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

[Oleg Nesterov]

On 07/10, Andy Lutomirski wrote:
>
> On Thu, Jul 7, 2016 at 4:11 AM, Dmitry Safonov <dsafonov@xxxxxxxxxxxxx> wrote:
> > On 07/06/2016 05:30 PM, Andy Lutomirski wrote:
> >>
> >> On Wed, Jun 29, 2016 at 3:57 AM, Dmitry Safonov <dsafonov@xxxxxxxxxxxxx>
> >> wrote:
> >>>
> >>> Add API to change vdso blob type with arch_prctl.
> >>> As this is usefull only by needs of CRIU, expose
> >>> this interface under CONFIG_CHECKPOINT_RESTORE.
> >>
> >>
> >>> +#ifdef CONFIG_CHECKPOINT_RESTORE
> >>> +       case ARCH_MAP_VDSO_X32:
> >>> +               return do_map_vdso(VDSO_X32, addr, false);
> >>> +       case ARCH_MAP_VDSO_32:
> >>> +               return do_map_vdso(VDSO_32, addr, false);
> >>> +       case ARCH_MAP_VDSO_64:
> >>> +               return do_map_vdso(VDSO_64, addr, false);
> >>> +#endif
> >>> +
> >>
> >>
> >> This will have an odd side effect: if the old mapping is still around,
> >> its .fault will start behaving erratically.

Yes but I am not sure I fully understand your concerns, so let me ask...

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?

The user of prctl(ARCH_MAP_VDSO) should understand what it does and unmap
the old vdso anyway.

> >> I wonder if we can either
> >> reliably zap the old vma (or check that it's not there any more)
> >> before mapping a new one

However, I think this is right anyway, please see below...

> >> or whether we can associate the vdso image
> >> with the vma (possibly by having a separate vm_special_mapping for
> >> each vdso_image.

Yes, I too thought it would be nice to do this, regardless.

But as you said we probably want to limit the numbet of special mappings
an application can create:

> >> I'm also a bit concerned that __install_special_mapping might not get
> >> all the cgroup and rlimit stuff right.  If we ensure that any old
> >> mappings are gone, then the damage is bounded, but otherwise someone
> >> might call this in a loop and fill their address space with arbitrary
> >> numbers of special mappings.

I think you are right, we should not allow user-space to abuse the special
mappings. Even if iiuc in this case only RLIMIT_AS does matter...

> Oleg, want to sanity-check us?  Do you believe that if .mremap ensures
> that only entire vma can be remapped

Yes I think this makes sense. And damn we should kill arch_remap() ;)

> and .close ensures that only the
> whole vma can be unmapped,

How? It can't return the error.

And do_munmap() doesn't necessarily call ->close(),

> Or will we have issues with
> mprotect?

Yes, __split_vma() doesn't call ->close() too. ->open() can't help...

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 ;)

Oleg.