Re: [PATCH 4.19 053/105] mm/mincore.c: make mincore() more conservative

[Kevin Easton]

On Wed, May 22, 2019 at 11:21:11AM +0200, Michal Hocko wrote:
> On Wed 22-05-19 10:57:41, Pavel Machek wrote:
> > Hi!
> > 
> > > commit 134fca9063ad4851de767d1768180e5dede9a881 upstream.
> > > 
> > > The semantics of what mincore() considers to be resident is not
> > > completely clear, but Linux has always (since 2.3.52, which is when
> > > mincore() was initially done) treated it as "page is available in page
> > > cache".
> > > 
> > > That's potentially a problem, as that [in]directly exposes
> > > meta-information about pagecache / memory mapping state even about
> > > memory not strictly belonging to the process executing the syscall,
> > > opening possibilities for sidechannel attacks.
> > > 
> > > Change the semantics of mincore() so that it only reveals pagecache
> > > information for non-anonymous mappings that belog to files that the
> > > calling process could (if it tried to) successfully open for writing;
> > > otherwise we'd be including shared non-exclusive mappings, which
> > > 
> > >  - is the sidechannel
> > > 
> > >  - is not the usecase for mincore(), as that's primarily used for data,
> > >    not (shared) text
> > 
> > ...
> > 
> > > @@ -189,8 +205,13 @@ static long do_mincore(unsigned long add
> > >  	vma = find_vma(current->mm, addr);
> > >  	if (!vma || addr < vma->vm_start)
> > >  		return -ENOMEM;
> > > -	mincore_walk.mm = vma->vm_mm;
> > >  	end = min(vma->vm_end, addr + (pages << PAGE_SHIFT));
> > > +	if (!can_do_mincore(vma)) {
> > > +		unsigned long pages = DIV_ROUND_UP(end - addr, PAGE_SIZE);
> > > +		memset(vec, 1, pages);
> > > +		return pages;
> > > +	}
> > > +	mincore_walk.mm = vma->vm_mm;
> > >  	err = walk_page_range(addr, end, &mincore_walk);
> > 
> > We normally return errors when we deny permissions; but this one just
> > returns success and wrong data.
> > 
> > Could we return -EPERM there? If not, should it at least get a
> > comment?
> 
> This was a deliberate decision AFAIR. We cannot return failure because
> this could lead to an unexpected userspace failure. We are pretendeing
> that those pages are present because that is the safest option -
> e.g. consider an application which tries to refault until the page is
> present...

Yes, in particular several userspace applications I found used mincore()
to find out whether a particular range is mapped at all or not, treating
any error as "unmapped" and any non-error return as "mapped".

    - Kevin