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

[Michal Hocko]

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...

Worth a comment? Probably yes, care to send a patch?
-- 
Michal Hocko
SUSE Labs