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    SubjectRe: interaction of MADV_PAGEOUT with CoW anonymous mappings?
    On Tue 10-03-20 19:08:28, 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?

    I have brought up side channel attacks earlier [1] but only in the
    context of shared page cache pages. I didn't really consider shared
    anonymous pages to be a real problem. I was under impression that CoW
    pages shouldn't be a real problem because any security sensible
    applications shouldn't allow untrusted code to be forked and CoW
    anything really important. I believe we have made this assumption
    in other places - IIRC on gup with FOLL_FORCE but I admit I have
    very happily forgot most details.

    [1] http://lkml.kernel.org/r/20190619132450.GQ2968@dhcp22.suse.cz

    --
    Michal Hocko
    SUSE Labs

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