[PATCH v5] ocfs2: validate bg_bits during freefrag scan

[PATCH v5] ocfs2: validate bg_bits during freefrag scan

[ZhengYuan Huang]

[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>
---
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

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

[Heming Zhao]

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>

> ---
> 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
> 
> 

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

[Joseph Qi]

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
>>
>>