Denial of Service Vulnerability in NFSv4 Client Kernel Module

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Denial of Service Vulnerability in NFSv4 Client Kernel Module:

1. Impact

A security vulnerability in the NFSv4 client kernel module may allow a local unprivileged user who cooperates with a remote privileged user on an NFSv4 server to be able to cause all NFSv4 mounts on client systems which have an NFSv4 mount of  the above NFSv4 server to become unresponsive.  This is a type of Denial of Service (DoS).

2. Contributing Factors

This issue can occur in the following releases:

SPARC Platform

• Solaris 10 without patch 137111-05
• OpenSolaris based upon builds snv_01 through snv_36

x86 Platform

• Solaris 10 without patch 137112-05
• OpenSolaris based upon builds snv_01 through snv_36

Note 1: Solaris 8 and 9 are not impacted by this issue.

Note 2: To determine what version of  NFS an NFS mount is using the nfsstat(1M) command can be used:

    $ nfsstat -m /mnt/point
    Flags: vers=4,proto=tcp,sec=sys,hard,intr,link,symlink,acl,rsize=1048576,wsize=1048576,retrans=10,timeo=600
    Attr cache: acregmin=3,acregmax=60,acdirmin=30,acdirmax=60

The number after the "vers=" string indicates the version of NFS in use for the mounted file system.

Note 3:  This issue only affects NFS environments where NFSv4 is in use as well as automountd(1M).  Solaris 10 and later NFS clients and servers default to NFSv4.  This is configurable by editing the /etc/default/nfs file (see nfs(4)).

To determine if the autofs mount/unmount daemon is enabled,  the following command can be run:

    $ svcs svc:/system/filesystem/autofs:default
    STATE          STIME    FMRI
    online         Aug_07   svc:/system/filesystem/autofs:default

Note 4:  OpenSolaris distributions may include additional bug fixes above and beyond the build from which it was derived. 

To determine the base build of  OpenSolaris, the following command can be used:

    $ uname -v
    snv_86

3. Symptoms

If the described issue is exploited, all NFSv4 mounts on systems which have an NFSv4 mount on an NFSv4 server which has been compromised will become unresponsive. Depending on the file system configuration, this may lead to a system hang.

4. Workaround

To work around the described issue, NFS client systems can be configured not to use NFSv4 by setting "NFS_CLIENT_VERSMAX=3" in "/etc/default/nfs". Please refer to nfs(4) documentation.

5. Resolution

This issue is resolved in the following releases:

SPARC Platform

• Solaris 10 with patch 137111-05 or later
• OpenSolaris based upon builds snv_37 or later

x86 Platform

• Solaris 10 with patch 137112-05 or later
• OpenSolaris based upon builds snv_37 or later
  

For more information on Security Sun Alerts, see Technical Instruction ID 213557.

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