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Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20190111040434.GN27534@dastard>
Date:   Thu, 10 Jan 2019 20:08:37 -0800
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Dominique Martinet <asmadeus@codewreck.org>,
        Jiri Kosina <jikos@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Jann Horn <jannh@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Michal Hocko <mhocko@suse.com>, Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>
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References: <20190109043906.GF27534@dastard> <CAHk-=wic28fSkwmPbBHZcJ3BGbiftprNy861M53k+=OAB9n0=w@mail.gmail.com> <20190110004424.GH27534@dastard> <CAHk-=wg1jSQ-gq-M3+HeTBbDs1VCjyiwF4gqnnBhHeWizyrigg@mail.gmail.com> <20190110070355.GJ27534@dastard> <CAHk-=wigwXV_G-V1VxLs6BAvVkvW5=Oj+xrNHxE_7yxEVwoe3w@mail.gmail.com> <20190110122442.GA21216@nautica> <CAHk-=wip2CPrdOwgF0z4n2tsdW7uu+Egtcx9Mxxe3gPfPW_JmQ@mail.gmail.com> <20190111020340.GM27534@dastard> <CAHk-=wgLgAzs42=W0tPrTVpu7H7fQ=BP5gXKnoNxMxh9=9uXag@mail.gmail.com> <20190111040434.GN27534@dastard>
To:     Dave Chinner <david@fromorbit.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Jan 10, 2019, at 8:04 PM, Dave Chinner <david@fromorbit.com> wrote:
>=20
>> On Thu, Jan 10, 2019 at 06:18:16PM -0800, Linus Torvalds wrote:
>>> On Thu, Jan 10, 2019 at 6:03 PM Dave Chinner <david@fromorbit.com> wrote=
:
>>>=20
>>>> On Thu, Jan 10, 2019 at 02:11:01PM -0800, Linus Torvalds wrote:
>>>> And we *can* do sane things about RWF_NOWAIT. For example, we could
>>>> start async IO on RWF_NOWAIT, and suddenly it would go from "probe the
>>>> page cache" to "probe and fill", and be much harder to use as an
>>>> attack vector..
>>>=20
>>> We can only do that if the application submits the read via AIO and
>>> has an async IO completion reporting mechanism.
>>=20
>> Oh, no, you misunderstand.
>>=20
>> RWF_NOWAIT has a lot of situations where it will potentially return
>> early (the DAX and direct IO ones have their own), but I was thinking
>> of the one in generic_file_buffered_read(), which triggers when you
>> don't find a page mapping. That looks like the obvious "probe page
>> cache" case.
>>=20
>> But we could literally move that test down just a few lines. Let it
>> start read-ahead.
>>=20
>> .. and then it will actually trigger on the *second* case instead, where w=
e have
>>=20
>>                if (!PageUptodate(page)) {
>>                        if (iocb->ki_flags & IOCB_NOWAIT) {
>>                                put_page(page);
>>                                goto would_block;
>>                        }
>>=20
>> and that's where RWF_MNOWAIT would act.
>>=20
>> It would still return EAGAIN.
>>=20
>> But it would have started filling the page cache. So now the act of
>> probing would fill the page cache, and the attacker would be left high
>> and dry - the fact that the page cache now exists is because of the
>> attack, not because of whatever it was trying to measure.
>>=20
>> See?
>=20
> Except for fadvise(POSIX_FADV_RANDOM) which triggers this code in
> page_cache_sync_readahead():
>=20
>        /* be dumb */
>        if (filp && (filp->f_mode & FMODE_RANDOM)) {
>                force_page_cache_readahead(mapping, filp, offset, req_size)=
;
>                return;
>        }
>=20
> So it will only read the single page we tried to access and won't
> perturb the rest of the message encoded into subsequent pages in
> file.
>=20

There are two types of attacks.  One is an intentional side channel where tw=
o cooperating processes communicate. This is, under some circumstances, a pr=
oblem, but it=E2=80=99s not one we=E2=80=99re about to solve in general. The=
 other is an attacker monitoring an unwilling process. I think we care a lot=
 more about that, and Linus=E2=80=99 idea will help.=