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

[Laurent Dufour]

Le 03/11/2020 à 22:08, Dmitry Safonov a écrit :
> Hi Laurent, Christophe, Michael, all,
>
> On 11/3/20 5:11 PM, Laurent Dufour wrote:
> > Le 23/10/2020 à 14:28, Christophe Leroy a écrit :
> [..]
> > > > > 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.
> >
> > Thanks Christophe for the advise.
> >
> > That is covering all the cases, and indeed is similar to the Michael's
> > proposal I missed last year.
> >
> > I'll send a patch fixing this issue following your proposal.
>
> It's probably not necessary anymore. I've sent patches [1], currently in
> akpm, the last one forbids splitting of vm_special_mapping.
> So, a user is able munmap() or mremap() vdso as a whole, but not partly.

Hi Dmitry,

That's a good thing too, but I think my patch is still valid in the PowerPC 
code, fixing a bad check, even if some corner cases are handled earlier in the code.

> [1]:
> https://lore.kernel.org/linux-mm/20201013013416.390574-1-dima@xxxxxxxxxx/
>
> Thanks,
>            Dmitry