Re: [linus:master] [mseal] 8be7258aad: stress-ng.pagemove.page_remaps_per_sec -4.4% regression

[Jeff Xu]

On Mon, Aug 5, 2024 at 6:33 AM Pedro Falcato <pedro.falcato@xxxxxxxxx> wrote:
>
> On Sun, Aug 4, 2024 at 9:33 PM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Sun, 4 Aug 2024 at 01:59, kernel test robot <oliver.sang@xxxxxxxxx> wrote:
> > >
> > > kernel test robot noticed a -4.4% regression of stress-ng.pagemove.page_remaps_per_sec on
> > > commit 8be7258aad44 ("mseal: add mseal syscall")
> >
> > Ok, it's basically just the vma walk in can_modify_mm():
> >
> > >       1.06            +0.1        1.18        perf-profile.self.cycles-pp.mas_next_slot
> > >       1.50            +0.5        1.97        perf-profile.self.cycles-pp.mas_find
> > >       0.00            +1.4        1.35        perf-profile.self.cycles-pp.can_modify_mm
> > >       3.13            +2.0        5.13        perf-profile.self.cycles-pp.mas_walk
> >
> > and looks like it's two different pathways. We have __do_sys_mremap ->
> > mremap_to -> do_munmap -> do_vmi_munmap -> can_modify_mm for the
> > destination mapping, but we also have mremap_to() calling
> > can_modify_mm() directly for the source mapping.
> >
> > And then do_vmi_munmap() will do it's *own* vma_find() after having
> > done arch_unmap().
> >
> > And do_munmap() will obviously do its own vma lookup as part of
> > calling vma_to_resize().
> >
> > So it looks like a large portion of this regression is because the
> > mseal addition just ends up walking the vma list way too much.
>
> Can we rollback the upfront checks "funny business" and just call
> can_modify_vma directly in relevant places? I still don't believe in
> the partial mprotect/munmap "security risks" that were stated in the
> mseal thread (and these operations can already fail for many other
> reasons than mseal) :)
>
In-place check and extra loop, implemented properly, will both prevent
changing to the sealed memory.

However, extra loop will make attacker difficult to call munmap(0,
random large-size), because  if one of vma in the range is sealed, the
whole operation will be no-op.

> I don't mind taking a look myself, just want to make sure I'm not
> stepping on anyone's toes here.
>
One thing that you can't walk around is that can_modify_mm must be
called prior to arch_unmap, that means in-place check for the munmap
is not possible.
( There are recent patch / refactor by Liam R. Howlett in this area,
but I am not sure if this restriction is removed)

> --
> Pedro