Re: CVE-2014-8159 kernel: infiniband: uverbs: unprotected physical memory access

[Yann Droneaud <ydroneaud@opteya.com>]

Hi,

Le vendredi 03 avril 2015 à 08:39 +0000, Haggai Eran a écrit :
> On Thursday, April 2, 2015 11:40 PM, Yann Droneaud <ydroneaud@opteya.com> wrote:
> > Le jeudi 02 avril 2015 à 16:44 +0000, Shachar Raindel a écrit :
> >> > -----Original Message-----
> >> > From: Yann Droneaud [mailto:ydroneaud@opteya.com]
> >> > Sent: Thursday, April 02, 2015 7:35 PM
> > 
> >> > Another related question: as the large memory range could be registered
> >> > by user space with ibv_reg_mr(pd, base, size, IB_ACCESS_ON_DEMAND),
> >> > what's prevent the kernel to map a file as the result of mmap(0, ...)
> >> > in this  region, making it available remotely through IBV_WR_RDMA_READ /
> >> > IBV_WR_RDMA_WRITE ?
> >> >
> >>
> >> This is not a bug. This is a feature.
> >>
> >> Exposing a file through RDMA, using ODP, can be done exactly like this.
> >> Given that the application explicitly requested this behavior, I don't
> >> see why it is a problem.
> > 
> > If the application cannot choose what will end up in the region it has
> > registered, it's an issue !
> > 
> > What might happen if one library in a program call mmap(0, size, ...) to
> > load a file storing a secret (a private key), and that file ends up
> > being mapped in an registered but otherwise free region (afaict, the
> > kernel is allowed to do it) ?
> > What might happen if one library in a program call call mmap(0,
> > size, ..., MAP_ANONYMOUS,...) to allocate memory, call mlock(), then
> > write in this location a secret (a passphrase), and that area ends up
> > in the memory region registered for on demand paging ?
> > 
> > The application haven't choose to disclose these confidential piece of
> > information, but they are available for reading/writing by remote
> > through RDMA given it knows the rkey of the memory region (which is a
> > 32bits value).
> > 
> > I hope I'm missing something, because I'm not feeling confident such
> > behavor is a feature.
> 
> What we are aiming for is the possibility to register the entire process' address 
> space for RDMA operations (if the process chooses to use this feature).
> This is similar to multiple threads accessing the same address space. I'm sure 
> you wouldn't be complaining about the ability of one thread to access the secret 
> passphrase mmapped by another thread in your example.
> 
> > I'm trying to understand how the application can choose what is exposed
> > through RDMA if it registers a very large memory region for later use
> > (but do not actually explicitly map something there yet): what's the
> > consequences ?
> > 
> >    void *start = sbrk(0);
> >    size_t size = ULONG_MAX - (unsigned long)start;
> > 
> >    ibv_reg_mr(pd, start, size, IB_ACCESS_ON_DEMAND)
> 
> The consequences are exactly as you wrote. Just as giving a non-ODP rkey 
> to a remote node allows the node to access the registered memory behind that 
> rkey, giving an ODP rkey to a remote node allows that node to access the 
> virtual address space behind that rkey.
> 

There's a difference: it's impossible to give a valid non-ODP rkey that
point to a memory region not already mapped (backed by a file for 
example), so the application *choose* the content of the memory to be
made accessible remotely before making it accessible.

As I understand the last explanation regarding ODP, at creation time,
an ODP rkey can point to a free, unused, unallocated memory portion.
At this point the kernel can happily map anything the application
(and its libraries) want to map at a (almost) *random* address that
could be in (or partially in) the ODP memory region.

And I have a problem with such random behavior. Allowing this is seems
dangerous and should be done with care.

I believe the application must kept the control of what's end up in its 
ODP registered memory region.

Especially for multi thread program: imagine one thread creating a large
memory region for its future purposes, then send the rkey to a remote 
peer and wait for some work to be done.
In the mean time another call mmap(0, ...) to map a file at a kernel 
chosen address, and that address happen to be in the memory region 
registered by the other thread:

1) the first thread is amputated from a portion of memory it was 
willing to use;
2) the data used by the second thread is accessible to the remote 
peer(s) while not expected.

Speculatively registering memory seems dangerous for any use case I
could think of.

Regards.

-- 
Yann Droneaud
OPTEYA