Security Advisory 2001-009: Race condition between sugid-exec and ptrace(2)

Type securityvulns
Reporter Securityvulns
Modified 2001-07-24T00:00:00



             NetBSD Security Advisory 2001-009

Topic: Race condition between sugid-exec and ptrace(2)

Version: All official releases up to and including 1.5

Severity: Local user may gain superuser privileges

Fixed: NetBSD-current: June 15, 2001 NetBSD-1.5 branch: June 17, 2001 (1.5.1 includes the fix) NetBSD-1.4 branch: July 19, 2001


A race condition between the setuid/setgid handling in the execve(2) system call and the ptrace(2) system call can allow a local user to cause a setuid-root executable to execute arbitrary code as the superuser.

Technical Details

The execve(2) system call causes a process to begin execution of a new program image. This system call has a facility known as sugid, which allows certain programs to run with the user and/or group ID indicated by the file system permissions. This facility is used by, e.g. the su(1) program to allow certain users to gain superuser privileges. Once a process uses the sugid facility, it is marked as having done so, in order for other kernel facilities to make decisions based on this information.

The ptrace(2) system call is a debugging facility. It provides the means for debuggers, such as gdb(1), to access the memory and registers of another process. The kernel allows a process to ptrace(2) another process only if they are owned by the same user ID, or if the tracing process has superuser privileges. If a process was exec'd using sugid, tracing of that process is disallowed unless done by the superuser.

The execve(2) code contains a check to skip sugid processing if a process is currently being traced by ptrace(2). However, in vulnerable systems, two problems exist:

    * The check for a traced process in the sugid-exec
      code is performed too early.  The check is made
      before the new executable image is opened, which
      may block.

    * The marking of a process as sugid happens too late.
      The process is marked as sugid after a call which
      allocates memory.  The memory allocation may block.

While the process exec'ing the new program image is asleep in either of these two places, another process owned by the same user can ptrace(2) the process. Once attached to the sugid process, the attacker can modify the sugid process's memory image and modify the program counter register, allowing the attacker to execute arbitrary code.

Solutions and Workarounds

Kernel sources must be updated and a new kernel built and installed. The instructions for updating your kernel sources depend upon which particular NetBSD release you are running.

  • NetBSD-current:
    Systems running NetBSD-current dated from before 2001-06-15
    should be upgraded to NetBSD-current dated 2001-06-15 or later.
    The following source directories need to be updated from
    the netbsd-current CVS branch (aka HEAD):
    Alternatively, apply the following patch (with potential offset

  • NetBSD 1.5:
    Systems running NetBSD 1.5 dated from before 2001-06-17 should be
    upgraded from NetBSD 1.5 sources dated 2001-06-17 or later.
    The following source directories need to be updated from the
    netbsd-1-5 CVS branch:
    Alternatively, apply the following patch (with potential offset

    NetBSD 1.5.1 is not vulnerable.
  • NetBSD 1.4, 1.4.1, 1.4.2, 1.4.3:
    It is believed the 1.4 versions are vulnerable to this issue, but
    a working exploit could not be produced. The following is recommended
    action for 1.4 systems.
    Systems running NetBSD 1.4 dated from before 2001-07-19 should be
    upgraded from NetBSD 1.4 sources dated 2001-07-19 or later.
    The following source directory needs to be updated from
    the netbsd-1-4 CVS branch:
    Alternatively, apply the following patch (with potential offset

Once the kernel sources have been updated, rebuild the kernel, install it, and reboot. For more information on how to do this, see:

Thanks To

Georgi Guninski for pointing out that the problem existed on the Bugtraq mailing list.

Artur Grabowski for some initial discussion about the problem.

Jason R. Thorpe for investigating and fixing the problem.

Bill Sommerfeld for additional discussion and code review.

Revision History

    2001-07-20      Initial revision

More Information

An up-to-date PGP signed copy of this release will be maintained at

Information about NetBSD and NetBSD security can be found at http://www.NetBSD.ORG/ and http://www.NetBSD.ORG/Security/.

Copyright 2001, The NetBSD Foundation, Inc. All Rights Reserved.

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