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

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apport-collect 1507463

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This change has been made by an automated script, maintained by the Ubuntu Kernel Team.

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I tried the following:

# 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

In a clean and fully updated Trusty VM and could not reproduce the issue with the trusty version of the packages.
ii lxc 1.0.7-0ubuntu0.7 amd64 Linux Containers userspace tools

I also tried the stable version of LXC (https://launchpad.net/~ubuntu-lxc/+archive/ubuntu/lxc-stable) and was able to get this working.
ii lxc 1.1.4-0ubuntu1~ubuntu14.04.1~ppa1 amd64 Linux Containers userspace tools

So seems to be fixed in 1.1.4, and I suspect this commit fixes the issue:
commit e228426cf483dc6a406276550b9cc7222e7c2602
Author: Serge Hallyn <email address hidden>
Date: Mon Sep 21 17:09:25 2015 +0000

    overlayfs_mount: create delta dir if it doesn't exist

Can you re-test with 1.1.4 and see if that fixes the issue? Thanks,
--chris

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 Trusty daily-builds, https://cloud-images.ubuntu.com/trusty/current.

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

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,

#!/bin/bash
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 following lxc commit introduced safe_mount(), which passes fd's as arguments to mount(),

CVE-2015-1335: Protect container mounts against symlinks
https://github.com/lxc/lxc/commit/6de26af93d3dd87c8b21a42fdf20f30fa1c1948d

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);

The strace of mount() shows,

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.

Related bug #1347746 : Overlayfs breaks /proc/self/exe link

This bug does still affect lxc on Ubuntu Trusty when using the 3.13 kernel in conjunction with the latest daily-build of lxc, 1.1.5+master~20151110-0623-0ubuntu1~trusty (from the lxc-daily ppa, https://launchpad.net/~ubuntu-lxc/+archive/ubuntu/daily).