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CVE-2026-89670
nfsd: hold rcu across localio cmpxchg retry
Published 2026-09-11 (37 days ago) by the Linux kernel CNA. Subsystem fs/nfsd. Debian urgency: unimportant.
What to do, per Debian release
Debian's own status for CVE-2026-89670 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | not affected | 6.1.190-1 | Nothing to do. bookworm never shipped the vulnerable code. |
| Debian 13trixie | not affected | 6.12.113-1 | Nothing to do. trixie never shipped the vulnerable code. |
| Debian 14forky | fixed | 7.2.6-1 | Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.2.6-1 | Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
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.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
fs/nfsd/localio.c
- What this is
- KERNEL NFSD, SUNRPC, AND LOCKD SERVERSMAINTAINERS section matched on fs/nfsd/ · linux-nfs@vger.kernel.org · Supported
Check it on the machine
grep -wE 'CONFIG_NFSD|CONFIG_NFS_LOCALIO' /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
Show whether Debian builds this, per release
Debian does not build this code in 3 releases. The build rule could not be established for 1 release, so nothing is claimed about those.
- bookworm?
- trixie, forky, sidn
- ?
- not established - no Makefile entry for the affected file
- n
- not enabled on any flavour - this release does not build the code
Where a release above reads "not established", no build rule for the affected file was found in that release's kernel tree. That is a gap in this lookup, not a finding that the code is absent.
- Build switches
- CONFIG_NFSD, CONFIG_NFS_LOCALIO
- Reads as
- all on means the file is compiled; any one off means it is not.
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-09-11 (37 days ago)source
- CVSS
- 7.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.17%, higher than 6.3% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- unimportantsource
- Subsystem
- fs/nfsd
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 6.16 | 6.18.50 | 763c0bad8723 |
| 6.16 | 7.2.4 | 559570f91a7d |
| 6.16 | 7.3 | 58884694978a |
| 6.15.3 | 0 | 0 |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:LAttack vector, local
- nfsd_open_local_fh is invoked only from the NFS client's LOCALIO path (nfs_generic_pg_pgios/nfs_commit_list → nfs_local_open_fh) after a same-host UUID handshake; a remote NFS peer never reaches this cmpxchg retry, so exploitation requires local access to a loopback NFS mount.
- AC:LAttack complexity, low
- The attacker controls both sides of the race: concurrent read/write/commit syscalls enter nfsd_open_local_fh while close/put_nfs_open_context runs nfsd_file_put_local on the same nfs_file_localio slot; the sequence is repeatable at will and can be widened by preemption.
- PR:LPrivileges required, low
- Any unprivileged local user who can open files on a LOCALIO-enabled NFS mount can reach nfs_generic_pg_pgios → nfsd_open_local_fh with no capability check; starting nfsd or creating the mount is a deployment precondition, not a privilege the attacker must hold.
- UI:NUser interaction, none
- The attacker triggers the use-after-free from their own threads via I/O and close syscalls; no separate victim action is required once the LOCALIO mount exists.
- S:UScope, unchanged
- The use-after-free corrupts nfsd_file slab objects in the host kernel, which is standard kernel memory corruption within one security authority, not a VM escape, IOMMU bypass, or sandbox boundary crossing.
- C:HConfidentiality impact, high
- nfsd_file is freed via call_rcu without SLAB_TYPESAFE_BY_RCU, so after the grace period nfsd_file_get and later nf_file/nf_net dereferences operate on recycled memory, enabling an arbitrary kernel read or disclosure of another exported file's contents.
- I:HIntegrity impact, high
- refcount_inc_not_zero writes into the recycled nfsd_file, and the stale pointer is then used for write_iter, corrupting another object's refcount and enabling a follow-on premature free and control-flow hijack.
- A:HAvailability impact, high
- Use-after-free on the nfsd_file slab causes slab corruption, refcount warnings, and oops/panic when nf_file is taken from freed memory, which is a repeatable local kernel denial of service.