[T/X/A/B/C][PATCH 1/1] ext4: fail ext4_iget for root directory if unallocated

[Khalid Elmously]

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

CVE-2018-1092

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 at gatech.edu>
Signed-off-by: Theodore Ts'o <tytso at mit.edu>
Cc: stable at vger.kernel.org
(cherry-picked from 8e4b5eae5decd9dfe5a4ee369c22028f90ab4c44)
Signed-off-by: Khalid Elmously <khalid.elmously at canonical.com>
---
 fs/ext4/inode.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 3d4d1dccc8a1..398faeff1938 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -4745,6 +4745,12 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
 		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 = -EFSCORRUPTED;
+		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 >
-- 
2.17.0