Subject: Re: mm: insure topdown mmap chooses addresses above security minimum

From: Ingo Molnar <[email-masked]>
Date: Wed, 25 Sep 2013 19:44:36 +0200

* Timothy Pepper <[email-masked]> wrote:

> On Wed 25 Sep at 09:30:49 +0200 [email-masked] 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.

Thanks,

	Ingo