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From:   Ben Hutchings <ben@decadent.org.uk>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
CC:     akpm@linux-foundation.org, "Theodore Ts'o" <tytso@mit.edu>,
        "Wen Xu" <wen.xu@gatech.edu>
Date:   Thu, 07 Jun 2018 15:05:21 +0100
Message-ID: <lsq.1528380321.743043835@decadent.org.uk>
Subject: [PATCH 3.16 018/410] ext4: fail ext4_iget for root directory if
 unallocated
In-Reply-To: <lsq.1528380320.647747352@decadent.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
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3.16.57-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Theodore Ts'o <tytso@mit.edu>

commit 8e4b5eae5decd9dfe5a4ee369c22028f90ab4c44 upstream.

If the root directory has an i_links_count of zero, then when the file
system is mounted, then when ext4_fill_super() notices the problem and
tries to call iput() the root directory in the error return path,
ext4_evict_inode() will try to free the inode on disk, before all of
the file system structures are set up, and this will result in an OOPS
caused by a NULL pointer dereference.

This issue has been assigned CVE-2018-1092.

https://bugzilla.kernel.org/show_bug.cgi?id=199179
https://bugzilla.redhat.com/show_bug.cgi?id=1560777

Reported-by: Wen Xu <wen.xu@gatech.edu>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
[bwh: Backported to 3.16: use EIO instead of EFSCORRUPTED]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 fs/ext4/inode.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -4198,6 +4198,12 @@ struct inode *ext4_iget(struct super_blo
 		goto bad_inode;
 	raw_inode = ext4_raw_inode(&iloc);
 
+	if ((ino == EXT4_ROOT_INO) && (raw_inode->i_links_count == 0)) {
+		EXT4_ERROR_INODE(inode, "root inode unallocated");
+		ret = -EIO;
+		goto bad_inode;
+	}
+
 	if (EXT4_INODE_SIZE(inode->i_sb) > EXT4_GOOD_OLD_INODE_SIZE) {
 		ei->i_extra_isize = le16_to_cpu(raw_inode->i_extra_isize);
 		if (EXT4_GOOD_OLD_INODE_SIZE + ei->i_extra_isize >