Re: [PATCHv2 3/6] x86/arch_prctl/vdso: add ARCH_MAP_VDSO_*

[Oleg Nesterov <oleg@redhat.com>]

On 07/11, Andy Lutomirski wrote:
>
> On Mon, Jul 11, 2016 at 11:26 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > Do we really care? I mean, the kernel can't crash or something like this,
> > just the old vdso mapping can faultin the "wrong" page from the new
> > vdso_image, right?
>
> That makes me nervous.  IMO a mapping should have well-defined
> semantics.

Perhaps. but map_vdso() will be special anyway, it also changes ->vdso.

For example, if a 32-bit application calls prctl(ARCH_MAP_VDSO) from a
signal handler and we unmap the old vdso mapping, it will crash later
trying to call the (unmapped) restorer == kernel_rt_sigreturn.

> If nothing else, could be really messy if the list of
> pages were wrong.

I do not see anything really wrong, but I can easily miss something.

And don't get me wrong, I agree that any cleanup (say, associate vdso
image with vma) makes sense.

> My real concern is DoS: I doubt that __install_special_mapping gets
> all the accounting right.

Yes, and if it was not clear I fully agree. Even if we forget about the
accounting, I feel that special mappings must not be abused by userspace.

> > So it seems that we should do this by hand somehow. But in fact, what
> > I actually think right now is that I am totally confused and got lost ;)
>
> I'm starting to wonder if we should finally suck it up and give
> special mappings a non-NULL vm_file so we can track them properly.
> Oleg, weren't you thinking of doing that for some other reason?

Yes, uprobes. Currently we can't probe vdso page(s).

Oleg.