interaction of MADV_PAGEOUT with CoW anonymous mappings?

[Jann Horn]

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?