Patch "ovl: fix regression caused by exclusive upper/work dir protection" has been added to the 4.13-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    ovl: fix regression caused by exclusive upper/work dir protection

to the 4.13-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     ovl-fix-regression-caused-by-exclusive-upper-work-dir-protection.patch
and it can be found in the queue-4.13 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 85fdee1eef1a9e48ad5716916677e0c5fbc781e3 Mon Sep 17 00:00:00 2001
From: Amir Goldstein <amir73il@gmail.com>
Date: Fri, 29 Sep 2017 10:21:21 +0300
Subject: ovl: fix regression caused by exclusive upper/work dir protection

From: Amir Goldstein <amir73il@gmail.com>

commit 85fdee1eef1a9e48ad5716916677e0c5fbc781e3 upstream.

Enforcing exclusive ownership on upper/work dirs caused a docker
regression: https://github.com/moby/moby/issues/34672.

Euan spotted the regression and pointed to the offending commit.
Vivek has brought the regression to my attention and provided this
reproducer:

Terminal 1:

  mount -t overlay -o workdir=work,lowerdir=lower,upperdir=upper none
        merged/

Terminal 2:

  unshare -m

Terminal 1:

  umount merged
  mount -t overlay -o workdir=work,lowerdir=lower,upperdir=upper none
        merged/
  mount: /root/overlay-testing/merged: none already mounted or mount point
         busy

To fix the regression, I replaced the error with an alarming warning.
With index feature enabled, mount does fail, but logs a suggestion to
override exclusive dir protection by disabling index.
Note that index=off mount does take the inuse locks, so a concurrent
index=off will issue the warning and a concurrent index=on mount will fail.

Documentation was updated to reflect this change.

Fixes: 2cac0c00a6cd ("ovl: get exclusive ownership on upper/work dirs")
Reported-by: Euan Kemp <euank@euank.com>
Reported-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 Documentation/filesystems/overlayfs.txt |    5 ++++-
 fs/overlayfs/ovl_entry.h                |    3 +++
 fs/overlayfs/super.c                    |   27 +++++++++++++++++++--------
 3 files changed, 26 insertions(+), 9 deletions(-)

--- a/Documentation/filesystems/overlayfs.txt
+++ b/Documentation/filesystems/overlayfs.txt
@@ -210,8 +210,11 @@ path as another overlay mount and it may
 beneath or above the path of another overlay lower layer path.
 
 Using an upper layer path and/or a workdir path that are already used by
-another overlay mount is not allowed and will fail with EBUSY.  Using
+another overlay mount is not allowed and may fail with EBUSY.  Using
 partially overlapping paths is not allowed but will not fail with EBUSY.
+If files are accessed from two overlayfs mounts which share or overlap the
+upper layer and/or workdir path the behavior of the overlay is undefined,
+though it will not result in a crash or deadlock.
 
 Mounting an overlay using an upper layer path, where the upper layer path
 was previously used by another mounted overlay in combination with a
--- a/fs/overlayfs/ovl_entry.h
+++ b/fs/overlayfs/ovl_entry.h
@@ -37,6 +37,9 @@ struct ovl_fs {
 	bool noxattr;
 	/* sb common to all layers */
 	struct super_block *same_sb;
+	/* Did we take the inuse lock? */
+	bool upperdir_locked;
+	bool workdir_locked;
 };
 
 /* private information held for every overlayfs dentry */
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -210,9 +210,10 @@ static void ovl_put_super(struct super_b
 
 	dput(ufs->indexdir);
 	dput(ufs->workdir);
-	ovl_inuse_unlock(ufs->workbasedir);
+	if (ufs->workdir_locked)
+		ovl_inuse_unlock(ufs->workbasedir);
 	dput(ufs->workbasedir);
-	if (ufs->upper_mnt)
+	if (ufs->upper_mnt && ufs->upperdir_locked)
 		ovl_inuse_unlock(ufs->upper_mnt->mnt_root);
 	mntput(ufs->upper_mnt);
 	for (i = 0; i < ufs->numlower; i++)
@@ -880,9 +881,13 @@ static int ovl_fill_super(struct super_b
 			goto out_put_upperpath;
 
 		err = -EBUSY;
-		if (!ovl_inuse_trylock(upperpath.dentry)) {
-			pr_err("overlayfs: upperdir is in-use by another mount\n");
+		if (ovl_inuse_trylock(upperpath.dentry)) {
+			ufs->upperdir_locked = true;
+		} else if (ufs->config.index) {
+			pr_err("overlayfs: upperdir is in-use by another mount, mount with '-o index=off' to override exclusive upperdir protection.\n");
 			goto out_put_upperpath;
+		} else {
+			pr_warn("overlayfs: upperdir is in-use by another mount, accessing files from both mounts will result in undefined behavior.\n");
 		}
 
 		err = ovl_mount_dir(ufs->config.workdir, &workpath);
@@ -900,9 +905,13 @@ static int ovl_fill_super(struct super_b
 		}
 
 		err = -EBUSY;
-		if (!ovl_inuse_trylock(workpath.dentry)) {
-			pr_err("overlayfs: workdir is in-use by another mount\n");
+		if (ovl_inuse_trylock(workpath.dentry)) {
+			ufs->workdir_locked = true;
+		} else if (ufs->config.index) {
+			pr_err("overlayfs: workdir is in-use by another mount, mount with '-o index=off' to override exclusive workdir protection.\n");
 			goto out_put_workpath;
+		} else {
+			pr_warn("overlayfs: workdir is in-use by another mount, accessing files from both mounts will result in undefined behavior.\n");
 		}
 
 		ufs->workbasedir = workpath.dentry;
@@ -1155,11 +1164,13 @@ out_put_lowerpath:
 out_free_lowertmp:
 	kfree(lowertmp);
 out_unlock_workdentry:
-	ovl_inuse_unlock(workpath.dentry);
+	if (ufs->workdir_locked)
+		ovl_inuse_unlock(workpath.dentry);
 out_put_workpath:
 	path_put(&workpath);
 out_unlock_upperdentry:
-	ovl_inuse_unlock(upperpath.dentry);
+	if (ufs->upperdir_locked)
+		ovl_inuse_unlock(upperpath.dentry);
 out_put_upperpath:
 	path_put(&upperpath);
 out_free_config:


Patches currently in stable-queue which might be from amir73il@gmail.com are

queue-4.13/ovl-fix-missing-unlock_rename-in-ovl_do_copy_up.patch
queue-4.13/ovl-fix-error-value-printed-in-ovl_lookup_index.patch
queue-4.13/ovl-fix-dput-of-err_ptr-in-ovl_cleanup_index.patch
queue-4.13/ovl-fix-regression-caused-by-exclusive-upper-work-dir-protection.patch
queue-4.13/ovl-fix-dentry-leak-in-ovl_indexdir_cleanup.patch