linux-kernel - [PATCH v2 0/2] hfsplus: avoid re-allocating header node 0 and stop on already-hashed nodes

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Message-Id: <20251224151347.1861896-1-shardul.b@mpiricsoftware.com>
Date: Wed, 24 Dec 2025 20:43:45 +0530
From: Shardul Bankar <shardul.b@...ricsoftware.com>
To: slava@...eyko.com,
	zippel@...ux-m68k.org,
	linux-fsdevel@...r.kernel.org,
	glaubitz@...sik.fu-berlin.de,
	frank.li@...o.com
Cc: akpm@...l.org,
	linux-kernel@...r.kernel.org,
	janak@...ricsoftware.com,
	shardulsb08@...il.com,
	stable@...r.kernel.org,
	syzbot+1c8ff72d0cd8a50dfeaa@...kaller.appspotmail.com,
	Shardul Bankar <shardul.b@...ricsoftware.com>
Subject: [PATCH v2 0/2] hfsplus: avoid re-allocating header node 0 and stop on already-hashed nodes

Hi,

syzbot reported a warning and crash when mounting a corrupted HFS+ image where
the on-disk B-tree bitmap has node 0 (header node) marked free. In that case
hfs_bmap_alloc() can try to allocate node 0 and reach hfs_bnode_create() with
an already-hashed node number.

Patch 1 prevents allocating the reserved header node (node 0) even if the bitmap
is corrupted.

Patch 2 follows Slava's review suggestion and changes the "already hashed" path
in hfs_bnode_create() to return ERR_PTR(-EEXIST) instead of returning the existing
node pointer, so we don't continue in a non-"business as usual" situation.

v2 changes:
  - Implement Slava's suggestion: return ERR_PTR(-EEXIST) for already-hashed nodes.
  - Keep the node-0 allocation guard as a minimal, targeted hardening measure.

Reported-by: syzbot+1c8ff72d0cd8a50dfeaa@...kaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=1c8ff72d0cd8a50dfeaa
Link: https://lore.kernel.org/all/20251213233215.368558-1-shardul.b@mpiricsoftware.com/

Shardul Bankar (2):
  hfsplus: skip node 0 in hfs_bmap_alloc
  hfsplus: return error when node already exists in hfs_bnode_create

 fs/hfsplus/bnode.c | 2 +-
 fs/hfsplus/btree.c | 3 +++
 2 files changed, 4 insertions(+), 1 deletion(-)

-- 
2.34.1