[PATCH v2 1/2] hfsplus: skip node 0 in hfs_bmap_alloc

[Shardul Bankar <shardul.b@mpiricsoftware.com>]

Node 0 is the header node in HFS+ B-trees and should always be allocated.
However, if a filesystem image has node 0's bitmap bit unset (e.g., due to
corruption or a buggy image generator), hfs_bmap_alloc() will find node 0
as free and attempt to allocate it. This causes a conflict because node 0
already exists as the header node, leading to a WARN_ON(1) in
hfs_bnode_create() when the node is found already hashed.

This issue can occur with syzkaller-generated HFS+ images or corrupted
real-world filesystems. Add a guard in hfs_bmap_alloc() to skip node 0
during allocation, providing defense-in-depth against such corruption.

Reported-by: syzbot+1c8ff72d0cd8a50dfeaa@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=1c8ff72d0cd8a50dfeaa
Signed-off-by: Shardul Bankar <shardul.b@mpiricsoftware.com>
---
 v2:
 - Keep the node-0 allocation guard as targeted hardening for corrupted images.
 fs/hfsplus/btree.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/hfsplus/btree.c b/fs/hfsplus/btree.c
index 229f25dc7c49..60985f449450 100644
--- a/fs/hfsplus/btree.c
+++ b/fs/hfsplus/btree.c
@@ -411,6 +411,9 @@ 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)) {
+						/* Skip node 0 (header node, always allocated) */
+						if (idx == 0 && i == 0)
+							continue;
 						idx += i;
 						data[off] |= m;
 						set_page_dirty(*pagep);
-- 
2.34.1