Re: interaction of MADV_PAGEOUT with CoW anonymous mappings?

[Minchan Kim]

Hi Jann,

On Tue, Mar 10, 2020 at 07:08:28PM +0100, Jann Horn wrote:
> Hi!
> 
> From looking at the source code, it looks to me as if using
> MADV_PAGEOUT on a CoW anonymous mapping will page out the page if
> possible, even if other processes still have the same page mapped. Is
> that correct?
> 
> If so, that's probably bad in environments where many processes (with
> different privileges) are forked from a single zygote process (like
> Android and Chrome), I think? If you accidentally call it on a CoW
> anonymous mapping with shared pages, you'll degrade the performance of
> other processes. And if an attacker does it intentionally, they could
> use that to aid with exploiting race conditions or weird
> microarchitectural stuff (e.g. the new https://lviattack.eu/lvi.pdf
> talks about "the assumption that attackers can provoke page faults or
> microcode assists for (arbitrary) load operations in the victim
> domain").
> 
> Should madvise_cold_or_pageout_pte_range() maybe refuse to operate on
> pages with mapcount>1, or something like that? Or does it already do
> that, and I just missed the check?

Originally, patchset had the mapcount check to filer out shared page
due to performance concern, not security stuff. However, the code
was removed because reviewer asked me "let the shared pages rely on
general LRU" because shared pages would have higher chance to be
touched compared to private pages so naturally, they will keep in
the memory. It did make sense for me.

I am not familiar with the security stuff but if it's really vulnerable
and everyone agree on that, it's easy to add mapcount check there.

Thanks.