[PATCH] ext4: fail ext4_iget for root directory if unallocated

[Theodore Ts'o <tytso@mit.edu>]

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.

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

Reported-by: Wen Xu <wen.xu@gatech.edu>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org
---
 fs/ext4/inode.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 951a3d69ed17..5adfd0980c71 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 >
-- 
2.16.1.72.g5be1f00a9a