Re: [PATCH 2/2] hfs/hfsplus: stop btree allocators from reusing node 0

[Viacheslav Dubeyko <slava@dubeyko.com>]

On Fri, 2026-08-14 at 10:31 +0800, Tao Yu wrote:
> The btree header node is permanently reserved as node 0. If the on-
> disk
> bitmap ever presents node 0 as free, the filesystem is already
> corrupted and the allocator must not try to instantiate it again.
> 
> Both HFS and HFS+ currently keep scanning the bitmap, set the bit,
> and
> hand node 0 to hfs_bnode_create()/hfsplus_bnode_create(). HFS+ then
> hits the "new node 0 already hashed?" warning reported by syzbot,
> while
> HFS risks continuing after the same corruption pattern.
> 
> Teach both allocators to treat attempts to allocate node 0 as btree
> map
> corruption. Force the filesystem read-only, emit the existing repair
> hint, and abort the allocation before the code reaches the hashed-
> node
> warning.
> 
> Reported-by: syzbot+2bf21610eea63cb2ce93@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=2bf21610eea63cb2ce93
> Signed-off-by: Tao Yu <tao1.yu@intel.com>
> ---
>  fs/hfs/btree.c     | 11 +++++++++++
>  fs/hfsplus/btree.c | 11 +++++++++++
>  2 files changed, 22 insertions(+)
> 
> diff --git a/fs/hfs/btree.c b/fs/hfs/btree.c
> index 14114318ec724..9b0b7418ddbd5 100644
> --- a/fs/hfs/btree.c
> +++ b/fs/hfs/btree.c
> @@ -376,6 +376,17 @@ struct hfs_bnode *hfs_bmap_alloc(struct
> hfs_btree *tree)
>  			if (byte != 0xff) {
>  				for (m = 0x80, i = 0; i < 8; m >>=
> 1, i++) {
>  					if (!(byte & m)) {
> +						if (unlikely(!(idx +
> i))) {
> +							pr_warn("(%s
> ): %s (cnid 0x%x) map record invalid or bitmap corruption detected,
> forcing read-only.\n",
> +								tree
> ->sb->s_id,
> +								hfs_
> btree_name(tree->cnid),
> +								tree
> ->cnid);
> +							pr_warn("Run
> fsck.hfs to repair.\n");
> +							tree->sb-
> >s_flags |= SB_RDONLY;
> +							kunmap_local
> (data);
> +							hfs_bnode_pu
> t(node);
> +							return
> ERR_PTR(-EIO);
> +						}
>  						idx += i;
>  						data[off] |= m;
>  						set_page_dirty(*page
> p);
> diff --git a/fs/hfsplus/btree.c b/fs/hfsplus/btree.c
> index 394542a47e600..3ee92248b2409 100644
> --- a/fs/hfsplus/btree.c
> +++ b/fs/hfsplus/btree.c
> @@ -561,6 +561,17 @@ struct hfs_bnode *hfs_bmap_alloc(struct
> hfs_btree *tree)
>  			if (byte != 0xff) {
>  				for (m = 0x80, i = 0; i < 8; m >>=
> 1, i++) {
>  					if (!(byte & m)) {
> +						if (unlikely(!(idx +
> i))) {
> +							pr_warn("(%s
> ): %s (cnid 0x%x) map record invalid or bitmap corruption detected,
> forcing read-only.\n",
> +								tree
> ->sb->s_id,
> +								hfs_
> btree_name(tree->cnid),
> +								tree
> ->cnid);
> +							pr_warn("Run
> fsck.hfsplus to repair.\n");
> +							tree->sb-
> >s_flags |= SB_RDONLY;
> +							kunmap_local
> (data);
> +							hfs_bnode_pu
> t(node);
> +							return
> ERR_PTR(-EIO);
> +						}
>  						idx += i;
>  						data[ctx.off] |= m;
>  						set_page_dirty(page)
> ;

We already have check [1] in hfs_btree_open():

	if (!hfs_bmap_test_bit(node, 0)) {
		pr_warn("(%s): %s (cnid 0x%x) map record invalid or
bitmap corruption detected, forcing read-only.\n",
				sb->s_id, hfs_btree_name(id), id);
		pr_warn("Run fsck.hfsplus to repair.\n");
		sb->s_flags |= SB_RDONLY;
	}

The likewise check has been implemented by patch [2] for HFS.

Thanks,
Slava.

[1]
https://elixir.bootlin.com/linux/v7.2-rc6/source/fs/hfsplus/btree.c#L388
[2]
https://lore.kernel.org/r/20260716074150.1660-1-aditya.ansh182@gmail.com