[PATCH v2 2/2] hfs: refuse catalog records that address the reserved B-tree CNIDs

[1sh1ro <yangguikun19@qq.com>]

The extent and catalog B-tree inodes are described by the MDB, not by
catalog records, but they are hashed by CNID like any other inode and
hfs_test_inode() compares i_ino only.  A crafted image with a catalog
record whose FlNum (or DirID) is HFS_EXT_CNID or HFS_CAT_CNID therefore
makes hfs_lookup() hand out the internal B-tree inode as a regular dentry,
bypassing the CNID validation that hfs_read_inode() performs: a read
exposes the B-tree file's contents, and a write would go through
hfs_write_inode(), which tries to update a catalog record that does not
exist.

Refuse such records in hfs_iget() by returning NULL for the reserved
B-tree CNIDs.  hfs_lookup() reports that as -EACCES and
hfs_fill_super() as a mount failure, the same as for any other
unaddressable record.

This is hardening on top of the file-type fix, not required for the
panic; it can be dropped if hfs_is_valid_cnid()'s acceptance of
HFS_CDR_FIL records for CNID 3/4 is intentional.

Signed-off-by: 1sh1ro <yangguikun19@qq.com>
---
  fs/hfs/inode.c |  11 +++++++++++
 1 file changed, 11 insertions(+), 0 deletions(-)

diff --git a/fs/hfs/inode.c b/fs/hfs/inode.c
index 4316d75..5190856 100644
--- a/fs/hfs/inode.c
+++ b/fs/hfs/inode.c
@@ -520,6 +520,17 @@ struct inode *hfs_iget(struct super_block *sb, struct hfs_cat_key *key, hfs_cat_
 	default:
 		return NULL;
 	}
+
+	/*
+	 * Never let a catalog record address one of the internal B-tree
+	 * objects.  hfs_test_inode() matches on i_ino alone, so such a
+	 * record would make hfs_lookup() hand out the cached B-tree inode as
+	 * a regular dentry, bypassing the CNID validation done by
+	 * hfs_read_inode().
+	 */
+	if (cnid == HFS_EXT_CNID || cnid == HFS_CAT_CNID)
+		return NULL;
+
 	inode = iget5_locked(sb, cnid, hfs_test_inode, hfs_read_inode, &data);
 	if (inode && (inode_state_read_once(inode) & I_NEW))
 		unlock_new_inode(inode);
-- 
2.39.5