Commit 8f687eee authored Apr 10, 2026 by ZhengYuan Huang Committed by Andrew Morton Apr 15, 2026

ocfs2: validate bg_bits during freefrag scan

[BUG]
A crafted filesystem can trigger an out-of-bounds bitmap walk when
OCFS2_IOC_INFO is issued with OCFS2_INFO_FL_NON_COHERENT.

BUG: KASAN: use-after-free in instrument_atomic_read include/linux/instrumented.h:68 [inline]
BUG: KASAN: use-after-free in _test_bit include/asm-generic/bitops/instrumented-non-atomic.h:141 [inline]
BUG: KASAN: use-after-free in test_bit_le include/asm-generic/bitops/le.h:21 [inline]
BUG: KASAN: use-after-free in ocfs2_info_freefrag_scan_chain fs/ocfs2/ioctl.c:495 [inline]
BUG: KASAN: use-after-free in ocfs2_info_freefrag_scan_bitmap fs/ocfs2/ioctl.c:588 [inline]
BUG: KASAN: use-after-free in ocfs2_info_handle_freefrag fs/ocfs2/ioctl.c:662 [inline]
BUG: KASAN: use-after-free in ocfs2_info_handle_request+0x1c66/0x3370 fs/ocfs2/ioctl.c:754
Read of size 8 at addr ffff888031bce000 by task syz.0.636/1435
Call Trace:
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0xbe/0x130 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:378 [inline]
 print_report+0xd1/0x650 mm/kasan/report.c:482
 kasan_report+0xfb/0x140 mm/kasan/report.c:595
 check_region_inline mm/kasan/generic.c:186 [inline]
 kasan_check_range+0x11c/0x200 mm/kasan/generic.c:200
 __kasan_check_read+0x11/0x20 mm/kasan/shadow.c:31
 instrument_atomic_read include/linux/instrumented.h:68 [inline]
 _test_bit include/asm-generic/bitops/instrumented-non-atomic.h:141 [inline]
 test_bit_le include/asm-generic/bitops/le.h:21 [inline]
 ocfs2_info_freefrag_scan_chain fs/ocfs2/ioctl.c:495 [inline]
 ocfs2_info_freefrag_scan_bitmap fs/ocfs2/ioctl.c:588 [inline]
 ocfs2_info_handle_freefrag fs/ocfs2/ioctl.c:662 [inline]
 ocfs2_info_handle_request+0x1c66/0x3370 fs/ocfs2/ioctl.c:754
 ocfs2_info_handle+0x18d/0x2a0 fs/ocfs2/ioctl.c:828
 ocfs2_ioctl+0x632/0x6e0 fs/ocfs2/ioctl.c:913
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl fs/ioctl.c:583 [inline]
 __x64_sys_ioctl+0x197/0x1e0 fs/ioctl.c:583
 ...

[CAUSE]
ocfs2_info_freefrag_scan_chain() uses on-disk bg_bits directly as the
bitmap scan limit. The coherent path reads group descriptors through
ocfs2_read_group_descriptor(), which validates the descriptor before
use. The non-coherent path uses ocfs2_read_blocks_sync() instead and
skips that validation, so an impossible bg_bits value can drive the
bitmap walk past the end of the block.

[FIX]
Compute the bitmap capacity from the filesystem format with
ocfs2_group_bitmap_size(), report descriptors whose bg_bits exceeds
that limit, and clamp the scan to the computed capacity. This keeps the
freefrag report going while avoiding reads beyond the buffer.

Link: https://lkml.kernel.org/r/20260410034220.3825769-1-gality369@gmail.com


Fixes: d24a10b9 ("Ocfs2: Add a new code 'OCFS2_INFO_FREEFRAG' for o2info ioctl.")
Signed-off-by: ZhengYuan Huang <gality369@gmail.com>
Reviewed-by: Heming Zhao <heming.zhao@suse.com>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Jun Piao <piaojun@huawei.com>
Cc: Heming Zhao <heming.zhao@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

parent d12f558e

fs/ocfs2/ioctl.c

+17 −1

Original line number	Diff line number	Diff line
		@@ -441,13 +441,16 @@ static int ocfs2_info_freefrag_scan_chain(struct ocfs2_super *osb,
		struct buffer_head *bh = NULL;
		struct ocfs2_group_desc *bg = NULL;

		unsigned int max_bits, num_clusters;
		unsigned int max_bits, max_bitmap_bits, num_clusters;
		unsigned int offset = 0, cluster, chunk;
		unsigned int chunk_free, last_chunksize = 0;

		if (!le32_to_cpu(rec->c_free))
		goto bail;

		max_bitmap_bits = 8 * ocfs2_group_bitmap_size(osb->sb, 0,
		osb->s_feature_incompat);

		do {
		if (!bg)
		blkno = le64_to_cpu(rec->c_blkno);
		@@ -479,6 +482,19 @@ static int ocfs2_info_freefrag_scan_chain(struct ocfs2_super *osb,
		continue;

		max_bits = le16_to_cpu(bg->bg_bits);

		/*
		* Non-coherent scans read raw blocks and do not get the
		* bg_bits validation from
		* ocfs2_read_group_descriptor().
		*/
		if (max_bits > max_bitmap_bits) {
		mlog(ML_ERROR,
		"Group desc #%llu has %u bits, max bitmap bits %u\n",
		(unsigned long long)blkno, max_bits, max_bitmap_bits);
		max_bits = max_bitmap_bits;
		}

		offset = 0;

		for (chunk = 0; chunk < chunks_in_group; chunk++) {