[PATCH 4.19 16/44] ovl: fix missing override creds in link of a metacopy upper

[Greg Kroah-Hartman]

4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Amir Goldstein <amir73il@xxxxxxxxx>

commit 91ff20f34e94424e586f57f4f593beae16504f86 upstream.

Theodore Ts'o reported a v4.19 regression with docker-dropbox:
https://marc.info/?l=linux-fsdevel&m=154070089431116&w=2

"I was rebuilding my dropbox Docker container, and it failed in 4.19
 with the following error:
 ...
 dpkg: error: error creating new backup file \
              '/var/lib/dpkg/status-old': Invalid cross-device link"

The problem did not reproduce with metacopy feature disabled.
The error was caused by insufficient credentials to set
"trusted.overlay.redirect" xattr on link of a metacopy file.

Reproducer:

 echo Y > /sys/module/overlay/parameters/redirect_dir
 echo Y > /sys/module/overlay/parameters/metacopy
 cd /tmp
 mkdir l u w m
 chmod 777 l u
 touch l/foo
 ln l/foo l/link
 chmod 666 l/foo
 mount -t overlay none -olowerdir=l,upperdir=u,workdir=w m
 su fsgqa
 ln m/foo m/bar
 [   21.455823] overlayfs: failed to set redirect (-1)
 ln: failed to create hard link 'm/bar' => 'm/foo':\
     Invalid cross-device link

Reported-by: Theodore Y. Ts'o <tytso@xxxxxxx>
Reported-by: Maciej ZiÄba <maciekz82@xxxxxxxxx>
Fixes: 4120fe64dce4 ("ovl: Set redirect on upper inode when it is linked")
Cc: <stable@xxxxxxxxxxxxxxx> # v4.19
Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
Acked-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/overlayfs/dir.c |   14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -652,6 +652,18 @@ static int ovl_symlink(struct inode *dir
 	return ovl_create_object(dentry, S_IFLNK, 0, link);
 }
 
+static int ovl_set_link_redirect(struct dentry *dentry)
+{
+	const struct cred *old_cred;
+	int err;
+
+	old_cred = ovl_override_creds(dentry->d_sb);
+	err = ovl_set_redirect(dentry, false);
+	revert_creds(old_cred);
+
+	return err;
+}
+
 static int ovl_link(struct dentry *old, struct inode *newdir,
 		    struct dentry *new)
 {
@@ -672,7 +684,7 @@ static int ovl_link(struct dentry *old,
 		goto out_drop_write;
 
 	if (ovl_is_metacopy_dentry(old)) {
-		err = ovl_set_redirect(old, false);
+		err = ovl_set_link_redirect(old);
 		if (err)
 			goto out_drop_write;
 	}