Add a check to verify the group descriptor has enough free bits before
attempting allocation in ocfs2_move_extent(). This prevents a kernel
BUG_ON crash in ocfs2_block_group_set_bits() when the move_extents ioctl
is called on a crafted or corrupted filesystem.

The existing validation in ocfs2_validate_gd_self() only checks static
metadata consistency (bg_free_bits_count <= bg_bits) when the descriptor
is first read from disk. However, during move_extents operations,
multiple allocations can exhaust the free bits count below the requested
allocation size, triggering BUG_ON(le16_to_cpu(bg->bg_free_bits_count)
< num_bits).

The debug trace shows the issue clearly:
  - Block group 32 validated with bg_free_bits_count=427
  - Repeated allocations decreased count: 427 -> 171 -> 43 -> ... -> 1
  - Final request for 2 bits with only 1 available triggers BUG_ON

By adding an early check in ocfs2_move_extent() right after
ocfs2_find_victim_alloc_group(), we return -ENOSPC gracefully instead of
crashing the kernel. This also avoids unnecessary work in
ocfs2_probe_alloc_group() and __ocfs2_move_extent() when the allocation
will fail.

Reported-by: syzbot+7960178e777909060224@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7960178e777909060224
Link: https://lore.kernel.org/all/20251231115801.293726-1-kartikey406@gmail.com/T/ [v1]
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
v2:
  - Fixed missing '<' in commit message (Joseph)
  - Moved check right after ocfs2_find_victim_alloc_group() to fail
    early and avoid unnecessary work (Joseph)
---
 fs/ocfs2/move_extents.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/ocfs2/move_extents.c b/fs/ocfs2/move_extents.c
index 99637e34d9da..c037fb34b1e3 100644
--- a/fs/ocfs2/move_extents.c
+++ b/fs/ocfs2/move_extents.c
@@ -662,6 +662,12 @@ static int ocfs2_move_extent(struct ocfs2_move_extents_context *context,
 		goto out_commit;
 	}
 
+	gd = (struct ocfs2_group_desc *)gd_bh->b_data;
+	if (le16_to_cpu(gd->bg_free_bits_count) < len) {
+		ret = -ENOSPC;
+		goto out_commit;
+	}
+
 	/*
 	 * probe the victim cluster group to find a proper
 	 * region to fit wanted movement, it even will perform
@@ -682,7 +688,6 @@ static int ocfs2_move_extent(struct ocfs2_move_extents_context *context,
 		goto out_commit;
 	}
 
-	gd = (struct ocfs2_group_desc *)gd_bh->b_data;
 	ret = ocfs2_alloc_dinode_update_counts(gb_inode, handle, gb_bh, len,
 					       le16_to_cpu(gd->bg_chain));
 	if (ret) {
-- 
2.43.0

On 2026/1/4 21:35, Deepanshu Kartikey wrote:
> Add a check to verify the group descriptor has enough free bits before
> attempting allocation in ocfs2_move_extent(). This prevents a kernel
> BUG_ON crash in ocfs2_block_group_set_bits() when the move_extents ioctl
> is called on a crafted or corrupted filesystem.
> 
> The existing validation in ocfs2_validate_gd_self() only checks static
> metadata consistency (bg_free_bits_count <= bg_bits) when the descriptor
> is first read from disk. However, during move_extents operations,
> multiple allocations can exhaust the free bits count below the requested
> allocation size, triggering BUG_ON(le16_to_cpu(bg->bg_free_bits_count)
> < num_bits).
> 
> The debug trace shows the issue clearly:
>   - Block group 32 validated with bg_free_bits_count=427
>   - Repeated allocations decreased count: 427 -> 171 -> 43 -> ... -> 1
>   - Final request for 2 bits with only 1 available triggers BUG_ON
> 
> By adding an early check in ocfs2_move_extent() right after
> ocfs2_find_victim_alloc_group(), we return -ENOSPC gracefully instead of
> crashing the kernel. This also avoids unnecessary work in
> ocfs2_probe_alloc_group() and __ocfs2_move_extent() when the allocation
> will fail.
> 
> Reported-by: syzbot+7960178e777909060224@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=7960178e777909060224
> Link: https://lore.kernel.org/all/20251231115801.293726-1-kartikey406@gmail.com/T/ [v1]
> Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>

Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
> ---
> v2:
>   - Fixed missing '<' in commit message (Joseph)
>   - Moved check right after ocfs2_find_victim_alloc_group() to fail
>     early and avoid unnecessary work (Joseph)
> ---
>  fs/ocfs2/move_extents.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ocfs2/move_extents.c b/fs/ocfs2/move_extents.c
> index 99637e34d9da..c037fb34b1e3 100644
> --- a/fs/ocfs2/move_extents.c
> +++ b/fs/ocfs2/move_extents.c
> @@ -662,6 +662,12 @@ static int ocfs2_move_extent(struct ocfs2_move_extents_context *context,
>  		goto out_commit;
>  	}
>  
> +	gd = (struct ocfs2_group_desc *)gd_bh->b_data;
> +	if (le16_to_cpu(gd->bg_free_bits_count) < len) {
> +		ret = -ENOSPC;
> +		goto out_commit;
> +	}
> +
>  	/*
>  	 * probe the victim cluster group to find a proper
>  	 * region to fit wanted movement, it even will perform
> @@ -682,7 +688,6 @@ static int ocfs2_move_extent(struct ocfs2_move_extents_context *context,
>  		goto out_commit;
>  	}
>  
> -	gd = (struct ocfs2_group_desc *)gd_bh->b_data;
>  	ret = ocfs2_alloc_dinode_update_counts(gb_inode, handle, gb_bh, len,
>  					       le16_to_cpu(gd->bg_chain));
>  	if (ret) {