On Tue, Mar 10, 2020 at 11:48 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > 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. I'm more worried about the performance implications. Consider libc.so's data section: that's a COW mapping, and we COW it during zygote initialization as we load and relocate libc.so. Child processes shouldn't be dirtying and re-COWing those relocated pages. If I understand Jann's message correctly, MADV_PAGEOUT would force the pages corresponding to the libc.so data segment out to zram just because we MADV_PAGEOUT-ed a single process that happened to use libc. We should leave those pages in memory, IMHO.
