> On Jan 10, 2019, at 8:04 PM, Dave Chinner <david@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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.
