[PATCH v2] hfs: Sanity check the root record

[Leo Stone]

In the syzbot reproducer, the hfs_cat_rec for the root dir has type
HFS_CDR_FIL after being read with hfs_bnode_read() in hfs_super_fill().
This indicates it should be used as an hfs_cat_file, which is 102 bytes.
Only the first 70 bytes of that struct are initialized, however,
because the entrylength passed into hfs_bnode_read() is still the length of
a directory record. This causes uninitialized values to be used later on,
when the hfs_cat_rec union is treated as the larger hfs_cat_file struct.

Add a check to make sure the retrieved record has the correct type
for the root directory (HFS_CDR_DIR), and make sure we load the correct
number of bytes for a directory record.

Reported-by: syzbot+2db3c7526ba68f4ea776@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=2db3c7526ba68f4ea776
Tested-by: syzbot+2db3c7526ba68f4ea776@xxxxxxxxxxxxxxxxxxxxxxxxx
Tested-by: Leo Stone <leocstone@xxxxxxxxx>
Signed-off-by: Leo Stone <leocstone@xxxxxxxxx>
---
v2: Made the check on fd.entrylength more strict. Tested with real HFS
    images.
v1: https://lore.kernel.org/all/20241123194949.9243-1-leocstone@xxxxxxxxx
---
 fs/hfs/super.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/hfs/super.c b/fs/hfs/super.c
index 3bee9b5dba5e..fe09c2093a93 100644
--- a/fs/hfs/super.c
+++ b/fs/hfs/super.c
@@ -349,11 +349,13 @@ static int hfs_fill_super(struct super_block *sb, struct fs_context *fc)
 		goto bail_no_root;
 	res = hfs_cat_find_brec(sb, HFS_ROOT_CNID, &fd);
 	if (!res) {
-		if (fd.entrylength > sizeof(rec) || fd.entrylength < 0) {
+		if (fd.entrylength != sizeof(rec.dir)) {
 			res =  -EIO;
 			goto bail_hfs_find;
 		}
 		hfs_bnode_read(fd.bnode, &rec, fd.entryoffset, fd.entrylength);
+		if (rec.type != HFS_CDR_DIR)
+			res = -EIO;
 	}
 	if (res)
 		goto bail_hfs_find;
-- 
2.43.0