vm_flags)
> > > +{
> > > +	return (vm_flags & VM_SHSTK);
> > > +}
> > > +#endif
> > > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > > index cbd98484c4f1..487c08df4365 100644
> > > --- a/include/linux/pgtable.h
> > > +++ b/include/linux/pgtable.h
> > > @@ -1470,6 +1470,17 @@ static inline pmd_t arch_maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma
> > >   #endif /* CONFIG_ARCH_MAYBE_MKWRITE */
> > >   #endif /* CONFIG_MMU */
> > > +#ifdef CONFIG_MMU
> > > +#ifdef CONFIG_ARCH_HAS_SHADOW_STACK
> > > +bool arch_shadow_stack_mapping(vm_flags_t vm_flags);
> > > +#else
> > > +static inline bool arch_shadow_stack_mapping(vm_flags_t vm_flags)
> > > +{
> > > +	return false;
> > > +}
> > > +#endif /* CONFIG_ARCH_HAS_SHADOW_STACK */
> > > +#endif /* CONFIG_MMU */
> > > +
> > >   /*
> > >    * Architecture PAGE_KERNEL_* fallbacks
> > >    *
> > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > index 3f287599a7a3..2ac67882ace2 100644
> > > --- a/mm/mmap.c
> > > +++ b/mm/mmap.c
> > > @@ -1718,6 +1718,9 @@ static inline int accountable_mapping(struct file *file, vm_flags_t vm_flags)
> > >   	if (file && is_file_hugepages(file))
> > >   		return 0;
> > > +	if (arch_shadow_stack_mapping(vm_flags))
> > > +		return 1;
> > > +
> > 
> > What's wrong with testing (vm_flags & VM_SHSTK) here? VM_SHSTK is 0 on
> > non-x86.
> > 
> > >   	return (vm_flags & (VM_NORESERVE | VM_SHARED | VM_WRITE)) == VM_WRITE;
> > >   }
> > > @@ -3387,6 +3390,8 @@ void vm_stat_account(struct mm_struct *mm, vm_flags_t flags, long npages)
> > >   		mm->stack_vm += npages;
> > >   	else if (is_data_mapping(flags))
> > >   		mm->data_vm += npages;
> > > +	else if (arch_shadow_stack_mapping(flags))
> > > +		mm->stack_vm += npages;
> > 
> > Ditto.
> > 
> 
> The thought was, here all testings are done with helpers, e.g.
> is_data_mapping(), so creating a helper for shadow stack is more inline with
> the existing code.  Or, maybe we can call it is_shadow_stack_mapping()?
> And, since we have a helper, use it in accountable_mapping() as well.  Or do
> you have other suggestions?

is_shadow_stack_mapping() sounds reasonable.

My point is that we already have ifdef around #define VM_SHSTK. No need in
duplicating it for helper.

-- 
 Kirill A. Shutemov