On Tue, Aug 13, 2019 at 05:34:16PM +0200, Daniel Gruss wrote: > 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. Will think more about it. If we limit it to private pages, then it could become useless. Consider a scenario where: A process allocates a some memory, then forks a bunch of worker processes that read that memory and perform some work with them. Per-PID page idle tracking is now run on the parent processes. Now it should appear that the pages are actively accessed (not-idle). If we don't track shared pages, then we cannot detect if those pages are really due to memory leaking, or if they are there for a purpose and are actively used. > To me the use case is not entirely clear. This is not a feature that > would normally be run in everyday computer usage, right? Generally, this to be used as a debugging feature that helps developers detect memory leaks in their programs. thanks, - Joel
