Re: [PATCH v2] hfsplus: Add a sanity check for btree node size

[Viacheslav Dubeyko]

On Thu, 2026-04-16 at 17:53 +0800, Edward Adam Davis wrote:
> 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
> 
>  fs/hfsplus/btree.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/hfsplus/btree.c b/fs/hfsplus/btree.c
> index 761c74ccd653..857705c3fe0d 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 < sb->s_blocksize || size > HFSPLUS_NODE_MXSZ)

Technically speaking, you are right that b-tree node size should be aligned on
logical block size. However, I am not sure that mkfs.hfsplus restricts the
creation of volume with b-tree's node size smaller than logical block size but
still in the required range of sizes.

Maybe, we need to declare the constant of HFSPLUS_NODE_MINSZ (512) and to check
this constant instead of logical block size. What do you think?

Thanks,
Slava. 

> +		goto fail_page;
>  	if (!is_power_of_2(size))
>  		goto fail_page;
>  	if (!tree->node_count)