linux-kernel - [PATCH 5.4 080/177] ovl: fix corner case of non-unique st_dev;st

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Message-Id: <20191216174836.969129558@linuxfoundation.org>
Date:   Mon, 16 Dec 2019 18:48:56 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Amir Goldstein <amir73il@...il.com>,
        Miklos Szeredi <mszeredi@...hat.com>
Subject: [PATCH 5.4 080/177] ovl: fix corner case of non-unique st_dev;st_ino

From: Amir Goldstein <amir73il@...il.com>

commit 9c6d8f13e9da10a26ad7f0a020ef86e8ef142835 upstream.

On non-samefs overlay without xino, non pure upper inodes should use a
pseudo_dev assigned to each unique lower fs and pure upper inodes use the
real upper st_dev.

It is fine for an overlay pure upper inode to use the same st_dev;st_ino
values as the real upper inode, because the content of those two different
filesystem objects is always the same.

In this case, however:
 - two filesystems, A and B
 - upper layer is on A
 - lower layer 1 is also on A
 - lower layer 2 is on B

Non pure upper overlay inode, whose origin is in layer 1 will have the same
st_dev;st_ino values as the real lower inode. This may result with a false
positive results of 'diff' between the real lower and copied up overlay
inode.

Fix this by using the upper st_dev;st_ino values in this case.  This breaks
the property of constant st_dev;st_ino across copy up of this case. This
breakage will be fixed by a later patch.

Fixes: 5148626b806a ("ovl: allocate anon bdev per unique lower fs")
Cc: stable@...r.kernel.org # v4.17+
Signed-off-by: Amir Goldstein <amir73il@...il.com>
Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/overlayfs/inode.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -200,8 +200,14 @@ int ovl_getattr(const struct path *path,
 			if (ovl_test_flag(OVL_INDEX, d_inode(dentry)) ||
 			    (!ovl_verify_lower(dentry->d_sb) &&
 			     (is_dir || lowerstat.nlink == 1))) {
-				stat->ino = lowerstat.ino;
 				lower_layer = ovl_layer_lower(dentry);
+				/*
+				 * Cannot use origin st_dev;st_ino because
+				 * origin inode content may differ from overlay
+				 * inode content.
+				 */
+				if (samefs || lower_layer->fsid)
+					stat->ino = lowerstat.ino;
 			}

 			/*