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[Bug Watch Updater <1783591@xxxxxxxxxxxxxxxxxx>]

** Changed in: lxc (Debian)
       Status: Confirmed => Fix Released

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https://bugs.launchpad.net/bugs/1783591

Title:
  lxc-user-nic allows unprivileged users to open arbitrary files

Status in lxc package in Ubuntu:
  Fix Released
Status in lxc source package in Xenial:
  Triaged
Status in lxc source package in Bionic:
  Fix Released
Status in lxc source package in Cosmic:
  Fix Released
Status in lxc package in Debian:
  Fix Released

Bug description:
  Matthias Gerstner from SUSE reported the following:

  ```
  Hello,

  following the lxc security reporting guidelines [1] I am reporting a
  finding in the lxc-user-nic setuid binary. I'm encrypting this mail as a
  best practice and because I found valid GPG keys for all of your
  adresses. Please find my public key attached to this mail.

  In the context of an openSUSE security audit of the lxc-user-nic setuid
  binary [2] (currently private bug) I came across an issue that should be
  adressed. In the "delete" case the program runs the following piece of
  code unconditionally with effective uid 0 (from lxc_user_nic.c):

  ```
          } else if (request == LXC_USERNIC_DELETE) {
                  netns_fd = open(args.pid, O_RDONLY);
                  if (netns_fd < 0) {
                          usernic_error("Could not open \"%s\": %s\n", args.pid,
                                        strerror(errno));
                          exit(EXIT_FAILURE);
                  }
          }
  ```

  `args.pid` is a user controlled parameter and can be an arbitrary path
  at the moment. Nothing is done with this file descriptor later on in the
  program except an attempt at `setns(fd, CLONE_NEWNET)` in
  `is_privileged_over_netns()`.  Still this allows the unprivileged caller
  of the setuid binary to achieve the following:

  - it can test for existence of files normally not accessible to the
    caller (information leak). Example:
    ```
    # this file is existing
    $ /usr/lib/lxc/lxc-user-nic delete path name /root/.bash_history type bridge nic
    lxc_user_nic.c: 1017: is_privileged_over_netns: Failed to setns() to network namespace Invalid argument
    lxc_user_nic.c: 1161: main: Process is not privileged over network namespace

    # this file is not existing
    $ /usr/lib/lxc/lxc-user-nic delete path name /root/.zsh_history type bridge nic
    lxc_user_nic.c: 1130: main: Could not open "/root/.zsh_history": No such file or directory
    ```

  - it allows to trigger code paths in the kernel that are normally not
    accessible to the caller. This can happen when opening special files
    like character and block devices or files in /proc or /sys. Opening
    some of these files can cause lock or alloc operations or even more
    complex things to happen like when opening /dev/ptmx, which causes the
    allocation of a new master/slave pseudo terminal. Therefore this can
    lead to DoS like situations or have further unspecified impact.

  For fixing this I suggest opening the file supplied in `args.pid` only
  with the permissions of the real user, since this is already done in
  `is_privileged_over_netns()` anyway. Another approach would be the
  normalization of the input path and then only allowing a path of the
  pattern /proc/<pid>/ns/net.

  [1] https://github.com/lxc/lxc/blob/master/README.md#reporting-security-issues
  [2] https://bugzilla.suse.com/show_bug.cgi?id=988348

  Best regards

  Matthias
  ```

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