Re: [PATCH -mm v9 0/8] idle memory tracking

[Kees Cook]

On Tue, Jul 21, 2015 at 4:34 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Sun, 19 Jul 2015 15:31:09 +0300 Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> wrote:
>> To mark a page idle one should set the bit corresponding to the
>>    page by writing to the file. A value written to the file is OR-ed with the
>>    current bitmap value. Only user memory pages can be marked idle, for other
>>    page types input is silently ignored. Writing to this file beyond max PFN
>>    results in the ENXIO error. Only available when CONFIG_IDLE_PAGE_TRACKING is
>>    set.
>>
>>    This file can be used to estimate the amount of pages that are not
>>    used by a particular workload as follows:
>>
>>    1. mark all pages of interest idle by setting corresponding bits in the
>>       /proc/kpageidle bitmap
>>    2. wait until the workload accesses its working set
>>    3. read /proc/kpageidle and count the number of bits set
>
> Security implications.  This interface could be used to learn about a
> sensitive application by poking data at it and then observing its
> memory access patterns.  Perhaps this is why the proc files are
> root-only (whcih I assume is sufficient).  Some words here about the
> security side of things and the reasoning behind the chosen permissions
> would be good to have.

As long as this stays true-root-only, I think it should be safe enough.

>>  * /proc/kpagecgroup.  This file contains a 64-bit inode number of the
>>    memory cgroup each page is charged to, indexed by PFN.
>
> Actually "closest online ancestor".  This also should be in the
> interface documentation.
>
>> Only available when CONFIG_MEMCG is set.
>
> CONFIG_MEMCG and CONFIG_IDLE_PAGE_TRACKING I assume?
>
>>
>>    This file can be used to find all pages (including unmapped file
>>    pages) accounted to a particular cgroup. Using /proc/kpageidle, one
>>    can then estimate the cgroup working set size.
>>
>> For an example of using these files for estimating the amount of unused
>> memory pages per each memory cgroup, please see the script attached
>> below.
>
> Why were these put in /proc anyway?  Rather than under /sys/fs/cgroup
> somewhere?  Presumably because /proc/kpageidle is useful in non-memcg
> setups.

Do we need a /proc/vm/ for holding these kinds of things? We're
collecting a lot there. Or invent some way for this to be sensible in
/sys?

-Kees

-- 
Kees Cook
Chrome OS Security
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