[PATCH] ocfs2: fix chunk number of the first chunk in a local quota file

[Jiaming Zhang <r772577952@gmail.com>]

The local quota file in OCFS2 is divided into chunks, and each chunk
begins with a header block holding a bitmap of the quota entries that
chunk has handed out.  Chunks are numbered from zero, and that number is
used to convert the file offset of an entry back into a bit position in
the bitmap. ocfs2_local_quota_add_chunk() appends a new chunk to the
in-memory list and numbers it one past the chunk that was last:

	list_add_tail(&chunk->qc_chunk, &oinfo->dqi_chunk);
	chunk->qc_num = list_entry(chunk->qc_chunk.prev,
				   struct ocfs2_quota_chunk,
				   qc_chunk)->qc_num + 1;

The predecessor is looked up after the new chunk is added to the list,
so if the list was empty, the prev pointer is the list head itself.  The
head is the dqi_chunk member of struct ocfs2_mem_dqinfo and is not a
chunk, so reading qc_num through it lands 16 bytes past the start of the
head, on the dqi_gqinode pointer that follows it.  The first chunk of
the file is then numbered with the lower half of a kernel pointer
instead of 0.

The list is empty when the local quota file header claims the file has
no chunks.  ocfs2_local_read_info() takes dqi_chunks from that header
without validating it, so an image with dqi_chunks == 0 takes this path
when the first quota entry is allocated.

ocfs2_create_local_dquot() turns the bad number into a file offset with
ol_dqblk_off(), which shifts a 32-bit block number left by the block
size bits, so the top bits of such a large block number are lost.
ocfs2_local_release_dquot() turns the offset back into a bit index with
ol_dqblk_chunk_off(), using the full chunk number, so the lost bits push
that index far outside the bitmap, and clearing it corrupts unrelated
memory.

Compute the chunk number before putting the chunk on the list, and use 0
when the list is empty.

Fixes: 9e33d69f553a ("ocfs2: Implementation of local and global quota file handling")
Closes: https://lore.kernel.org/lkml/CANypQFZ05tpth0Xc33gmP6jgPnkY4VuezyHcsajV-SqCsmN_gg@mail.gmail.com/
Cc: stable@vger.kernel.org
Assisted-by: Claude Code:claude-opus-5
Signed-off-by: Jiaming Zhang <r772577952@gmail.com>
---
 fs/ocfs2/quota_local.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/ocfs2/quota_local.c b/fs/ocfs2/quota_local.c
index f55810c59b1b..d351cda9211f 100644
--- a/fs/ocfs2/quota_local.c
+++ b/fs/ocfs2/quota_local.c
@@ -1071,10 +1071,13 @@ static struct ocfs2_quota_chunk *ocfs2_local_quota_add_chunk(
 		goto out;
 	}
 
+	if (list_empty(&oinfo->dqi_chunk))
+		chunk->qc_num = 0;
+	else
+		chunk->qc_num = list_entry(oinfo->dqi_chunk.prev,
+					   struct ocfs2_quota_chunk,
+					   qc_chunk)->qc_num + 1;
 	list_add_tail(&chunk->qc_chunk, &oinfo->dqi_chunk);
-	chunk->qc_num = list_entry(chunk->qc_chunk.prev,
-				   struct ocfs2_quota_chunk,
-				   qc_chunk)->qc_num + 1;
 	chunk->qc_headerbh = bh;
 	*offset = 0;
 	return chunk;
-- 
2.43.0