linux-kernel - Re: [PATCHv2 1/2] x86/mm: Do not allow non-MAP_FIXED mapping across DEFAULT_MAP

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Date:   Wed, 15 Nov 2017 00:00:46 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Nicholas Piggin <npiggin@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 1/2] x86/mm: Do not allow non-MAP_FIXED mapping across
 DEFAULT_MAP_WINDOW border

On Wed, 15 Nov 2017, Kirill A. Shutemov wrote:
> On Tue, Nov 14, 2017 at 09:54:52PM +0100, Thomas Gleixner wrote:
> > On Tue, 14 Nov 2017, Kirill A. Shutemov wrote:
> > 
> > > On Tue, Nov 14, 2017 at 05:01:50PM +0100, Thomas Gleixner wrote:
> > > > @@ -198,11 +199,14 @@ arch_get_unmapped_area_topdown(struct fi
> > > >  	/* requesting a specific address */
> > > >  	if (addr) {
> > > >  		addr = PAGE_ALIGN(addr);
> > > > +		if (!mmap_address_hint_valid(addr, len))
> > > > +			goto get_unmapped_area;
> > > > +
> > > 
> > > Here and in hugetlb_get_unmapped_area(), we should align the addr after
> > > the check, not before. Otherwise the alignment itself can bring us over
> > > the borderline as we align up.
> > 
> > Hmm, then I wonder whether the next check against vm_start_gap() which
> > checks against the aligned address is correct:
> > 
> >                 addr = PAGE_ALIGN(addr);
> >                 vma = find_vma(mm, addr);
> > 
> >                 if (end - len >= addr &&
> >                     (!vma || addr + len <= vm_start_gap(vma)))
> >                         return addr;
> 
> I think the check is correct. The check is against resulting addresses
> that end up in vm_start/vm_end. In our case we want to figure out what
> user asked for.

Well, but then checking just against the user supplied addr is only half of
the story.

    addr = boundary - PAGE_SIZE - PAGE_SIZE / 2;
    len = PAGE_SIZE - PAGE_SIZE / 2;

That fits, but then after alignment we end up with

    addr = boudary - PAGE_SIZE;

and due to len > PAGE_SIZE this will result in a mapping which crosses the
boundary, right? So checking against the PAGE_ALIGN(addr) should be the
right thing to do.

Thanks,

	tglx