ocfs2: add check for free bits before allocation in ocfs2_move_extent()

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.

Link: https://lkml.kernel.org/r/20260104133504.14810-1-kartikey406@gmail.com
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
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/
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>

diff --git a/fs/ocfs2/move_extents.c b/fs/ocfs2/move_extents.c
index 99637e34d9daf895d9deb6098b3e34905014433e..e3cdf87884843e6284e27a2311ce66f5e96a91f0 100644 (file)
--- 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) {