Re: [PATCH v1] mm: abstract shadow stack vma behind arch_is_shadow_stack_vma

[Deepak Gupta]

On Wed, Dec 27, 2023 at 1:45 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, 22 Dec 2023 15:51:04 -0800 Deepak Gupta <debug@xxxxxxxxxxxx> wrote:
>
> > x86 has used VM_SHADOW_STACK (alias to VM_HIGH_ARCH_5) to encode shadow
> > stack VMA. VM_SHADOW_STACK is thus not possible on 32bit. Some arches may
> > need a way to encode shadow stack on 32bit and 64bit both and they may
> > encode this information differently in VMAs.
>
> Is such a patch in the pipeline?  Otherwise we're making a change that
> serves no purpose.

Yes I do have patches in the pipeline for riscv.
On riscv, presence of only `VM_WRITE` (i.e. (flags & (VM_READ |
VM_WRITE | VM_EXEC))
== VM_WRITE) would mean a shadow stack.
And yes there would be  relevant patches to ensure that existing consumers using
`PROT_WRITE` gets translated to (VM_WRITE | VM_READ)

>
> > This patch changes checks of VM_SHADOW_STACK flag in generic code to call
> > to a function `arch_is_shadow_stack_vma` which will return true if arch
> > supports shadow stack and vma is shadow stack else stub returns false.
> >
> > ...
> >
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -352,8 +352,21 @@ extern unsigned int kobjsize(const void *objp);
> >   * for more details on the guard size.
> >   */
> >  # define VM_SHADOW_STACK     VM_HIGH_ARCH_5
> > +
> > +static inline bool arch_is_shadow_stack_vma(vm_flags_t vm_flags)
> > +{
> > +     return (vm_flags & VM_SHADOW_STACK) ? true : false;
> > +}
>
> The naming seems a little wrong.  I'd expect it to take a vma* arg.
> Maybe just drop the "_vma"?

Well I did start with taking vma* argument but then realized that
`is_stack_mapping`
only takes vma flags. And in order to change that I would have to
change `vm_stat_account`
and every place it's called.

In the next version I'll either do that or drop `_vma` from the
proposed function name.

>