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SubjectRe: [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





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