Re: [PATCH 10/10] svcrdma: Documentation update for the FastReg memory model

[Tom Tucker <tom@opengridcomputing.com>]

J. Bruce Fields wrote:
> Thanks, I think this is much more helpful.
> 
> On Tue, Sep 30, 2008 at 03:17:21PM -0500, Tom Tucker wrote:
>> +Security
>> +--------
>> +
>> +  NFSRDMA exploits the RDMA capabilities of the IB and iWARP
>> +  transports to more efficiently exchange RPC data between the client
>> +  and the server. This section discusses the security implications of
>> +  the exchange of memory information on the wire when the wire may be
>> +  monitorable by an untrusted application. The identifier that
>> +  encapsulates this memory information is called an RKEY.
>> +
>> +  A principal exploit is that a node on the local network could snoop
>> +  RDMA packets containing RKEY and then forge a packet with this RKEY
>> +  to write and/or read the memory of the peer to which the RKEY
>> +  referred.
>> +
>> +  If the underlying RDMA device is capable of Fast Memory
>> +  Registration, then NFSRDMA is no less secure than TCP with
>> +  auth_unix. However, if the device does not support Fast Memory
>> +  Registration, then such a node could write anywhere in the server's
>> +  memory using the method above. At mount time, the server sends a
> 
> The server doesn't really know about mounts, especially not at this
> level, so I assume you mean either server start time or client connect
> time?

Right, client connect time, I'll fix. Thanks.

> 
>> +  string to the message log to indicate whether or not Fast Memory
>> +  Registration is being used. If Fast Memory Registration is being
>> +  used, the string
>> +
>> +	"svcrdma: Using Fast Memory Registration"
>> +
>> +  is logged, otherwise,
>> +
>> +	"svcrdma: Using a Global DMA MR"
>> +
>> +  will be logged.
> 
> It'd be nicer to have something that can be queried by a program--a file
> in proc or nfsd, for example--without having to grep through log files.
> (Or is it possible the drivers already export enough information under
> sysfs someplace to figure this out with a simple script?)

Yes, it's gross. But I was trying to keep it simple for the first go-round and
since it is conceivable that you have two adapters, one that supports FRMR and
the other doesn't, you would need a proc file per adapter. All my systems have
both iWARP and IB adapters in them. So half my connections are DMA MR and the
other FRMR.

> 
> Or maybe the non-fast registration stuff should be under a separate
> configuration option entirely?  Distro's could eventually enable only
> the safer configurations and people doing testing could build their own
> kernels with the rest enabled.

Perhaps, or maybe a module option that specifically disables DMA_MR. Also
note that with IB the DMA MR is RKEY is not put on the wire so I think I
need to qualify the warning somewhat.

> 
> My initial impulse is to be a bit scared of the non-fast-registration
> case, but maybe I don't understand how this hardware is deployed.
> 

In practice, I think the exposure is real, but somewhat academic.
Obviously as this sees wider adoption the likelihood that this could be
deployed on a network with untrusted hosts grows significantly. Today
I don't believe that's the case.

I would lean towards the module option and a perhaps a Kconfig option that
allows you to tweak the default. I also think the policy should be transport
dependent. IOW, DMA MR is OK for IB, but verboten for iWARP.

Thanks for the feedback,
Tom

> --b.
> 
>> +
>> +  The sections below provide additional information on this issue.
>> +
>> +  The NFSRDMA protocol is defined such that a) only the server
>> +  initiates RDMA, and b) only the client's memory is exposed via
>> +  RKEY. This is why the server reads to fetch RPC data from the client
>> +  even though it would be more efficient for the client to write the
>> +  data to the server's memory. This design goal is not entirely
>> +  realized with iWARP, however, because the RKEY (called an STag on
>> +  iWARP) for the data sink of an RDMA_READ is actually placed on the
>> +  wire, and this RKEY has Remote Write permission. This means that the
>> +  server's memory is exposed by virtue of having placed the RKEY for
>> +  its local memory on the wire in order to receive the result of the
>> +  RDMA_READ.
>> +
>> +  By contrast, IB uses an opaque transaction ID# to associate the
>> +  READ_RPL with the READ_REQ and the data sink of an READ_REQ does not
>> +  require remote access. That said, the byzantine node in question
>> +  could forge a packet with this transaction ID and corrupt the target
>> +  memory, however, the scope of the exploit is bounded to the lifetime
>> +  of this single RDMA_READ request and to the memory mapped by the
>> +  data sink of the READ_REQ.
>> +
>> +  The newer RDMA adapters (both iWARP and IB) support "Fast Memory
>> +  Registration". This capability allows memory to be quickly
>> +  registered (i.e. made available for remote access) and de-registered
>> +  by submitting WR on the SQ. These capabilities provide a mechanism
>> +  to reduce the exposure discused above by limiting the scope of the
>> +  exploit. The idea is to create an RKEY that only maps the single RPC
>> +  and whose effective lifetime is only the exchange of this single
>> +  RPC. This is the default memory model that is employed by the server
>> +  when supported by the adapter and by the client when the
>> +  rdma_memreg_strategy is set to 6. Note that the client and server
>> +  may use different memory registration strategies, however,
>> +  performance is better when both the client and server use the
>> +  FastReg memory registration strategy.
>> +
>> +  This approach has two benefits, a) it restricts the domain of the
>> +  exploit to the memory of a single RPC, and b) it limits the duration
>> +  of the exploit to the time it takes to satisfy the RDMA_READ.
>> +
>> +  It is arguable that a one-shot STag/RKEY is no less secure than RPC
>> +  on the TCP transport. Consider that the exact same byzantine
>> +  application could more easily corrupt TCP RPC payload by simply
>> +  forging a packet with the correct TCP sequence number -- in fact
>> +  it's easier than the RDMA exploit because the RDMA exploit requires
>> +  that you correctly forge both the TCP packet and the RDMA
>> +  payload. In addition the duration of the TCP exploit is the lifetime
>> +  of the connection, not the lifetime of a single WR/RPC data transfer.
>> +
>> +  RDMA on IB or iWARP using Fast Reg is no less secure than TCP.
>> +
>>
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