Re: [RFC] Adding a SECURITY policy for GDB

[Andrew Burgess <aburgess@redhat.com>]

Hello!

Apologies for taking so long to get a second version of this document
prepared.  I've been through several iterations of this text since I
last posted trying to get something semi-reasonable... It would be great
to get your feedback on this new text.

This is basically a complete rewrite since V1, I'll try cover how this
new text addresses the feedback on V1:

 1. A part of the original hope was that I could write some text that
    would allow bugs triggered by just loading a binary into GDB (but not
    running it) as non-security bugs.  Why?  Well, most of the bugs we
    see in this area are from folk throwing fuzzed binaries at GDB,
    raising a bug, and then raising a CVE on the back of that.  The
    first part of this process is useful for sure.  The CVE part is just
    a resource drain for downstream distributions.

    I did make several attempts to write some text that would classify
    bugs in this way ... but, to be honest, after about my fourth
    attempt, all I'd managed to do was convince myself that this was not
    a good idea.

    So, instead, I've just given up on that.  I think Simon, Paul, and
    Felix should find (at least this part of) the new text satisfactory;
    loading a binary, but not executing it, should be safe to do, and if
    that's not the case, then this is a security issue.

 2. As an aside, but not really relevant to the new text, given my
    evolving position, I would say that I now disagree with the binutils
    SECURITY position w.r.t. tools like objdump and readelf that Felix
    mentioned.  I think it's not unreasonable that a user might run an
    analysis tool on an untrusted binary, and if such a tool can be
    tricked (via a bug) into executing code, even with the permissions
    of the original user, then I would see this as a security risk.

 3. Kevin asked for discussion about running crash diagnostic tools that
    wrap around GDB.  I believe this is covered by the new text which
    discusses in more detail how to safely run against trusted vs
    untrusted binaries.  Kevin: If you feel there are still things
    missing that you'd like to see, then maybe you could suggest at
    least some bullet points for what should be said, and I'll have a go
    at fleshing it out.

One last thing, while writing this, I did wonder if this text would be
better moved into the GDB manual, and the gdb/SECURITY.txt document
should just say "See the GDB manual", but I figure that's a problem for
future me, for now I just need to find some words we can all agree on.

Thanks,
Andrew

----

GNU Debugger Security Policy
============================

Introduction
------------

  The GNU Debugger (GDB) is a tool for diagnosing issues "inside"
  another program.  This can be done by controlling the execution of
  the program being debugged and allowing the user to inspect and
  modify the state of the running program.  Or GDB can be used to
  analyse the program or a core file generated from the program
  without needing to execute the program.

  The program being debugged may be local i.e. on the system on which
  GDB runs or remote, on a different system.

Policy Objectives
-----------------

  The objective of this policy is define what the GDB project
  considers a security bug and what is a non-security bug, and how
  each category of bug should be reported.

  Additionally, this policy will give some general advice to users on
  how to use GDB securely.

Scope Of This Policy
--------------------

  This policy covers all currently supported versions of GDB as
  released from the official GDB website and covers gdb, gdbserver, as
  well as gcore and gdb-add-index, which are packaged with each GDB
  release.  The official GDB website can be found here:

    https://sourceware.org/gdb/

  Remote debugging uses GDB to connect to a remote target.  GDB sends
  commands to the remote target which then controls the remote target.
  The GDB project provides one remote target, gdbserver, which is
  included with official GDB releases.  Bugs within gdbserver are in
  scope for this policy.

  Other projects also implement remote targets to which GDB can
  connect.  Any bugs in these remote targets are out of scope for this
  policy and should be reported to the relevant project.  Any bugs in
  GDB caused by a misbehaving remote target, even when that target is
  not gdbserver, are in scope for this policy.

What Is Not A Security Bug?
---------------------------

  In the context of local debugging, when GDB is used to execute a
  program, the program runs with the same privileges as GDB itself.
  Running a program through GDB is no different than running a program
  directly.  If debugging an untrusted program then the user should
  ensure that this is done in a secure environment.  Additionally, it
  is possible for a program to detect when it is run under GDB and to
  change its behaviour, unwanted behaviour may only appear when a
  program is run under GDB.

  Any issues that arise from running an untrusted binary are not
  security bugs in GDB.

  In the context of remote debugging, the program being debugged is
  run with the same privileges as gdbserver.  As with GDB in the local
  debugging case, any issues that arise from running an untrusted
  binary via gdbserver are not not security bugs in gdbserver.

