kernel-packages team mailing list archive

[Jamie Strandboge <jamie@xxxxxxxxxx>]

** Summary changed:

- allow defining the attach root for attach_disconnected
+ attach_disconnected not sufficient for overlayfs

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https://bugs.launchpad.net/bugs/1408106

Title:
  attach_disconnected not sufficient for overlayfs

Status in AppArmor Linux application security framework:
  In Progress
Status in apparmor package in Ubuntu:
  Confirmed
Status in linux package in Ubuntu:
  Confirmed

Bug description:
  With the following use of overlayfs, we get a disconnected path:

  $ cat ./profile
  #include <tunables/global>
  profile foo {
    #include <abstractions/base>

    capability sys_admin,
    capability sys_chroot,
    mount,
    pivot_root,
  }

  $ cat ./overlay.c
  #include <alloca.h>
  #include <linux/sched.h>
  #include <stdio.h>
  #include <string.h>
  #include <sys/mount.h>
  #include <fcntl.h>
  #include <unistd.h>

  int main(int argc, char* argv[]) {
      int i = 0;
      int len = 0;
      int ret = 0;
      char* options;

      if (geteuid())
          unshare(CLONE_NEWUSER);
      unshare(CLONE_NEWNS);

      for (i = 1; i < argc; i++) {
          if (i == 1) {
              len = strlen(argv[i]) + strlen("upperdir=,lowerdir=/") + 2;
              options = alloca(len);
              ret = snprintf(options, len, "upperdir=%s,lowerdir=/", argv[i]);
          }
          else {
              len = strlen(argv[i]) + strlen("upperdir=,lowerdir=/mnt") + 2;
              options = alloca(len);
              ret = snprintf(options, len, "upperdir=%s,lowerdir=/mnt", argv[i]);
          }

          mount("overlayfs", "/mnt", "overlayfs", MS_MGC_VAL, options);
      }

      chdir("/mnt");
      pivot_root(".", ".");
      chroot(".");

      chdir("/");
      execl("/bin/bash", "/bin/bash", NULL);
  }

  $ sudo apparmor_parser -r ./profile && aa-exec -p foo -- ./a.out /tmp
  [255]
  ...
  Dec 12 14:31:38 localhost kernel: [57278.040216] audit: type=1400 audit(1418387498.613:712): apparmor="DENIED" operation="exec" info="Failed name lookup - disconnected path" error=-13 profile="foo" name="/bin/bash" pid=18255 comm="a.out" requested_mask="x" denied_mask="x" fsuid=1000 ouid=0

  With the above, the expectation was for the denial to be /mnt/bin/bash. There are three ways forward:
  1. the correct solution is to patch overlayfs to properly track the loopback, but this will take a while, may ultimately be unachievable (investigation is needed) and is likely non-upstreamable
  2. we could rely on the fact that overlayfs creates a private unshared submount, and provide a way to not mediate the path when that is present, and tagged. This would take a bit of time, and might be the preferred method over 1 longer term
  3. we could extend attach_disconnected so that we can define the attach root. Eg, we can use profile foo (attach_disconnected=/mnt) {} such that '/bin/bash' maps to '/mnt/bin/bash'.

  While attach_disconnected should in general be discouraged, this method:
   * is doable in a short time frame,
   * is generally useful even when the proper fix is in place
   * would help lxc in a few cases
   * would be sufficient for snappy

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