Re: [GIT PULL] kdbus for 4.1-rc1

[Michal Hocko <mhocko@suse.cz>]

On Wed 22-04-15 12:36:12, Andy Lutomirski wrote:
> On Apr 22, 2015 7:57 AM, "Michal Hocko" <mhocko@suse.cz> wrote:
> >
> > On Tue 21-04-15 11:11:35, Andy Lutomirski wrote:
> > > On Tue, Apr 21, 2015 at 7:27 AM, Michal Hocko <mhocko@suse.cz> wrote:
> > > > On Tue 21-04-15 16:01:01, David Herrmann wrote:
> > > >> Hi
> > > >>
> > > >> On Tue, Apr 21, 2015 at 2:20 PM, Michal Hocko <mhocko@suse.cz> wrote:
> > > >> > On Tue 21-04-15 12:17:49, David Herrmann wrote:
> > > >> >> Hi
> > > >> >>
> > > >> >> On Tue, Apr 21, 2015 at 11:35 AM, One Thousand Gnomes
> > > >> >> <gnomes@lxorguk.ukuu.org.uk> wrote:
> > > >> >> >> On top of that, I think that someone into resource management needs to
> > > >> >> >> seriously consider whether having a broadcast send do get_user_pages
> > > >> >> >> or the equivalent on pages supplied by untrusted recipients (plural!)
> > > >> >> >> is a good idea.
> > > >> >> >
> > > >> >> > Oh but its so much fun if you pass pages belonging to a device driver, or
> > > >> >> > pass bits of a GEM object thereby keeping entire graphics textures
> > > >> >> > referenced 8)
> > > >> >>
> > > >> >> We do not use GUP, nor do we pass around pinned pages. All we use is
> > > >> >> __vfs_read() / __vfs_write() on shmem. Whether generic_file_write() /
> > > >> >> copy_from_user() internally relies on GUP or not, is an orthogonal
> > > >> >> issue that does not belong here.
> > > >> >
> > > >> > It kind of does AFAIU.
> > > >>
> > > >> No, it is not. The issue with GUP is that you elevate the page
> > > >> ref-count and thus prevent lru isolation, sealing, whatsoever.
> > > >
> > > > The point was that such a memory might be not present yet and need a
> > > > page fault with all the side effects - memory reclaim, memcg charge...
> > > >
> > > >> I cannot see how it is related to kdbus. However, ...
> > > >>
> > > >> > If for nothing else then the memcg reasons mentioned in
> > > >> > other email (http://marc.info/?l=linux-kernel&m=142953380508188). If an
> > > >> > untrusted user is allowed to hand over a shmem backed buffer which hasn't
> > > >> > been charged yet (read faulted in) and then kdbus forced to fault it in
> > > >> > a different user's context then you basically allow to hide memory
> > > >> > allocations from the memcg. That is a clear show stopper.
> > > >> >
> > > >> > Or have I misunderstood the way how shmem buffers are used here?
> > > >>
> > > >> ..as you mentioned memcg, lets figure that out here. shmem buffers are
> > > >> used as receive-buffers by kdbus peers. They are read-only to
> > > >> user-space. All allocations are done by the kernel on message passing.
> > > >
> > > > OK, so the shmem buffer is allocated on the kernels behalf and under
> > > > its control and no userspace can hand over one to kdbus. Do I get
> > > > it right? If yes then the memcg escape I was describing above is
> > > > not possible of course. This wasn't clear to me from the previous
> > > > discussion. Thanks for the clarification!
> > >
> > > I'm still missing something here, I think.  At the time of pool
> > > creation, the kernel calls shmem_file_setup in the context of the
> > > untrusted user.  Then, when a privileged daemon broadcasts, the kernel
> > > calls vfs_iter_write or similar, thus allocating the page, right?  I
> > > don't see why the page would be allocated early or why vfs_iter_write
> > > and the associated shmem code would care what memcg created the shmem
> > > file -- all of that code seems to use current's memcg on brief
> > > inspection.
> >
> > Yes it is the current task on the first charge or the original memcg on
> > the swap in. But my understanding from the above, and I haven't read the
> > code yet, is that the untrusted userspace is only reader from the buffer
> > and isn't allowed to modify the buffer.
> >
> > > Bear in mind that the bad guy gets to use madvise, etc to mess around
> > > with the page cache state.
> >
> > How can an untrusted user play with shmem when it is read-only?
> > shmem_file_setup shouls create an unlinked file so no process can access
> > it via tmpfs AFAIU and potentially fault the memory before the producent
> > will fill it up (thus fault in in the trusted context). I have no idea
> > how the receiver gets to the buffer though.
> >
> 
> The receiver gets to mmap the buffer.  I'm not sure what protection they get.

OK, so I've checked the code. kdbus_pool_new sets up a shmem file
(unlinked) so not visible externally. The consumer will get it via mmap
on the endpoint file by kdbus_pool_mmap and it refuses VM_WRITE and
clears VM_MAYWRITE. The receiver even doesn't have access to the shmem
file directly.

It is ugly that kdbus_pool_mmap replaces the original vm_file and make
it point to the shmem file. I am not sure whether this is safe all the
time and it would deserve a big fat comment. On the other hand, it seems
some drivers are doing this already (e.g. dma_buf_mmap).

> The thing I'm worried about is that the receiver might deliberately
> avoid faulting in a bunch of pages and instead wait for the producer
> to touch them, causing pages that logically belong to the receiver to
> be charged to the producer instead.

Hmm, now that I am looking into the code it seems you are right. E.g.
kdbus_cmd_send runs in the context of the sender AFAIU. This gets down
to kdbus_pool_slice_copy_iovec which does vfs_iter_write and this
is where we get to charge the memory. AFAIU the terminology all the
receivers will share the same shmem file when mmaping the endpoint.

This, however, doesn't seem to be exploitable to hide memory charges
because the receiver cannot make the buffer writable. A nasty process
with a small memcg limit could still pre-fault the memory before any
writer gets sends a message and slow the whole endpoint traffic. But
that wouldn't be a completely new thing because processes might hammer
on memory even without memcg... It is just that this would be kind of
easier with memcg.
If that is the concern then the buffer should be pre-charged at the time
when it is created.

-- 
Michal Hocko
SUSE Labs