This patch adds security related documentation to the nfs-rdma.txt file
that describes the memory registration model, the potential security
exploits, and compares these exploits to a similar threat when using TCP
as the transport.
Signed-off-by: Tom Tucker <tom@xxxxxxxxxxxxxxxxxxxxx>
---
Documentation/filesystems/nfs-rdma.txt | 126 ++++++++++++++++++++++++++++++++
1 files changed, 126 insertions(+), 0 deletions(-)
diff --git a/Documentation/filesystems/nfs-rdma.txt b/Documentation/filesystems/nfs-rdma.txt
index 44bd766..6abdc6d 100644
--- a/Documentation/filesystems/nfs-rdma.txt
+++ b/Documentation/filesystems/nfs-rdma.txt
@@ -269,3 +269,129 @@ NFS/RDMA Setup
the "proto" field for the given mount.
Congratulations! You're using NFS/RDMA!
+
+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
+ or the transport is IB, then NFSRDMA is no less secure than TCP.
+ However, if the transport is iWARP and the device does not support
+ Fast Memory Registration, then the node above could write anywhere
+ in the server's memory using the method described.
+
+ When the client connects, NFSRDMA sends a string to the message log
+ providing information about the new connection. Examples of this
+ string are shown below:
+
+ svcrdma: New connection accepted with the following attributes:
+ transport : iWARP
+ local_ip : 192.168.70.121
+ local_port : 20049
+ remote_ip : 192.168.70.122
+ remote_port : 44286
+ max_sge : 4
+ sq_depth : 128
+ max_requests : 16
+ ord : 8
+ using fastreg? : Y
+ memory exposure : Safe. Only RPC memory exposed.
+
+ This connection is safe. iWARP is the transport, however, the device
+ supports Fast Memory Registration.
+
+ svcrdma: New connection accepted with the following attributes:
+ transport : iWARP
+ local_ip : 192.168.70.121
+ local_port : 20049
+ remote_ip : 192.168.70.122
+ remote_port : 44286
+ max_sge : 4
+ sq_depth : 128
+ max_requests : 16
+ ord : 8
+ using fastreg? : N
+ memory exposure : Unsafe. Server memory exposed.
+
+ This conneciton is unsafe, the transport is iWARP and Fast Memory
+ Registration is not supported.
+
+ svcrdma: New connection accepted with the following attributes:
+ transport : IB
+ local_ip : 192.168.79.121
+ local_port : 20049
+ remote_ip : 192.168.79.122
+ remote_port : 44287
+ max_sge : 27
+ sq_depth : 128
+ max_requests : 16
+ ord : 4
+ using fastreg? : N
+ memory exposure : Safe. Only RPC memory exposed.
+
+ This connection is safe because although the adapter does not
+ support Fast Memory Registration, the transport is IB.
+
+ These messages can be disabled as follows:
+
+ # echo 0 > /proc/sys/sunrpc/svc_rdma/show_conn_info
+
+ The text below provides 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 still 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.
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