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

[Tao Yu <tao1.yu@intel.com>]

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_put(node);
+							return ERR_PTR(-EIO);
+						}
 						idx += i;
 						data[off] |= m;
 						set_page_dirty(*pagep);
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_put(node);
+							return ERR_PTR(-EIO);
+						}
 						idx += i;
 						data[ctx.off] |= m;
 						set_page_dirty(page);
-- 
2.34.1