Re: [PATCH v1 RFC Zisslpcfi 11/20] mmu: maybe_mkwrite updated to manufacture shadow stack PTEs

[David Hildenbrand]

On 13.02.23 15:37, Deepak Gupta wrote:
> On Mon, Feb 13, 2023 at 01:05:16PM +0100, David Hildenbrand wrote:
> > On 13.02.23 05:53, Deepak Gupta wrote:
> > > maybe_mkwrite creates PTEs with WRITE encodings for underlying arch if
> > > VM_WRITE is turned on in vma->vm_flags. Shadow stack memory is a write-
> > > able memory except it can only be written by certain specific
> > > instructions. This patch allows maybe_mkwrite to create shadow stack PTEs
> > > if vma is shadow stack VMA. Each arch can define which combination of VMA
> > > flags means a shadow stack.
> > >
> > > Additionally pte_mkshdwstk must be provided by arch specific PTE
> > > construction headers to create shadow stack PTEs. (in arch specific
> > > pgtable.h).
> > >
> > > This patch provides dummy/stub pte_mkshdwstk if CONFIG_USER_SHADOW_STACK
> > > is not selected.
> > >
> > > Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
> > > ---
> > >   include/linux/mm.h      | 23 +++++++++++++++++++++--
> > >   include/linux/pgtable.h |  4 ++++
> > >   2 files changed, 25 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > index 8f857163ac89..a7705bc49bfe 100644
> > > --- a/include/linux/mm.h
> > > +++ b/include/linux/mm.h
> > > @@ -1093,6 +1093,21 @@ static inline unsigned long thp_size(struct page *page)
> > >   void free_compound_page(struct page *page);
> > >   #ifdef CONFIG_MMU
> > > +
> > > +#ifdef CONFIG_USER_SHADOW_STACK
> > > +bool arch_is_shadow_stack_vma(struct vm_area_struct *vma);
> > > +#endif
> > > +
> > > +static inline bool
> > > +is_shadow_stack_vma(struct vm_area_struct *vma)
> > > +{
> > > +#ifdef CONFIG_USER_SHADOW_STACK
> > > +	return arch_is_shadow_stack_vma(vma);
> > > +#else
> > > +	return false;
> > > +#endif
> > > +}
> > > +
> > >   /*
> > >    * Do pte_mkwrite, but only if the vma says VM_WRITE.  We do this when
> > >    * servicing faults for write access.  In the normal case, do always want
> > > @@ -1101,8 +1116,12 @@ void free_compound_page(struct page *page);
> > >    */
> > >   static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
> > >   {
> > > -	if (likely(vma->vm_flags & VM_WRITE))
> > > -		pte = pte_mkwrite(pte);
> > > +	if (likely(vma->vm_flags & VM_WRITE)) {
> > > +		if (unlikely(is_shadow_stack_vma(vma)))
> > > +			pte = pte_mkshdwstk(pte);
> > > +		else
> > > +			pte = pte_mkwrite(pte);
> > > +	}
> > >   	return pte;
> >
> > Exactly what we are trying to avoid in the x86 approach right now.
> > Please see the x86 series on details, we shouldn't try reinventing the
> > wheel but finding a core-mm approach that fits multiple architectures.
> >
> > https://lkml.kernel.org/r/20230119212317.8324-1-rick.p.edgecombe@xxxxxxxxx
>
> Thanks David for comment here. I looked at x86 approach. This patch
> actually written in a way which is not re-inventing wheel and is following
> a core-mm approach that fits multiple architectures.
>
> Change above checks `is_shadow_stack_vma` and if it returns true then only
> it manufactures shadow stack pte else it'll make a regular writeable mapping.
>
> Now if we look at `is_shadow_stack_vma` implementation, it returns false if
> `CONFIG_USER_SHADOW_STACK` is not defined. If `CONFIG_USER_SHADOW_STACK is
> defined then it calls `arch_is_shadow_stack_vma` which should be implemented
> by arch specific code. This allows each architecture to define their own vma
> flag encodings for shadow stack (riscv chooses presence of only `VM_WRITE`
> which is analogous to choosen PTE encodings on riscv W=1,R=0,X=0)
>
> Additionally pte_mkshdwstk will be nop if not implemented by architecture.
>
> Let me know if this make sense. If I am missing something here, let me know.

See the discussion in that thread. The idea is to pass a VMA to 
pte_mkwrite() and let it handle how to actually set it writable.

--
Thanks,

David / dhildenb