Re: [PATCH] ocfs2: validate global bitmap cl_bpc before resize

[Heming Zhao <heming.zhao@suse.com>]

On Mon, Aug 03, 2026 at 11:11:16AM +0800, ZhengYuan Huang wrote:
> [BUG]
> A corrupted global bitmap inode can make online group extension scan past
> the end of a group descriptor bitmap:
> 
> BUG: KASAN: use-after-free in _find_next_bit+0xef/0x120 lib/find_bit.c:157
> Read of size 8 at addr ffff888021b52000 by task syz.0.34/409
> Call Trace:
>  <TASK>
>  ...
>  _find_next_bit+0xef/0x120 lib/find_bit.c:157
>  find_next_bit include/linux/find.h:73 [inline]
>  find_next_bit_le include/linux/find.h:518 [inline]
>  ocfs2_find_max_contig_free_bits+0x53/0xb0 fs/ocfs2/suballoc.c:1292
>  ocfs2_update_last_group_and_inode fs/ocfs2/resize.c:127 [inline]
>  ocfs2_group_extend+0x83e/0x1ae0 fs/ocfs2/resize.c:350
>  ocfs2_ioctl+0x175/0x6e0 fs/ocfs2/ioctl.c:869
>  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_group_extend() trusts the global bitmap dinode's cl_bpc value.
> If its high byte corrupted from zero to 0xc9 makes cl_bpc 51457.
> Extending by seven clusters narrows their product to a u16 value of
> 32519 and raises a 2048-bit group to 34567 bits, beyond its 32256-bit
> bitmap. The subsequent maximum-free-run scan then reads into the next
> page.
> 
> [FIX]
> Reject a global bitmap whose cl_bpc is not one before using it in any
> resize arithmetic. The global allocator has exactly one bitmap bit per
> cluster, so this validates the invariant at the cold online-resize
> boundary and reports metadata corruption instead of enlarging bg_bits
> past the descriptor.
> 
> Fixes: d659072f7368 ("[PATCH 1/2] ocfs2: Add group extend for online resize")
> Signed-off-by: ZhengYuan Huang <gality369@gmail.com>
> ---
>  fs/ocfs2/resize.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ocfs2/resize.c b/fs/ocfs2/resize.c
> index 6375d5035972..556aaa319621 100644
> --- a/fs/ocfs2/resize.c
> +++ b/fs/ocfs2/resize.c
> @@ -311,6 +311,14 @@ int ocfs2_group_extend(struct inode * inode, int new_clusters)
>  		goto out_unlock;
>  	}
>  
> +	cl_bpc = le16_to_cpu(fe->id2.i_chain.cl_bpc);
> +	if (cl_bpc != 1) {

cl_bpc is not a fixed value. Refer from mkfs.ocfs2(8), only both block size
and cluster size are 4K, cl_bpc is 1. Otherwise, cl_bpc is 2, 4, .... 

Btw, is it any possible to put the check in ocfs2_validate_inode_block()?

Thanks,
Heming
> +		ret = ocfs2_error(main_bm_inode->i_sb,
> +				  "Invalid global bitmap bits per cluster %u\n",
> +				  cl_bpc);
> +		goto out_unlock;
> +	}
> +
>  	if (le16_to_cpu(fe->id2.i_chain.cl_cpg) !=
>  		ocfs2_group_bitmap_size(osb->sb, 0,
>  					osb->s_feature_incompat) * 8) {
> @@ -332,7 +340,6 @@ int ocfs2_group_extend(struct inode * inode, int new_clusters)
>  	}
>  	group = (struct ocfs2_group_desc *)group_bh->b_data;
>  
> -	cl_bpc = le16_to_cpu(fe->id2.i_chain.cl_bpc);
>  	if (le16_to_cpu(group->bg_bits) / cl_bpc + new_clusters >
>  		le16_to_cpu(fe->id2.i_chain.cl_cpg)) {
>  		ret = -EINVAL;
> -- 
> 2.43.0
> 
>