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[Bug 1507463] Re: OverlayFS: Wrong mnt_id and path reported in /proc in linux-3.13


I retested using lxc-1.1.4 from the lxc-stable ppa
(https://launchpad.net/~ubuntu-lxc/+archive/ubuntu/lxc-stable) which I
added to a fresh vm image from the daily-builds, https://cloud-

"lxc-start -n trusty_overlay" fails as before, with the same error

ERROR    lxc_utils - utils.c:safe_mount:1641 - Permission denied - Failed to mount /home/ubuntu/test.txt onto /usr/lib/x86_64-linux-gnu/lxc/home/ubuntu/test.txt
ERROR    lxc_conf - conf.c:mount_entry:1731 - Permission denied - failed to mount '/home/ubuntu/test.txt' on '/usr/lib/x86_64-linux-gnu/lxc/home/ubuntu/test.txt'
ERROR    lxc_conf - conf.c:lxc_setup:3745 - failed to setup the mount entries for 'trusty_overlay'

Chris, it would be of interest to see whether you can reproduce the
underlying kernel bug using this script,

fatal() { echo "error: $@"; exit 1; }

echo -e "testing for overlayfs kernel bug in kernels <= 3.17.";
echo -e "(This script does not work for the newer version of overlayfs in kernels 3.18+)";

mkdir test_dir                    || fatal "mkdir test_dir";
sudo mount -t tmpfs none test_dir || fatal "mount tmpfs";
cd test_dir;
mkdir lowerdir upperdir overlayfs;
sudo mount -t overlayfs -o lowerdir=lowerdir,upperdir=upperdir none overlayfs || fatal "mount overlayfs";

exec 6> file_tmpfs.txt;
exec 7> lowerdir/file_lowerdir.txt;
exec 8> upperdir/file_upperdir.txt;
exec 9> overlayfs/file_overlayfs.txt;

echo -e "\ncontents of /proc/$BASHPID/fd/ .  A broken symbolic link to file_overlayfs.txt indicates a kernel bug";
echo "--------------------------------------------------------------------------------------------------------";
file /proc/$BASHPID/fd/[6-9];

# cleanup
exec 9>&-;
exec 8>&-;
exec 7>&-;
exec 6>&-;
sudo umount overlayfs;
cd ..;
sudo umount test_dir && rmdir test_dir;
#----------end of script

Running the script as root, sudo ./script, I get the following output,

  testing for overlayfs kernel bug in kernels <= 3.17.
  (This script does not work for the newer version of overlayfs in kernels 3.18+)

  contents of /proc/1916/fd/ .  A broken symbolic link to file_overlayfs.txt indicates a kernel bug
  /proc/1916/fd/6: symbolic link to `/home/ubuntu/test_dir/file_tmpfs.txt' 
  /proc/1916/fd/7: symbolic link to `/home/ubuntu/test_dir/lowerdir/file_lowerdir.txt' 
  /proc/1916/fd/8: symbolic link to `/home/ubuntu/test_dir/upperdir/file_upperdir.txt' 
  /proc/1916/fd/9: broken symbolic link to `/file_overlayfs.txt'

It seems that the kernel bug is causing fd 9 to point to
/file_overlayfs.txt, when it should point to
/home/ubuntu/test_dir/overlayfs/file_overlayfs.txt .

The lxc commit which introduced safe_mount(), which passes fd's as
arguments to mount() is,

CVE-2015-1335: Protect container mounts against symlinks

Here is the relevant part of the commit, which creates a file descriptor
and passes it to mount().

+	destfd = open_without_symlink(dest, rootfs);
+	if (destfd < 0) {
+		if (srcfd != -1)
+			close(srcfd);
+		return destfd;
+	}
+	ret = snprintf(destbuf, 50, "/proc/self/fd/%d", destfd);
+	if (ret < 0 || ret > 50) {
+		if (srcfd != -1)
+			close(srcfd);
+		close(destfd);
+		ERROR("Out of memory");
+		return -EINVAL;
+	}
+	ret = mount(mntsrc, destbuf, fstype, flags, data);

** CVE added: http://www.cve.mitre.org/cgi-

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  OverlayFS: Wrong mnt_id and path reported in /proc in linux-3.13

Status in linux package in Ubuntu:
Status in lxc package in Ubuntu:

Bug description:
  I am running Ubuntu Trusty (3.13.0-65-generic #106-Ubuntu SMP Fri Oct
  2 22:08:27 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux).

  There are two issues in OverlayFS as follows:

   1. /proc/<pid>/fdinfo/<fd> reports wrong mnt_id (non-existent in

   2. /proc/<pid>/fd/<fd> shows incorrect path for the symlink target

  These issues were previously reported and patched for the Ubuntu Vivid
  kernel (3.19) in bug #1479468, but the patch was not applied to the
  3.13 kernel, possibly because it contains an older version of

  These issues are now adversely affecting some versions of lxc when
  used in conjunction with the 3.13 kernel, because lxc has recently
  been patched to pass /proc/<pid>/fd/<fd> to the mount syscall.  This
  change to lxc was made in response to cve-2015-1335; see bug #1476662.

  The kernel issues cause some overlayfs-backed containers to fail to
  start, with error message,

  strace: mount("/home/ubuntu/test.txt", "/proc/self/fd/16",
  0x7ffd4c3f7fdb, MS_BIND, NULL) = -1 EACCES (Permission denied)

  The mount fails because /proc/self/fd/16, which should point to a file
  in overlayfs, has an invalid value.

  The version of lxc in the trusty-updates repository, lxc-1.0.7-0ubuntu0.7,
  is not affected.  The following versions of lxc are affected:

  repository                         lxc version
  ppa:ubuntu-lxc/lxc-git-stable-1.0  1.0.7+stable~20151014-2143-0ubuntu1~trusty
  ppa:ubuntu-lxc/lxc-git-stable-1.1  1.1.3+stable~20151007-2301-0ubuntu1~trusty
  ppa:ubuntu-lxc/lxc-git-master      1.1.3+master~20151012-0541-0ubuntu1~trusty

  To reproduce the lxc failure-to-start issue:

  # Install the 3.13 kernel and also an affected version of lxc.
  lxc-create -t download -n trusty -- -d ubuntu -r trusty -a amd64;
  touch $HOME/test.txt;
  echo "lxc.mount.entry = $HOME/test.txt home/ubuntu/test.txt none bind,create=file 0 0" >> $HOME/.local/share/lxc/trusty/config;
  lxc-clone -s -B overlayfs trusty trusty_overlay;
  lxc-start -n trusty_overlay; # fails to start

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