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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Vivek Goyal <vgoyal@redhat.com>,
        Miklos Szeredi <mszeredi@redhat.com>
Subject: [PATCH 4.15 039/146] ovl: redirect_dir=nofollow should not follow redirect for opaque lower
Date:   Tue, 13 Mar 2018 16:23:26 +0100
Message-Id: <20180313152323.579717003@linuxfoundation.org>
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4.15-stable review patch.  If anyone has any objections, please let me know.

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

From: Vivek Goyal <vgoyal@redhat.com>

commit d1fe96c0e4de78ba0cd336ea3df3b850d06b9b9a upstream.

redirect_dir=nofollow should not follow a redirect. But in a specific
configuration it can still follow it.  For example try this.

$ mkdir -p lower0 lower1/foo upper work merged
$ touch lower1/foo/lower-file.txt
$ setfattr -n "trusted.overlay.opaque" -v "y" lower1/foo
$ mount -t overlay -o lowerdir=lower1:lower0,workdir=work,upperdir=upper,redirect_dir=on none merged
$ cd merged
$ mv foo foo-renamed
$ umount merged

# mount again. This time with redirect_dir=nofollow
$ mount -t overlay -o lowerdir=lower1:lower0,workdir=work,upperdir=upper,redirect_dir=nofollow none merged
$ ls merged/foo-renamed/
# This lists lower-file.txt, while it should not have.

Basically, we are doing redirect check after we check for d.stop. And
if this is not last lower, and we find an opaque lower, d.stop will be
set.

ovl_lookup_single()
        if (!d->last && ovl_is_opaquedir(this)) {
                d->stop = d->opaque = true;
                goto out;
        }

To fix this, first check redirect is allowed. And after that check if
d.stop has been set or not.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Fixes: 438c84c2f0c7 ("ovl: don't follow redirects if redirect_dir=off")
Cc: <stable@vger.kernel.org> #v4.15
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/overlayfs/namei.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -678,9 +678,6 @@ struct dentry *ovl_lookup(struct inode *
 		stack[ctr].layer = lower.layer;
 		ctr++;
 
-		if (d.stop)
-			break;
-
 		/*
 		 * Following redirects can have security consequences: it's like
 		 * a symlink into the lower layer without the permission checks.
@@ -697,6 +694,9 @@ struct dentry *ovl_lookup(struct inode *
 			goto out_put;
 		}
 
+		if (d.stop)
+			break;
+
 		if (d.redirect && d.redirect[0] == '/' && poe != roe) {
 			poe = roe;