Re: [PATCH] x86/mpx: fix recursive munmap() corruption

[Christophe Leroy]

Hi Laurent

Le 07/05/2019 à 18:35, Laurent Dufour a écrit :
> Le 01/05/2019 à 12:32, Michael Ellerman a écrit :
> > Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx> writes:
> > > Le 23/04/2019 à 18:04, Dave Hansen a écrit :
> > > > On 4/23/19 4:16 AM, Laurent Dufour wrote:
> > ...
> > > > > There are 2 assumptions here:
> > > > >    1. 'start' and 'end' are page aligned (this is guaranteed by __do_munmap().
> > > > >    2. the VDSO is 1 page (this is guaranteed by the union vdso_data_store on powerpc)
> > > >
> > > > Are you sure about #2?  The 'vdso64_pages' variable seems rather
> > > > unnecessary if the VDSO is only 1 page. ;)
> > >
> > > Hum, not so sure now ;)
> > > I got confused, only the header is one page.
> > > The test is working as a best effort, and don't cover the case where
> > > only few pages inside the VDSO are unmmapped (start >
> > > mm->context.vdso_base). This is not what CRIU is doing and so this was
> > > enough for CRIU support.
> > >
> > > Michael, do you think there is a need to manage all the possibility
> > > here, since the only user is CRIU and unmapping the VDSO is not a so
> > > good idea for other processes ?
> >
> > Couldn't we implement the semantic that if any part of the VDSO is
> > unmapped then vdso_base is set to zero? That should be fairly easy, eg:
> >
> >     if (start < vdso_end && end >= mm->context.vdso_base)
> >         mm->context.vdso_base = 0;
> >
> >
> > We might need to add vdso_end to the mm->context, but that should be OK.
> >
> > That seems like it would work for CRIU and make sense in general?
>
> Sorry for the late answer, yes this would make more sense.
>
> Here is a patch doing that.

In your patch, the test seems overkill:

+	if ((start <= vdso_base && vdso_end <= end) ||  /* 1   */
+	    (vdso_base <= start && start < vdso_end) || /* 3,4 */
+	    (vdso_base < end && end <= vdso_end))       /* 2,3 */
+		mm->context.vdso_base = mm->context.vdso_end = 0;

What about

	if (start < vdso_end && vdso_start < end)
		mm->context.vdso_base = mm->context.vdso_end = 0;

This should cover all cases, or am I missing something ?


And do we really need to store vdso_end in the context ?
I think it should be possible to re-calculate it: the size of the VDSO should be (&vdso32_end - 
&vdso32_start) + PAGE_SIZE for 32 bits VDSO, and (&vdso64_end - &vdso64_start) + PAGE_SIZE for the 
64 bits VDSO.

Christophe