[PATCH v2] ext4: don't remove reserved inodes in ext4_unlink()

[Eryu Guan <guaneryu@gmail.com>]

Corrupted ext4_dir_entry_2 struct on disk may have wrong inode number,
when the inode number is 8 (EXT4_JOURNAL_INO) and the file is deleted,
the journal inode is gone, and unmounting such a fs could trigger the
following BUG_ON() in start_this_handle().

	BUG_ON(journal->j_flags & JBD2_UNMOUNT);

	------------[ cut here ]------------
	kernel BUG at fs/jbd2/transaction.c:307!
	...
	CPU: 1 PID: 1535 Comm: umount Not tainted 3.13.0+ #14
	...
	Call Trace:
	 [<ffffffff8119f17a>] ? kmem_cache_alloc+0x1ca/0x1f0
	 [<ffffffff812850f0>] ? jbd2__journal_start+0x90/0x1e0
	 [<ffffffff81285153>] jbd2__journal_start+0xf3/0x1e0
	 [<ffffffff81242a62>] ? ext4_evict_inode+0x1b2/0x4f0
	 [<ffffffff8126d039>] __ext4_journal_start_sb+0x69/0xe0
	 [<ffffffff81242a62>] ext4_evict_inode+0x1b2/0x4f0
	 [<ffffffff811d3b8e>] evict+0x9e/0x190
	 [<ffffffff811d4373>] iput+0xf3/0x180
	 [<ffffffff8128f301>] jbd2_journal_destroy+0x191/0x220
	 [<ffffffff810b0ae0>] ? abort_exclusive_wait+0xb0/0xb0
	 [<ffffffff8125d004>] ext4_put_super+0x64/0x340
	 [<ffffffff811bbae2>] generic_shutdown_super+0x72/0xf0
	 [<ffffffff811bbd77>] kill_block_super+0x27/0x70
	 [<ffffffff811bc05d>] deactivate_locked_super+0x3d/0x60
	 [<ffffffff811bc606>] deactivate_super+0x46/0x60
	 [<ffffffff811d7f47>] mntput_no_expire+0xa7/0x140
	 [<ffffffff811d939e>] SyS_umount+0x8e/0x100
	 [<ffffffff81690c29>] system_call_fastpath+0x16/0x1b

Check inode number in ext4_unlink() and return error if the inode number
is reserved or nonexistent(except EXT4_ROOT_INO, as Ted pointed out that
it's a security hole).

Tested by removing a reserved inode(modify the ondisk structure by hand)
and unmounting the fs. Inodes 1-10 have been tested. Also tested by
xfstests.

Signed-off-by: Eryu Guan <guaneryu@gmail.com>
---

(This is a v2 of an old patch, I forgot about the patch..)

v2: exempt the root inode as Ted suggested, although unlink("/") would be
catched by vfs and unlink a corrupt file with root inode number would be
catched by ext4_lookup, and won't reach ext4_unlink() in both cases

	EXT4-fs error (device loop0): ext4_lookup:1441: inode #2: comm rm: 'testfile' linked to parent dir
	Aborting journal on device loop0-8.
	EXT4-fs (loop0): Remounting filesystem read-only
	EXT4-fs error (device loop0): ext4_lookup:1441: inode #2: comm rm: 'testfile' linked to parent dir

 fs/ext4/namei.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 603e4eb..6e6b312 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -2796,9 +2796,11 @@ end_rmdir:
 static int ext4_unlink(struct inode *dir, struct dentry *dentry)
 {
 	int retval;
+	unsigned long ino;
 	struct inode *inode;
 	struct buffer_head *bh;
 	struct ext4_dir_entry_2 *de;
+	struct super_block *sb;
 	handle_t *handle = NULL;
 
 	trace_ext4_unlink_enter(dir, dentry);
@@ -2815,13 +2817,20 @@ static int ext4_unlink(struct inode *dir, struct dentry *dentry)
 		goto end_unlink;
 
 	inode = dentry->d_inode;
+	ino = inode->i_ino;
+	sb = dir->i_sb;
 
 	retval = -EIO;
-	if (le32_to_cpu(de->inode) != inode->i_ino)
+	if (le32_to_cpu(de->inode) != ino)
 		goto end_unlink;
+	if ((ino < EXT4_FIRST_INO(sb) && ino != EXT4_ROOT_INO) ||
+	    ino > le32_to_cpu(EXT4_SB(sb)->s_es->s_inodes_count)) {
+		ext4_error(sb, "reserved or nonexistent inode %lu", ino);
+		goto end_unlink;
+	}
 
 	handle = ext4_journal_start(dir, EXT4_HT_DIR,
-				    EXT4_DATA_TRANS_BLOCKS(dir->i_sb));
+				    EXT4_DATA_TRANS_BLOCKS(sb));
 	if (IS_ERR(handle)) {
 		retval = PTR_ERR(handle);
 		handle = NULL;
-- 
1.8.3.1