[PATCH v6 0/2] hfsplus: prevent b-tree allocator corruption

Shardul Bankar
2026-03-15T17:20:37Z

Hi all,

This series addresses a Syzkaller-reported vulnerability where fuzzed
HFS+ images mark the B-tree Header Node (Node 0) as free in the
allocation bitmap. This violates a core filesystem invariant and
leads to allocator corruption and kernel panics.

To fix this safely and cleanly, the series is split into two parts:

Patch 1 introduces a unified API for B-tree map record access
(struct hfs_bmap_ctx, hfs_bmap_get_map_page, and hfs_bmap_clear_bit) and
refactors the boilerplate page mapping logic out of
hfs_bmap_alloc() and hfs_bmap_free().
Patch 2 utilizes this new API to perform a mount-time validation of
Node 0 via hfs_bmap_test_bit(), forcing a safe read-only mount if structural
or bit-level corruption is detected.

Note on Allocator Optimization: Following discussions in v4, there is
a recognized opportunity to optimize hfs_bmap_alloc() from a first-fit
to a next-fit allocator by caching an in-core allocation hint (roving
pointer) and bounding the scan with tree->node_count. To keep the
scope of this series strictly aligned with the Syzkaller corruption
fix, that architectural optimization is deferred to a separate,
follow-up patchset/thread.

Link: https://lore.kernel.org/all/20260228122305.1406308-1-shardul.b@mpiricsoftware.com/

v6:
- Symmetric Mapping: Updated hfs_bmap_get_map_page() to return an unmapped
struct page * instead of a mapped pointer. This ensures the caller
explicitly handles both kmap_local_page() and kunmap_local(), preventing
dangerous asymmetric mapping lifecycles.

- Bisectability: Moved the introduction of hfs_bmap_test_bit() from Patch 1
to Patch 2 where it is actually consumed, preventing a -Wunused-function
compiler warning and keeping the Git history perfectly bisectable.

- API Clarity: Renamed the bit_idx parameter to node_bit_idx in the bit-level
helpers to explicitly clarify that the index is strictly relative to the
target hfs_bnode's map record, preventing future absolute-index misuse.

- Naming & Style: Replaced hardcoded 8s with BITS_PER_BYTE, updated local
variable names (m to mask, data to bmap inside the new helpers), and added
kernel-doc field descriptions to struct hfs_bmap_ctx.

- Minimal Diff Scope: Restored the original variable names (data, m) inside
the legacy hfs_bmap_alloc() loop to keep the diff surgically focused on the
logical changes and preserve git blame history.

- Error Codes: Changed the error return in hfs_bmap_clear_bit() from
-EALREADY to -EINVAL.

- CNID String Lookup: Replaced the sparse string array with #define macros
and a standard switch statement for cleaner subsystem visibility, per
Slava's preference.

v5:
- API Encapsulation: Introduced struct hfs_bmap_ctx to cleanly bundle
offset, length, and page index state instead of passing multiple
pointers, addressing reviewer feedback.

- Bit-Level Helpers: Added hfs_bmap_test_bit() and hfs_bmap_clear_bit()
to safely encapsulate mapping/unmapping for single-bit accesses
(like the mount-time check and node freeing).

- Performance Retention: Retained the page-level mapping approach for
the linear scan inside hfs_bmap_alloc() to prevent the severe
performance regression of mapping/unmapping on a per-byte basis,
while refactoring it to use the new ctx struct.

- Hexagon Overflow Fix: Fixed a 0-day Kernel Test Robot warning on
architectures with 256KB page sizes by upgrading the offset variables
in the new struct hfs_bmap_ctx to unsigned int, preventing 16-bit shift
overflows.
Link: https://lore.kernel.org/all/202602270310.eBmeD8VX-lkp@intel.com/

- Map Record Spanning: Added a byte_offset parameter to the page mapper
to correctly handle large map records that span across multiple 4KB
pages.

- Loop Mask Revert: Reverted the 0x80 bitmask in the alloc() inner loop
back to its original state (and dropped the HFSPLUS_BTREE_NODE0_BIT
macro), as it represents a generic sliding mask, not specifically
Node 0.

- String Array Cleanup: Replaced the verbose switch(id) block in the
mount validation with a clean static array of constant strings for
the CNID names, per reviewer feedback.

v4:
- Split the changes into a 2-patch series (Refactoring + Bug Fix).
- Extracted map node traversal into a generic helper (hfs_bmap_get_map_page)
as per Slava's feedback, replacing manual offset/page management.
- Added node-type validation (HFS_NODE_HEADER vs HFS_NODE_MAP) inside the
helper to defend against structurally corrupted linkages.
- Replaced hardcoded values with named macros (HFSPLUS_BTREE_NODE0_BIT, etc).
- Handled invalid map offsets/lengths as corruption, continuing the mount
as SB_RDONLY instead of failing it completely to preserve data recovery.

v3:
- Moved validation logic inline into hfs_btree_open() to allow
reporting the specific corrupted tree ID.
- Replaced custom offset calculations with existing hfs_bnode_find()
and hfs_brec_lenoff() infrastructure to handle node sizes and
page boundaries correctly.
- Removed temporary 'btree_bitmap_corrupted' superblock flag; setup
SB_RDONLY directly upon detection.
- Moved logging to hfs_btree_open() to include the specific tree ID in
the warning message
- Used explicit bitwise check (&) instead of test_bit() to ensure
portability. test_bit() bit-numbering is architecture-dependent
(e.g., bit 0 vs bit 7 can swap meanings on BE vs LE), whereas
masking 0x80 consistently targets the MSB required by the HFS+
on-disk format.

v2:
- Fix compiler warning about comparing u16 bitmap_off with PAGE_SIZE which
can exceed u16 maximum on some architectures
- Cast bitmap_off to unsigned int for the PAGE_SIZE comparison to avoid
tautological constant-out-of-range comparison warning.
- Link: https://lore.kernel.org/oe-kbuild-all/202601251011.kJUhBF3P-lkp@intel.com/

Shardul Bankar (2):
hfsplus: refactor b-tree map page access and add node-type validation
hfsplus: validate b-tree node 0 bitmap at mount time

fs/hfsplus/btree.c | 236 +++++++++++++++++++++++++++++--------
include/linux/hfs_common.h | 2 +
2 files changed, 191 insertions(+), 47 deletions(-)

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2.34.1