Linux RDS Protocol
Local Privilege Escalation
| Release Date | 2010-10-19 |
| Application | Linux Kernel |
| Versions | 2.6.30 - 2.6.36-rc8 |
| Severity | High |
| Discovered by | Dan Rosenberg <drosenberg {at} vsecurity.com> |
| Vendor Status | Patch Released [3] |
| CVE Candidate | CVE-2010-3904 |
| Reference | Original Advisory |
Product Description
From [1]:
"Linux is a free Unix-type operating system originally created by Linus Torvalds with the assistance of developers around the world. Developed under the GNU General Public License, the source code for Linux is freely available to everyone."
From [2]:
"Reliable Datagram Sockets (RDS) provide in order, non-duplicating, highly available, low overhead, reliable delivery of datagrams between hundreds of thousands of non-connected endpoints."
Vulnerability Overview
On October 13th, VSR identified a vulnerability in the RDS protocol, as implemented in the Linux kernel. Because kernel functions responsible for copying data between kernel and user space failed to verify that a user-provided address actually resided in the user segment, a local attacker could issue specially crafted socket function calls to write abritrary values into kernel memory. By leveraging this capability, it is possible for unprivileged users to escalate privileges to root.
Vulnerability Details
On Linux, recvmsg() style socket calls are performed using iovec structs, which
allow a user to specify a base address and size for a buffer used to receive
socket data. Each packet family is responsible for defining functions that
copy socket data, which is received by the kernel, back to user space to allow
user programs to process and handle received network data.
When performing this copying of data to user space, the RDS protocol failed to
verify that the base address of a user-provided iovec struct pointed to a valid
userspace address before using the __copy_to_user_inatomic() function to copy
the data. As a result, by providing a kernel address as an iovec base and
issuing a recvmsg() style socket call, a local user could write arbitrary data
into kernel memory. This can be leveraged to escalate privileges to root.
Proof-of-Concept Exploit
VSR has developed a proof-of-concept exploit [4] to
both demonstrate the severity of this issue as well as allow users and
administrators to verify the existence of the vulnerability. The exploit
leverages the ability to write into kernel memory to reset the kernel's
security operations structure and gain root privileges. The exploit requires
that kernel symbol resolution is available to unprivileged users, via
/proc/kallsyms or similar, as is the case on most stock
distributions. It has been tested on both 32-bit and 64-bit x86 platforms.
While this exploit has been reliable during testing, it is not advised to run
kernel exploits on production systems, as there is a risk of causing system
instability and crashing the affected machine.
Versions Affected
This vulnerability affects unpatched versions of the Linux kernel, starting
from 2.6.30, where the RDS protocol was first included. Installations are only
vulnerable if the CONFIG_RDS kernel configuration option is set, and if there
are no restrictions on unprivileged users loading packet family modules, as is
the case on most stock distributions.
Vendor Response
The following timeline details Linux's response to the reported issue:
| 2010-10-13 | Vulnerability reported to Linux security team |
| 2010-10-13 | Response, agreement on disclosure date |
| 2010-10-19 | Fix publicly committed [3] |
| 2010-10-19 | Coordinated disclosure |
Recommendation
Users should install updates provided by downstream distributions or apply the committed patch [3] and recompile their kernel.
Preventing the RDS kernel module from loading is an effective workaround. This can be accomplished by executing the following command as root:
echo "alias net-pf-21 off" > /etc/modprobe.d/disable-rds
Common Vulnerabilities and Exposures (CVE) Information
The Common Vulnerabilities and Exposures (CVE) project has assigned the number CVE-2010-3904 to this issue. This is a candidate for inclusion in the CVE list (http://cve.mitre.org), which standardizes names for security problems.
Acknowledgements
Thanks to Andrew Morton, Linus Torvalds, Andy Grove, and Eugene Teo for their prompt responses and patch.
References
| 1. | Linux kernel http://www.linux.org |
| 2. | Reliable Datagram Sockets http://oss.oracle.com/pipermail/rds-devel/2007-November/000228.html |
| 3. | GIT patch http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=799c10559d60f159ab2232203f222f18fa3c4a5f |
| 4. | RDS protocol privilege escalation exploit http://www.vsecurity.com/download/tools/linux-rds-exploit.c |
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Copyright 2010 Virtual Security Research, LLC. All rights reserved.
