CVE-2026-89670
HighCVSS 7.8Exploitation Probability (EPSS)
Low risk6th percentile - higher than 6% of all known CVEs
Summary
A vulnerability was fixed in the Linux kernel's nfsd module where nfsd_open_local_fh() dereferenced an nfsd_file pointer without holding an RCU read lock. The object could be freed and reallocated by another thread, leading to a use-after-free condition. The fix wraps the cmpxchg and reference acquisition in an rcu_read_lock() section.
Risk Assessment
This use-after-free in the nfsd module can lead to kernel memory corruption, system crash (panic), or potentially privilege escalation by a local attacker. It affects systems acting as NFS servers with the localio feature enabled.
Recommendation
Update the Linux kernel to a version containing the fix for CVE-2026-89670. If immediate patching is not possible, consider disabling the localio feature in the NFS server configuration until the patch is applied.
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Original NVD description (English source)
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.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS