Re: [PATCH 1/3] mm/mincore: make mincore() more conservative

[Jiri Kosina]

On Wed, 6 Mar 2019, Andrew Morton wrote:

> > 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.
> 
> "for writing" comes as a bit of a surprise.  Why not for reading?

I guess this is a rhetorical question from you :) but fair enough, good 
point, I'll explain this a bit more in the changelog and in the code 
comments.

> > @@ -189,8 +197,13 @@ static long do_mincore(unsigned long addr, unsigned long pages, unsigned char *v
> >  	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 = (end - addr) >> PAGE_SHIFT;
> 
> I'm not sure this is correct in all cases.   If
> 
> 	addr = 4095
> 	vma->vm_end = 4096
> 	pages = 1000
> 
> then `end' is 4096 and `(end - addr) << PAGE_SHIFT' is zero, but it
> should have been 1.

Good catch! It should rather be something like

	unsigned long pages = (end >> PAGE_SHIFT) - (addr >> PAGE_SHIFT);

I'll fix that up and resend tomorrow.

Thanks,

-- 
Jiri Kosina
SUSE Labs