[PATCH] ocfs2: validate rl_used against rl_count in refcount block validator

[Ibrahim Hashimov <security@auditcode.ai>]

ocfs2_find_refcount_rec_in_rl() walks the on-disk refcount record
array with:

	for (; i < le16_to_cpu(rb->rf_records.rl_used); i++) {
		rec = &rb->rf_records.rl_recs[i];
		...

rl_recs[] lives in a single metadata block (4096 bytes on the common
configuration), so its real capacity is fixed by
ocfs2_refcount_recs_per_rb(sb) (247 records for a 4K block with the
16-byte ocfs2_refcount_rec). rl_used and rl_count are both read
directly off disk by ocfs2_validate_refcount_block() and are never
checked against that capacity, nor against each other, before any
refcount/reflink/CoW operation walks the array.

A crafted (or corrupted) refcount block with rl_used == 0xffff makes
the loop above walk far past the end of the block, dereferencing
rl_recs[i] for i up to 65534. The resulting index is then handed to
the sibling ocfs2_insert_refcount_rec(), whose insert-shift does:

	if (index < le16_to_cpu(rf_list->rl_used))
		memmove(&rf_list->rl_recs[index + 1],
			&rf_list->rl_recs[index],
			(le16_to_cpu(rf_list->rl_used) - index) *
			 sizeof(struct ocfs2_refcount_rec));

i.e. a memmove() of up to (0xffff - index) * 16 bytes (~1 MiB) from an
offset already past the block. This is reachable from an ordinary
reflink (FICLONE) against a crafted/corrupted ocfs2 image: attaching
an extent whose cpos sorts past every real record in the leaf forces
the lookup to run off the end instead of returning early on a match.
The attacker model is local: CAP_SYS_ADMIN mounting a crafted or
corrupted ocfs2 image, or a raw write to the block device backing an
already-mounted ocfs2 filesystem.

ocfs2_validate_refcount_block() already validates the block's ECC,
signature, rf_blkno and rf_fs_generation, but never rl_count/rl_used
against the block's actual on-disk capacity. This is the same class
of gap that ocfs2_validate_extent_block() (fs/ocfs2/alloc.c) already
closes for the sibling extent-list header, which checks both the
record capacity and the "used" bound before any code walks
h_list.l_recs[]:

	if (le16_to_cpu(eb->h_list.l_count) != ocfs2_extent_recs_per_eb(sb)) {
		rc = ocfs2_error(...);
		goto bail;
	}

	if (le16_to_cpu(eb->h_list.l_next_free_rec) >
	    le16_to_cpu(eb->h_list.l_count)) {
		rc = ocfs2_error(...);
		goto bail;
	}

Add the equivalent pair of checks to ocfs2_validate_refcount_block():
reject a refcount block whose rl_count does not match the fixed
per-block capacity returned by ocfs2_refcount_recs_per_rb(), and
reject rl_used > rl_count. Both checks are skipped when
OCFS2_REFCOUNT_TREE_FL is set, because in that case the same union
bytes hold an ocfs2_extent_list (rf_list), not the refcount record
list (rf_records) -- that layout is already validated separately by
ocfs2_validate_extent_block() when the referenced extent block is
read. This mirrors the existing
"!(rb->rf_flags & OCFS2_REFCOUNT_TREE_FL)" guard used elsewhere in
this file (e.g. ocfs2_get_refcount_rec()) to decide whether
rf_records or rf_list is the live member of the union.

With this in place, a forged rl_used/rl_count is caught at block
validation time (ocfs2_error()), consistent with every other
corruption check in this function, instead of driving an
out-of-bounds read in ocfs2_find_refcount_rec_in_rl() and a
subsequent out-of-bounds memmove() in ocfs2_insert_refcount_rec().

Verified against a crafted image on a v6.19 KASAN (KASAN_GENERIC)
build: replaying the same reflink (FICLONE) reliably hit a KASAN
report in __ocfs2_increase_refcount()/ocfs2_insert_refcount_rec()
before this patch, and triggers no report once
ocfs2_validate_refcount_block() rejects the forged rl_used/rl_count.

Fixes: f2c870e3b12e ("ocfs2: Add ocfs2_read_refcount_block.")
Cc: stable@vger.kernel.org
Signed-off-by: Ibrahim Hashimov <security@auditcode.ai>
Assisted-by: AuditCode-AI:2026.07
---
 fs/ocfs2/refcounttree.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/fs/ocfs2/refcounttree.c b/fs/ocfs2/refcounttree.c
index 7323bde70caa..63d6cb326e30 100644
--- a/fs/ocfs2/refcounttree.c
+++ b/fs/ocfs2/refcounttree.c
@@ -116,6 +116,33 @@ static int ocfs2_validate_refcount_block(struct super_block *sb,
 				 le32_to_cpu(rb->rf_fs_generation));
 		goto out;
 	}
+
+	/*
+	 * rf_records (rl_count/rl_used/rl_recs[]) is only meaningful when
+	 * this block is not an interior tree block (OCFS2_REFCOUNT_TREE_FL);
+	 * in that case the same union bytes hold an extent list (rf_list)
+	 * instead, which is validated by ocfs2_validate_extent_block().
+	 */
+	if (!(le32_to_cpu(rb->rf_flags) & OCFS2_REFCOUNT_TREE_FL)) {
+		if (le16_to_cpu(rb->rf_records.rl_count) !=
+		    ocfs2_refcount_recs_per_rb(sb)) {
+			rc = ocfs2_error(sb,
+					 "Refcount block #%llu has an invalid rl_count of %u\n",
+					 (unsigned long long)bh->b_blocknr,
+					 le16_to_cpu(rb->rf_records.rl_count));
+			goto out;
+		}
+
+		if (le16_to_cpu(rb->rf_records.rl_used) >
+		    le16_to_cpu(rb->rf_records.rl_count)) {
+			rc = ocfs2_error(sb,
+					 "Refcount block #%llu has an invalid rl_used of %u (rl_count %u)\n",
+					 (unsigned long long)bh->b_blocknr,
+					 le16_to_cpu(rb->rf_records.rl_used),
+					 le16_to_cpu(rb->rf_records.rl_count));
+			goto out;
+		}
+	}
 out:
 	return rc;
 }
-- 
2.50.1 (Apple Git-155)