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

[Yann Droneaud]

Hi,

Le jeudi 02 avril 2015 Ã 16:44 +0000, Shachar Raindel a Ãcrit :
> > -----Original Message-----
> > From: Yann Droneaud [mailto:ydroneaud@xxxxxxxxxx]
> > 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.


> Actually, some of our tests use such flows.
> The mmu notifiers mechanism allow us to do this safely. When the page is
> written back to disk, it is removed from the ODP mapping. When it is
> accessed by the HCA, it is brought back to RAM.
> 

I'm not discussing about the benefit of On Demand Paging and why it's a
very good feature to expose files through RDMA.

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)


Regards.

-- 
Yann Droneaud
OPTEYA


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