[PATCH v3] hfsplus: Add a sanity check for btree node size

[Edward Adam Davis]

Syzbot reported an uninit-value bug in [1] with a corrupted HFS+ image,
during the file system mounting process, specifically while loading the
catalog, a corrupted node_size value of 1 caused the rec_off argument
passed to hfs_bnode_read_u16() (within hfs_bnode_find()) to be excessively
large. Consequently, the function failed to return a valid value to
initialize the off variable, triggering the bug [1].

Every node starts from BTree node descriptor: struct hfs_bnode_desc.
So, the size of node cannot be lesser than that. However, technical
specification declares that: "The node size (which is expressed in bytes)
must be power of two, from 512 through 32,768, inclusive." Add a check
for btree node size base on technical specification.

[1]
BUG: KMSAN: uninit-value in hfsplus_bnode_find+0x141c/0x1600 fs/hfsplus/bnode.c:584
 hfsplus_bnode_find+0x141c/0x1600 fs/hfsplus/bnode.c:584
 hfsplus_btree_open+0x169a/0x1e40 fs/hfsplus/btree.c:382
 hfsplus_fill_super+0x111f/0x2770 fs/hfsplus/super.c:553
 get_tree_bdev_flags+0x6e6/0x920 fs/super.c:1694
 get_tree_bdev+0x38/0x50 fs/super.c:1717
 hfsplus_get_tree+0x35/0x40 fs/hfsplus/super.c:709
 vfs_get_tree+0xb3/0x5d0 fs/super.c:1754
 fc_mount fs/namespace.c:1193 [inline]

Fixes: 8ad2c6a36ac4 ("hfsplus: validate b-tree node 0 bitmap at mount time")
Reported-by: syzbot+217eb327242d08197efb@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=217eb327242d08197efb
Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
---
v1 -> v2: change check base on technical specification
v2 -> v3: using const min size

 fs/hfsplus/btree.c         | 2 ++
 include/linux/hfs_common.h | 1 +
 2 files changed, 3 insertions(+)

diff --git a/fs/hfsplus/btree.c b/fs/hfsplus/btree.c
index 761c74ccd653..394542a47e60 100644
--- a/fs/hfsplus/btree.c
+++ b/fs/hfsplus/btree.c
@@ -365,6 +365,8 @@ struct hfs_btree *hfs_btree_open(struct super_block *sb, u32 id)
 	}
 
 	size = tree->node_size;
+	if (size < HFSPLUS_NODE_MINSZ || size > HFSPLUS_NODE_MXSZ)
+		goto fail_page;
 	if (!is_power_of_2(size))
 		goto fail_page;
 	if (!tree->node_count)
diff --git a/include/linux/hfs_common.h b/include/linux/hfs_common.h
index 07dfc39630ab..45fb4c9ff9f5 100644
--- a/include/linux/hfs_common.h
+++ b/include/linux/hfs_common.h
@@ -513,6 +513,7 @@ struct hfs_btree_header_rec {
 /* HFS+ BTree misc info */
 #define HFSPLUS_TREE_HEAD			0
 #define HFSPLUS_NODE_MXSZ			32768
+#define HFSPLUS_NODE_MINSZ			512
 #define HFSPLUS_ATTR_TREE_NODE_SIZE		8192
 #define HFSPLUS_BTREE_HDR_NODE_RECS_COUNT	3
 #define HFSPLUS_BTREE_HDR_MAP_REC_INDEX		2	/* Map (bitmap) record in Header node */
-- 
2.43.0