[PATCH] ocfs2: bound-check dir entries in the readdir re-validation scan

[Zhan Xusheng <zhanxusheng1024@gmail.com>]

When the inode version changed since the last readdir(),
ocfs2_dir_foreach_blk_el() re-scans the directory block from its start to
relocate the current position:

	for (i = 0; i < sb->s_blocksize && i < offset; ) {
		de = (struct ocfs2_dir_entry *)(bh->b_data + i);
		if (le16_to_cpu(de->rec_len) < OCFS2_DIR_REC_LEN(1))
			break;
		i += le16_to_cpu(de->rec_len);
	}

The loop dereferences de->rec_len (at byte offset 8 within the entry)
guarded only by i < sb->s_blocksize.  `offset` is derived from ctx->pos,
which userspace controls via lseek() on the directory fd, so i can reach
the last bytes of the block; reading de->rec_len then reads a few bytes
past the s_blocksize-sized block buffer (an out-of-bounds read).

The main emit loop below already guards this via ocfs2_check_dir_entry(),
which rejects entries too close to the buffer end before touching de.
Apply the same lower bound to the re-validation scan so that a full
minimal directory entry is known to fit before de is dereferenced.  For a
consistent directory this changes nothing: entries are at least
OCFS2_DIR_REC_LEN(1) bytes, so no valid entry starts in the excluded tail.

Found by the sashiko review tool; fix approach suggested by Joseph Qi.

Suggested-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Fixes: ccd979bdbce9 ("[PATCH] OCFS2: The Second Oracle Cluster Filesystem")
Cc: stable@vger.kernel.org
Link: https://sashiko.dev/#/patchset/20260806022044.167962-1-zhanxusheng@xiaomi.com
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
---
 fs/ocfs2/dir.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/ocfs2/dir.c b/fs/ocfs2/dir.c
index d7fc3cccf2f4..3b606b4c04dd 100644
--- a/fs/ocfs2/dir.c
+++ b/fs/ocfs2/dir.c
@@ -1903,7 +1903,8 @@ static int ocfs2_dir_foreach_blk_el(struct inode *inode,
 		 * dirent right now.  Scan from the start of the block
 		 * to make sure. */
 		if (!inode_eq_iversion(inode, *f_version)) {
-			for (i = 0; i < sb->s_blocksize && i < offset; ) {
+			for (i = 0; i + OCFS2_DIR_REC_LEN(1) <= sb->s_blocksize &&
+			     i < offset;) {
 				de = (struct ocfs2_dir_entry *) (bh->b_data + i);
 				/* It's too expensive to do a full
 				 * dirent test each time round this
-- 
2.43.0