Messages in this thread | | | Date | Tue, 12 Feb 2019 04:44:30 +0100 (CET) | From | Jiri Kosina <> | Subject | Re: [PATCH 3/3] mm/mincore: provide mapped status when cached status is not allowed |
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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.
Thanks,
-- Jiri Kosina SUSE Labs
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