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

[Laurent Dufour]

Le 20/04/2019 Ã 12:31, Michael Ellerman a ÃcritÂ:
> Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes:
> > On Mon, 1 Apr 2019, Dave Hansen wrote:
> > > diff -puN mm/mmap.c~mpx-rss-pass-no-vma mm/mmap.c
> > > --- a/mm/mmap.c~mpx-rss-pass-no-vma	2019-04-01 06:56:53.409411123 -0700
> > > +++ b/mm/mmap.c	2019-04-01 06:56:53.423411123 -0700
> > > @@ -2731,9 +2731,17 @@ int __do_munmap(struct mm_struct *mm, un
> > >   		return -EINVAL;
> > >   
> > >   	len = PAGE_ALIGN(len);
> > > +	end = start + len;
> > >   	if (len == 0)
> > >   		return -EINVAL;
> > >   
> > > +	/*
> > > +	 * arch_unmap() might do unmaps itself.  It must be called
> > > +	 * and finish any rbtree manipulation before this code
> > > +	 * runs and also starts to manipulate the rbtree.
> > > +	 */
> > > +	arch_unmap(mm, start, end);
> >
> > ...
> >    
> > > -static inline void arch_unmap(struct mm_struct *mm, struct vm_area_struct *vma,
> > > -			      unsigned long start, unsigned long end)
> > > +static inline void arch_unmap(struct mm_struct *mm, unsigned long start,
> > > +			      unsigned long end)
> >
> > While you fixed up the asm-generic thing, this breaks arch/um and
> > arch/unicorn32. For those the fixup is trivial by removing the vma
> > argument.
> >
> > But itt also breaks powerpc and there I'm not sure whether moving
> > arch_unmap() to the beginning of __do_munmap() is safe. Micheal???
>
> I don't know for sure but I think it should be fine. That code is just
> there to handle CRIU unmapping/remapping the VDSO. So that either needs
> to happen while the process is stopped or it needs to handle races
> anyway, so I don't see how the placement within the unmap path should
> matter.

My only concern is the error path.
Calling arch_unmap() before handling any error case means that it will 
have to be undo and there is no way to do so.

I don't know what is the rational to move arch_unmap() to the beginning 
of __do_munmap() but the error paths must be managed.

> > 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 ?

Cheers,
Laurent.