linux-kernel - [PATCH] ocfs2: add check for free bits before allocation in ocfs2_move

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Message-ID: <20251231115801.293726-1-kartikey406@gmail.com>
Date: Wed, 31 Dec 2025 17:28:01 +0530
From: Deepanshu Kartikey <kartikey406@...il.com>
To: mark@...heh.com,
	jlbec@...lplan.org,
	joseph.qi@...ux.alibaba.com
Cc: ocfs2-devel@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	Deepanshu Kartikey <kartikey406@...il.com>,
	syzbot+7960178e777909060224@...kaller.appspotmail.com
Subject: [PATCH] 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() before calling
ocfs2_block_group_set_bits(), we return -ENOSPC gracefully instead of
crashing the kernel.

Reported-by: syzbot+7960178e777909060224@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7960178e777909060224
Tested-by: syzbot+7960178e777909060224@...kaller.appspotmail.com
Signed-off-by: Deepanshu Kartikey <kartikey406@...il.com>
---
 fs/ocfs2/move_extents.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/ocfs2/move_extents.c b/fs/ocfs2/move_extents.c
index 99637e34d9da..2548a8908a1b 100644
--- a/fs/ocfs2/move_extents.c
+++ b/fs/ocfs2/move_extents.c
@@ -690,6 +690,11 @@ static int ocfs2_move_extent(struct ocfs2_move_extents_context *context,
 		goto out_commit;
 	}

+	if (le16_to_cpu(gd->bg_free_bits_count) < len) {
+		ret = -ENOSPC;
+		goto out_commit;
+	}
+
 	ret = ocfs2_block_group_set_bits(handle, gb_inode, gd, gd_bh,
 					 goal_bit, len, 0, 0);
 	if (ret) {
-- 
2.43.0