From: Daniel Colascione <email@example.com> To: Jann Horn <firstname.lastname@example.org> Cc: Michal Hocko <email@example.com>, Minchan Kim <firstname.lastname@example.org>, Linux-MM <email@example.com>, kernel list <firstname.lastname@example.org>, Dave Hansen <email@example.com>, "Joel Fernandes (Google)" <firstname.lastname@example.org> Subject: Re: interaction of MADV_PAGEOUT with CoW anonymous mappings? Date: Tue, 10 Mar 2020 14:52:55 -0700 Message-ID: <CAKOZues-ejm8kKOp9YaZwDy_HNhvWK12x2cqtqHBYj4DCQtXpQ@mail.gmail.com> (raw) In-Reply-To: <CAG48ez2EWzn_xm0Q3YaPjSXrhVAAOUUvNDyDQ+jpgavmN6wLvQ@mail.gmail.com> On Tue, Mar 10, 2020 at 2:40 PM Jann Horn <email@example.com> wrote: > > On Tue, Mar 10, 2020 at 9:19 PM Daniel Colascione <firstname.lastname@example.org> wrote: > > On Tue, Mar 10, 2020 at 11:48 AM Michal Hocko <email@example.com> wrote: > > > On Tue 10-03-20 19:08:28, Jann Horn wrote: > > > > >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  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. > > Actually, the libc.so data section is a file mapping, so I think > can_do_pageout() would decide whether the caller is allowed to force > pageout based on whether the caller is the owner of (or capable over) > libc (in other words, root, basically). But I think the bss section, > as well as heap memory, could have pageout forced by anyone. lmkd would have that capability though, right? But the point stands regardless. It sounds like both security and performance suggest a behavior change here. Thanks for bringing it up!
next prev parent reply index Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-03-10 18:08 Jann Horn 2020-03-10 18:48 ` Michal Hocko 2020-03-10 19:11 ` Jann Horn 2020-03-10 21:09 ` Michal Hocko 2020-03-10 22:48 ` Dave Hansen 2020-03-11 8:45 ` Michal Hocko 2020-03-11 22:02 ` Minchan Kim 2020-03-11 23:53 ` Shakeel Butt 2020-03-12 0:18 ` Minchan Kim 2020-03-12 2:03 ` Daniel Colascione 2020-03-12 15:15 ` Shakeel Butt 2020-03-10 20:19 ` Daniel Colascione 2020-03-10 21:40 ` Jann Horn 2020-03-10 21:52 ` Daniel Colascione [this message] 2020-03-10 22:14 ` Minchan Kim 2020-03-12 8:22 ` Michal Hocko 2020-03-12 15:40 ` Vlastimil Babka 2020-03-12 20:16 ` Minchan Kim 2020-03-12 20:26 ` Dave Hansen 2020-03-12 20:41 ` Michal Hocko 2020-03-13 2:08 ` Minchan Kim 2020-03-13 8:05 ` Michal Hocko 2020-03-13 20:59 ` Minchan Kim 2020-03-16 9:20 ` Michal Hocko 2020-03-17 1:43 ` Minchan Kim 2020-03-17 7:12 ` Michal Hocko 2020-03-17 15:00 ` Minchan Kim 2020-03-17 15:58 ` Michal Hocko 2020-03-17 17:20 ` Minchan Kim 2020-03-12 21:41 ` Dave Hansen 2020-03-13 2:00 ` Minchan Kim 2020-03-13 16:59 ` Dave Hansen 2020-03-13 21:13 ` Minchan Kim 2020-03-12 23:29 ` Jann Horn
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