Re: [PATCH v6 22/41] mm/mmap: Add shadow stack pages to memory accounting
From: Edgecombe, Rick P
Date: Mon Feb 20 2023 - 17:44:25 EST
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/