[PATCH 6.6.y 1/2] erofs: avoid infinite loops due to corrupted subpage compact indexes

[Artem Dinaburg]

From: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>

[ Upstream commit e13d315ae077bb7c3c6027cc292401bc0f4ec683 ]

Robert reported an infinite loop observed by two crafted images.

The root cause is that `clusterofs` can be larger than `lclustersize`
for !NONHEAD `lclusters` in corrupted subpage compact indexes, e.g.:

  blocksize = lclustersize = 512   lcn = 6   clusterofs = 515

Move the corresponding check for full compress indexes to
`z_erofs_load_lcluster_from_disk()` to also cover subpage compact
compress indexes.

It also fixes the position of `m->type >= Z_EROFS_LCLUSTER_TYPE_MAX`
check, since it should be placed right after
`z_erofs_load_{compact,full}_lcluster()`.

[ Backport to 6.6.y: preserve the switch-based full/compact loader and
  perform the type and cluster-offset validation after either loader
  using z_logical_clusterbits. ]

Fixes: 8d2517aaeea3 ("erofs: fix up compacted indexes for block size < 4096")
Fixes: 1a5223c182fd ("erofs: do sanity check on m->type in z_erofs_load_compact_lcluster()")
Reported-by: Robert Morris <rtm@xxxxxxxxxxxxx>
Closes: https://lore.kernel.org/r/35167.1760645886@localhost
Reviewed-by: Hongbo Li <lihongbo22@xxxxxxxxxx>
Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>
Assisted-by: LLM
Signed-off-by: Artem Dinaburg <artem@xxxxxxxxxxxxxxx>
---
This is patch 1 of 2 in the ordered 6.6.y backport series.
This change addresses CVE-2025-68251. 6.6 validates clusterofs only in the
full-index loader, leaving corrupted subpage compact NONHEAD records able
to drive non-progressing lookback.
This needed a target-specific adjustment; I called it out in the bracketed
backport note above.

The fix is already present in 6.12.y, 6.18.y, and 7.2.y, but not in 6.6.y.

 fs/erofs/zmap.c | 30 +++++++++++++++++++++++-------
 1 file changed, 23 insertions(+), 7 deletions(-)

diff --git a/fs/erofs/zmap.c b/fs/erofs/zmap.c
index 76566c2cbf63..e53d5239f04d 100644
--- a/fs/erofs/zmap.c
+++ b/fs/erofs/zmap.c
@@ -67,10 +67,6 @@ static int z_erofs_load_full_lcluster(struct z_erofs_maprecorder *m,
 		if (advise & Z_EROFS_LI_PARTIAL_REF)
 			m->partialref = true;
 		m->clusterofs = le16_to_cpu(di->di_clusterofs);
-		if (m->clusterofs >= 1 << vi->z_logical_clusterbits) {
-			DBG_BUGON(1);
-			return -EFSCORRUPTED;
-		}
 		m->pblk = le32_to_cpu(di->di_u.blkaddr);
 		break;
 	default:
@@ -276,14 +272,34 @@ static int z_erofs_load_compact_lcluster(struct z_erofs_maprecorder *m,
 static int z_erofs_load_lcluster_from_disk(struct z_erofs_maprecorder *m,
 					   unsigned int lcn, bool lookahead)
 {
-	switch (EROFS_I(m->inode)->datalayout) {
+	struct erofs_inode *vi = EROFS_I(m->inode);
+	int err;
+
+	switch (vi->datalayout) {
 	case EROFS_INODE_COMPRESSED_FULL:
-		return z_erofs_load_full_lcluster(m, lcn);
+		err = z_erofs_load_full_lcluster(m, lcn);
+		break;
 	case EROFS_INODE_COMPRESSED_COMPACT:
-		return z_erofs_load_compact_lcluster(m, lcn, lookahead);
+		err = z_erofs_load_compact_lcluster(m, lcn, lookahead);
+		break;
 	default:
 		return -EINVAL;
 	}
+	if (err)
+		return err;
+
+	if (m->type >= Z_EROFS_LCLUSTER_TYPE_MAX) {
+		erofs_err(m->inode->i_sb, "unknown type %u @ lcn %u of nid %llu",
+			  m->type, lcn, vi->nid);
+		DBG_BUGON(1);
+		return -EOPNOTSUPP;
+	}
+	if (m->type != Z_EROFS_LCLUSTER_TYPE_NONHEAD &&
+	    m->clusterofs >= (1 << vi->z_logical_clusterbits)) {
+		DBG_BUGON(1);
+		return -EFSCORRUPTED;
+	}
+	return 0;
 }
 
 static int z_erofs_extent_lookback(struct z_erofs_maprecorder *m,
-- 
2.39.5