# CVE-2026-89493

## Summary

- **CVE ID:** CVE-2026-89493
- **Severity:** HIGH
- **CVSS Score:** 8.8 (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H)
- **CWE:** N/A
- **Published:** Sep 11, 2026
- **Last Modified:** Sep 14, 2026

## Description

In the Linux kernel, the following vulnerability has been resolved:

ocfs2: validate rl_used against rl_count in refcount block validator

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.

## Affected Products

- Linux — Linux (f2c870e3b12e38da6d9b5b17c4c8ae56a0ed68e4)
- Linux — Linux (2.6.32)
- Linux — Linux (0)
- Linux — Linux (6.12.109)
- Linux — Linux (6.18.50)
- Linux — Linux (7.2.4)
- Linux — Linux (7.3-rc1)
- Linux — Linux (5.10.270)
- Linux — Linux (5.15.221)
- Linux — Linux (6.1.188)
- Linux — Linux (6.6.157)

## References

- [CNA](https://git.kernel.org/stable/c/af56e90cb546cb0c47bef059335365283f61d6a4)
- [CNA](https://git.kernel.org/stable/c/0761d2c9494424469a0d30a9da3496ca010b0b2d)
- [CNA](https://git.kernel.org/stable/c/04ead708e13ddcb9c39319cbe02a18cef99cb823)
- [CNA](https://git.kernel.org/stable/c/4ca62df6bc0708947b48da3f6a712ecb8e73929c)
- [CNA](https://git.kernel.org/stable/c/f882e9e32d017a6569b2996c63ba3864717788e6)
- [CNA](https://git.kernel.org/stable/c/5593b63028cb43a6cda75a65ce1db95f00ea6857)
- [CNA](https://git.kernel.org/stable/c/c8b02f4ee91e757b3fff556c7168b24b7ae55132)
- [CNA](https://git.kernel.org/stable/c/97584e93fc49a2d59252ac60467e9279f11b1b5b)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.63%
- **EPSS Percentile:** 48.7

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-18._