Re: [PATCH v6 22/41] mm/mmap: Add shadow stack pages to memory accounting

[David Hildenbrand]

On 20.02.23 23:44, Edgecombe, Rick P wrote:
> On Mon, 2023-02-20 at 13:58 +0100, David Hildenbrand wrote:
> > On 18.02.23 22:14, Rick Edgecombe wrote:
> > > From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> > >
> > > The x86 Control-flow Enforcement Technology (CET) feature includes
> > > a new
> > > type of memory called shadow stack. This shadow stack memory has
> > > some
> > > unusual properties, which requires some core mm changes to function
> > > properly.
> > >
> > > Account shadow stack pages to stack memory.
> > >
> > > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > > Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
> > > Tested-by: John Allen <john.allen@xxxxxxx>
> > > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> > > Co-developed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> > > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> > > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > >
> > > ---
> > > v3:
> > >     - Remove unneeded VM_SHADOW_STACK check in accountable_mapping()
> > >       (Kirill)
> > >
> > > v2:
> > >     - Remove is_shadow_stack_mapping() and just change it to
> > > directly bitwise
> > >       and VM_SHADOW_STACK.
> > >
> > > Yu-cheng v26:
> > >     - Remove redundant #ifdef CONFIG_MMU.
> > >
> > > Yu-cheng v25:
> > >     - Remove #ifdef CONFIG_ARCH_HAS_SHADOW_STACK for
> > > is_shadow_stack_mapping().
> > > ---
> > >     mm/mmap.c | 2 ++
> > >     1 file changed, 2 insertions(+)
> > >
> > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > index 425a9349e610..9f85596cce31 100644
> > > --- a/mm/mmap.c
> > > +++ b/mm/mmap.c
> > > @@ -3290,6 +3290,8 @@ void vm_stat_account(struct mm_struct *mm,
> > > vm_flags_t flags, long npages)
> > >                 mm->exec_vm += npages;
> > >         else if (is_stack_mapping(flags))
> > >                 mm->stack_vm += npages;
> > > +     else if (flags & VM_SHADOW_STACK)
> > > +             mm->stack_vm += npages;
> >
> > Why not modify is_stack_mapping() ?
>
> It kind of sticks out a little in this conditional, but
> is_stack_mapping() has this comment:
> /*
>   * Stack area - automatically grows in one direction
>   *
>   * VM_GROWSUP / VM_GROWSDOWN VMAs are always private anonymous:
>   * do_mmap() forbids all other combinations.
>   */
>
> Shadow stack don't grow, so it doesn't quite fit. There used to be an
> is_shadow_stack_mapping(), but it was removed because all that was
> needed (for the time being) was the simple bitwise AND:
>
> https://lore.kernel.org/lkml/804adbac-61e6-0fd2-f726-5735fb290199@xxxxxxxxx/

As there is only a single user of is_stack_mapping(), I'd simply have 
adjusted the doc of is_stack_mapping() to include shadow stacks.

--
Thanks,

David / dhildenb