Re: [PATCH v5 1/6] mm/page_idle: Add per-pid idle page tracking using virtual index

[Daniel Gruss]

On 8/13/19 5:29 PM, Jann Horn wrote:
> On Tue, Aug 13, 2019 at 12:09 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>> On Mon 12-08-19 20:14:38, Jann Horn wrote:
>>> On Wed, Aug 7, 2019 at 7:16 PM Joel Fernandes (Google)
>>> <joel@xxxxxxxxxxxxxxxxx> wrote:
>>>> The page_idle tracking feature currently requires looking up the pagemap
>>>> for a process followed by interacting with /sys/kernel/mm/page_idle.
>>>> Looking up PFN from pagemap in Android devices is not supported by
>>>> unprivileged process and requires SYS_ADMIN and gives 0 for the PFN.
>>>>
>>>> This patch adds support to directly interact with page_idle tracking at
>>>> the PID level by introducing a /proc/<pid>/page_idle file.  It follows
>>>> the exact same semantics as the global /sys/kernel/mm/page_idle, but now
>>>> looking up PFN through pagemap is not needed since the interface uses
>>>> virtual frame numbers, and at the same time also does not require
>>>> SYS_ADMIN.
>>>>
>>>> In Android, we are using this for the heap profiler (heapprofd) which
>>>> profiles and pin points code paths which allocates and leaves memory
>>>> idle for long periods of time. This method solves the security issue
>>>> with userspace learning the PFN, and while at it is also shown to yield
>>>> better results than the pagemap lookup, the theory being that the window
>>>> where the address space can change is reduced by eliminating the
>>>> intermediate pagemap look up stage. In virtual address indexing, the
>>>> process's mmap_sem is held for the duration of the access.
>>>
>>> What happens when you use this interface on shared pages, like memory
>>> inherited from the zygote, library file mappings and so on? If two
>>> profilers ran concurrently for two different processes that both map
>>> the same libraries, would they end up messing up each other's data?
>>
>> Yup PageIdle state is shared. That is the page_idle semantic even now
>> IIRC.
>>
>>> Can this be used to observe which library pages other processes are
>>> accessing, even if you don't have access to those processes, as long
>>> as you can map the same libraries? I realize that there are already a
>>> bunch of ways to do that with side channels and such; but if you're
>>> adding an interface that allows this by design, it seems to me like
>>> something that should be gated behind some sort of privilege check.
>>
>> Hmm, you need to be priviledged to get the pfn now and without that you
>> cannot get to any page so the new interface is weakening the rules.
>> Maybe we should limit setting the idle state to processes with the write
>> status. Or do you think that even observing idle status is useful for
>> practical side channel attacks? If yes, is that a problem of the
>> profiler which does potentially dangerous things?
> 
> I suppose read-only access isn't a real problem as long as the
> profiler isn't writing the idle state in a very tight loop... but I
> don't see a usecase where you'd actually want that? As far as I can
> tell, if you can't write the idle state, being able to read it is
> pretty much useless.
> 
> If the profiler only wants to profile process-private memory, then
> that should be implementable in a safe way in principle, I think, but
> since Joel said that they want to profile CoW memory as well, I think
> that's inherently somewhat dangerous.

I agree that allowing profiling of shared pages would leak information.
To me the use case is not entirely clear. This is not a feature that
would normally be run in everyday computer usage, right?