CVE-2026-89670

Status: UPDATED  |  Advisory ID: CVE-2026-89670

Key Details

Affected Products, Subsystems & Sectors

What to Know

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

nfsd: hold rcu across localio cmpxchg retry

nfsd_file objects are freed via call_rcu (filecache.c:296), and

nfsd_file_slab is created without SLAB_TYPESAFE_BY_RCU

(KMEM_CACHE(nfsd_file, 0) at filecache.c:789), so the slab page

backing a freed nfsd_file becomes freely reclaimable once the RCU

grace period elapses.

The again: retry block in nfsd_open_local_fh() loads a pointer with

cmpxchg and then calls nfsd_file_get(new) (which is

refcount_inc_not_zero) without holding rcu_read_lock. The sole caller

nfs_open_local_fh() drops rcu_read_lock before invoking this helper,

so no outer reader-side critical section covers the load.

CPU 0 (nfsd_open_local_fh) CPU 1 (nfsd_file_put_local)

----- -----

new = cmpxchg(pnf, NULL, ...)

nf = xchg(pnf, NULL)

nfsd_file_put(nf)

last ref -> call_rcu()

/* grace period elapses;

slab page recycled */

nfsd_file_get(new)

refcount_inc_not_zero(&new->nf_ref)

/* operates on recycled memory */

A non-zero word at the nf_ref offset of the recycled object makes the

refcount bump appear to succeed, and the caller then dereferences

new->nf_net and new->nf_file out of freed memory.

Fix by taking rcu_read_lock() immediately before the cmpxchg and

releasing it on all three exits of the if (new) block: the goto-again

retry, the lost-race cleanup path, and the install-succeeded path.

nfsd_file_put() and nfsd_net_put() stay outside the RCU section so

they remain free to block. (NVD)

What to Do

Monitor Linux's web page for any future patch releases.

References