  The connection between GDB and a remote target is not protected by
  either authentication, or encryption.  Connecting to a remote target
  allows for arbitrary code execution on the remote system with the
  same privileges as the remote user, and any resource that the remote
  user can access can be read by GDB, and downloaded to the local
  machine on which GDB is running.  As such users need to take
  independent measures to secure the connection between GDB and the
  remote target.

  Any issues that arise due to a failure to protect the connection
  between GDB and a remote target are not security bugs.

What Is A Security Bug?
-----------------------

  Any bugs in GDB or gdbserver that result in an unexpected crossing
  of a privilege boundary are considered security bugs.  Some examples
  of crossing a privilege boundary include; being able to execute code
  as an arbitrarily different user, or accessing resources
  (e.g. files, sockets, etc) for which the original user would not
  normally have access.

  Any bugs in GDB that result in execution of the program being
  debugged without a user issued GDB command triggering execution
  (either from the GDB command line, a GDB configuration file, or from
  the GDB prompt) are considered security bugs.

  When gdbserver is started, if it is passed a program on its command
  line then that program will be started, but paused before executing
  its first instruction.  Any bug which results in execution of the
  program being debugged without GDB first connecting to the target
  and sending a command that is intended to trigger execution is a
  security bug in gdbserver.

  Any bug in GDB or gdbserver that can trigger arbitrary code
  execution without the program being debugged having been executed,
  for example, by simply loading the program into GDB or gdbserver, is
  considered a security bug.

  The additional tools gcore and gdb-add-index are scripts that wrap
  around GDB.  Any issue in these tools that meet the above
  definitions of a security bug, are considered a security bug.

Security Considerations
-----------------------

  Within this section, mention of GDB should be read as meaning GDB,
  gdbserver, gcore, or gdb-add-index, unless specifically stated
  otherwise.

  The most common use case for GDB is a developer trying to resolve
  issues within a program that they have either written themselves, or
  within a program that they trust not to be malicious.  In this
  situation we would say GDB is being used to debug trusted code.
  There is no greater security risk from running the program to debug
  through GDB than there is running the program directly.  Additional
  process isolation for the GDB process is only needed if additional
  isolation would have been applied anyway when running the program to
  debug.

  In some cases a developer may be given a program from an untrusted
  source and be asked to debug an issue.  In this situation we would
  say GDB is being used to debug untrusted code.  In this case the
  user should take all the precautions when running GDB that they
  would normally take when running an untrusted program outside of
  GDB, e.g. running within a sandboxed environment.

  When using GDB or to examine an untrusted program (with gdbserver,
  the program can be started, but paused at the first instruction and
  never run), there should be no security risk, however, there are two
  things to consider:

  1. The risk of inadvertently executing the untrusted program.  It
     would be easy for a user to accidentally issue one of the many
     commands that would start the untrusted program executing.  Even
     the 'start' command, which stops the program being debugged at
     the start of the main function might not be safe, a program can
     embed a malicious payload before the main function is reached.

  2. It is possible that a malicious executable could trigger a bug in
     GDB to perform a malicious action.  Such a bug would be a
     security bug, but it would be best practice to guard against such
     a bug if possible.

  Given these risks, the advice of the GDB project is, when using GDB
  with an untrusted binary, to do so in a sandboxed environment.

  When using GDB and gdbserver to perform remote debug, the connection
  between the two components in by design insecure.  It is up to the
  user to protect this connection, for example, only starting
  gdbserver within a secure network.

Reporting Non-Security Bugs
---------------------------

  NOTE: All bugs reported in the GDB Bugzilla are public.

  Non-security bugs, as well as any security bugs that pose limited
  risk to users should be reported in GDB's bugzilla system.  Bugs
  reported in this way will be public.  The bugzilla system can be
  found here:

    https://sourceware.org/bugzilla/

Reporting Security Bugs
-----------------------

  In order to report a private security bug that is not immediately
  public, please contact one of the downstream distributions with
  security teams.  The following teams have volunteered to handle
  such bugs:

     Red Hat: secalert@redhat.com
     SUSE:    security@suse.de

  Please report the bug to just one of these teams.  It will be shared
  with other teams as necessary.

  The team contacted will take care of details such as vulnerability
  rating and CVE assignment (http://cve.mitre.org/about/).  It is likely
  that the team will ask to file a public bug because the issue is
  sufficiently minor and does not warrant an embargo.  An embargo is not
  a requirement for being credited with the discovery of a security
  vulnerability.