linux-kernel - Re: [PATCH] mm/mincore: allow for making sys

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Date:   Thu, 10 Jan 2019 20:08:37 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Dave Chinner <david@...morbit.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Jiri Kosina <jikos@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>, Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged



> On Jan 10, 2019, at 8:04 PM, Dave Chinner <david@...morbit.com> wrote:
> 
>> 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@...morbit.com> wrote:
>>> 
>>>> 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..
>>> 
>>> We can only do that if the application submits the read via AIO and
>>> has an async IO completion reporting mechanism.
>> 
>> Oh, no, you misunderstand.
>> 
>> 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.
>> 
>> But we could literally move that test down just a few lines. Let it
>> start read-ahead.
>> 
>> .. and then it will actually trigger on the *second* case instead, where we have
>> 
>>                if (!PageUptodate(page)) {
>>                        if (iocb->ki_flags & IOCB_NOWAIT) {
>>                                put_page(page);
>>                                goto would_block;
>>                        }
>> 
>> and that's where RWF_MNOWAIT would act.
>> 
>> It would still return EAGAIN.
>> 
>> 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.
>> 
>> See?
> 
> Except for fadvise(POSIX_FADV_RANDOM) which triggers this code in
> page_cache_sync_readahead():
> 
>        /* be dumb */
>        if (filp && (filp->f_mode & FMODE_RANDOM)) {
>                force_page_cache_readahead(mapping, filp, offset, req_size);
>                return;
>        }
> 
> 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.
> 

There are two types of attacks.  One is an intentional side channel where two cooperating processes communicate. This is, under some circumstances, a problem, but it’s not one we’re 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’ idea will help.