[PATCH 0/1][SRU][D/E/F] Fix uuid-less squashfs overlayfs corruption issue (LP: #1824407)

[Colin King]

From: Colin Ian King <colin.king at canonical.com>

BugLink: https://bugs.launchpad.net/bugs/1824407

== SRU Justification Disco, Eoan, Focal ==

Multiple squashfs filesystems with overlayfs cause file corruption issues
when modifying zero sized files

== Fix ==

The current fix is pending in https://github.com/amir73il/linux/commit/b2d4f0ea5af42e16e154254de99da064f3ac551a
and should be landing in Linux v5.5

== Test case ==

With an Ubuntu ISO on the cdrom drive, use:

#!/bin/bash -x
mkdir -p /cdrom
mount -t iso9660 -o ro,noatime /dev/sr0 /cdrom
sleep 1
mkdir -p /cow
mount -t tmpfs -o 'rw,noatime,mode=755' tmpfs /cow
sleep 1
mkdir -p /cow/upper
mkdir -p /cow/work
modprobe -q -b overlay
sleep 1
modprobe -q -b loop
sleep 1
dev=$(losetup -f)
mkdir -p /filesystem.squashfs
losetup $dev /cdrom/casper/filesystem.squashfs
mount -t squashfs -o ro,noatime $dev /filesystem.squashfs
sleep 1

dev=$(losetup -f)
mkdir -p /installer.squashfs
losetup $dev /cdrom/casper/installer.squashfs
mount -t squashfs -o ro,noatime $dev /installer.squashfs
sleep 1

mkdir -p /root-tmp
mount -t overlay -o 'upperdir=/cow/upper,lowerdir=/installer.squashfs:/filesystem.squashfs,workdir=/cow/work' /cow /root-tmp

FILE=/root-tmp/etc/.pwd.lock

echo foo > $FILE
cat $FILE
sync
#
# dropping caches or remounting causes the bug
#
echo 3 > /proc/sys/vm/drop_caches
cat $FILE

Without the fix the cat of the file will produce an error. With the the
cat will work correctly.

== Regression Potential ==

There is an unhandled corner case:
    - two filesystems, A and B, both have null uuid
    - upper layer is on A
    - lower layer 1 is also on A
    - lower layer 2 is on B

However, since this is an issue without the fix and will be addressed later
with subsequent fixes once they are OK with upstream I think the risk is
minimal considering nobody is complaining about these corner cases with the
current broken overlayfs squashfs layering.

----

Amir Goldstein (1):
  ovl: fix lookup failure on multi lower squashfs

 fs/overlayfs/namei.c     |  8 ++++++++
 fs/overlayfs/ovl_entry.h |  2 ++
 fs/overlayfs/super.c     | 24 +++++++++++++++++-------
 3 files changed, 27 insertions(+), 7 deletions(-)

-- 
2.24.0