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

[Laurent Dufour]

Le 23/10/2020 à 14:28, Christophe Leroy a écrit :
> 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.

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.

Cheers,
Laurent.