Re: [PATCH 3/3] mm/mincore: provide mapped status when cached status is not allowed

From: Jiri Kosina
Date: Mon Feb 11 2019 - 22:44:39 EST

On Fri, 1 Feb 2019, Vlastimil Babka wrote:

> >> After "mm/mincore: make mincore() more conservative" we sometimes restrict the
> >> information about page cache residency, which we have to do without breaking
> >> existing userspace, if possible. We thus fake the resulting values as 1, which
> >> should be safer than faking them as 0, as there might theoretically exist code
> >> that would try to fault in the page(s) until mincore() returns 1.
> >>
> >> Faking 1 however means that such code would not fault in a page even if it was
> >> not in page cache, with unwanted performance implications. We can improve the
> >> situation by revisting the approach of 574823bfab82 ("Change mincore() to count
> >> "mapped" pages rather than "cached" pages") but only applying it to cases where
> >> page cache residency check is restricted. Thus mincore() will return 0 for an
> >> unmapped page (which may or may not be resident in a pagecache), and 1 after
> >> the process faults it in.
> >>
> >> One potential downside is that mincore() will be again able to recognize when a
> >> previously mapped page was reclaimed. While that might be useful for some
> >> attack scenarios, it's not as crucial as recognizing that somebody else faulted
> >> the page in, and there are also other ways to recognize reclaimed pages anyway.
> >
> > Is this really worth it? Do we know about any specific usecase that
> > would benefit from this change? TBH I would rather wait for the report
> > than add a hard to evaluate side channel.
> Well it's not that complicated IMHO. Linus said it's worth trying, so
> let's see how he likes the result. The side channel exists anyway as
> long as process can e.g. check if its rss shrinked, and I doubt we are
> going to remove that possibility.

So, where do we go from here?

Either Linus and Andrew like the mincore() return value tweak, or this
could be further discussed (*). But in either of the cases, I think
patches 1 and 2 should be at least queued for 5.1.

(*) I'd personally include it as well, as I don't see how it would break
anything, it's pretty straightforward, and brings back some sanity to
mincore() return value.


Jiri Kosina