Re: [PATCH v5] ocfs2: validate bg_bits during freefrag scan

[Joseph Qi <joseph.qi@linux.alibaba.com>]

On 4/10/26 11:56 AM, Heming Zhao wrote:
> On Fri, Apr 10, 2026 at 11:42:20AM +0800, ZhengYuan Huang wrote:
>> [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.
>>
>> Fixes: d24a10b9f8ed ("Ocfs2: Add a new code 'OCFS2_INFO_FREEFRAG' for o2info ioctl.")
>> Signed-off-by: ZhengYuan Huang <gality369@gmail.com>
> 
> LGTM.
> Reviewed-by: Heming Zhao <heming.zhao@suse.com>

Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
> 
>> ---
>> v5:
>> - move max_bitmap_bits computation out of the group loop
>> - restore the untouched blank line after assigning bg
>>
>> v4:
>> - add the freefrag introduction commit as a Fixes tag
>> - use cluster allocator bitmap sizing with ocfs2_group_bitmap_size(..., 0, ...)
>> - drop the unnecessary 8U suffix and tighten the mlog() formatting
>>
>> v3:
>> - restore the empty-group fast path before computing bitmap limits
>> - move the bg_bits clamp below bg_free_bits_count to skip extra work
>>   for empty groups
>>
>> v2:
>> - use ocfs2_group_bitmap_size() instead of the on-disk bg_size field
>> - clamp bg_bits to the computed bitmap capacity and continue scanning
>> ---
>>  fs/ocfs2/ioctl.c | 19 ++++++++++++++++++-
>>  1 file changed, 18 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/ocfs2/ioctl.c b/fs/ocfs2/ioctl.c
>> index b6864602814c..10f573a8b6b6 100644
>> --- a/fs/ocfs2/ioctl.c
>> +++ b/fs/ocfs2/ioctl.c
>> @@ -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++) {
>> -- 
>> 2.43.0
>>
>>