mm: insure topdown mmap chooses addresses above security minimum

[Timothy Pepper]

On Wed 25 Sep at 19:44:36 +0200 mingo at kernel.org said:
> 
> * Timothy Pepper <timothy.c.pepper at linux.intel.com> wrote:
> 
> > On Wed 25 Sep at 09:30:49 +0200 mingo at kernel.org said:
> > > >  	info.flags = VM_UNMAPPED_AREA_TOPDOWN;
> > > >  	info.length = len;
> > > > -	info.low_limit = PAGE_SIZE;
> > > > +	info.low_limit = max(PAGE_SIZE, PAGE_ALIGN(mmap_min_addr));
> > > >  	info.high_limit = mm->mmap_base;
> > > >  	info.align_mask = filp ? get_align_mask() : 0;
> > > >  	info.align_offset = pgoff << PAGE_SHIFT;
> > > 
> > > There appears to be a lot of repetition in these methods - instead of 
> > > changing 6 places it would be more future-proof to first factor out the 
> > > common bits and then to apply the fix to the shared implementation.
> > 
> > Besides that existing redundancy in the multiple somewhat similar
> > arch_get_unmapped_area_topdown() functions, I was expecting people might
> > question the added redundancy of the six instances of:
> > 
> > 	max(PAGE_SIZE, PAGE_ALIGN(mmap_min_addr));
> 
> That redundancy would be automatically addressed by my suggestion.

Yes.

I'm looking at the cleanup and will post a bisectable series that
introduces a common helper, addes the calls to use that helper where
applicable (looks like it might be a few dozen per arch locations), and
then the single line change for the topdown case within the common helper
to do:

	info->low_limit = max(PAGE_SIZE, PAGE_ALIGN(mmap_min_addr));

-- 
Tim Pepper <timothy.c.pepper at linux.intel.com>
Intel Open Source Technology Center