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

[Thomas Gleixner]

On Tue, 23 Apr 2019, Laurent Dufour wrote:
> Le 20/04/2019 Ã 12:31, Michael Ellerman a ÃcritÂ:
> > Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes:
> > > Aside of that the powerpc variant looks suspicious:
> > > 
> > > static inline void arch_unmap(struct mm_struct *mm,
> > >                                unsigned long start, unsigned long end)
> > > {
> > >   	if (start <= mm->context.vdso_base && mm->context.vdso_base < end)
> > >                  mm->context.vdso_base = 0;
> > > }
> > > 
> > > Shouldn't that be:
> > > 
> > >   	if (start >= mm->context.vdso_base && mm->context.vdso_base < end)
> > > 
> > > Hmm?
> > 
> > Yeah looks pretty suspicious. I'll follow-up with Laurent who wrote it.
> > Thanks for spotting it!
> 
> I've to admit that I had to read that code carefully before answering.
> 
> 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).
> 
> The idea is to handle a munmap() call surrounding the VDSO area:
>       | VDSO |
>  ^start         ^end
> 
> This is covered by this test, as the munmap() matching the exact boundaries of
> the VDSO is handled too.
> 
> Am I missing something ?

Well if this is the intention, then you missed to add a comment explaining it :)

Thanks,

	tglx