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CVE-2026-89681
nfsd: fix layout fence worker double-reference race
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-2026-89681 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
The workqueue core clears WORK_STRUCT_PENDING before the callback is invoked, so delayed_work_pending() in lm_breaker_timedout() can return false while the fence worker is already running. This lets the breaker take a duplicate sc_count reference and schedule a new worker that coalesces with the in-progress one. The extra reference is never put, leaking the layout stateid. Replace the racy delayed_work_pending() check with an ls_fence_inflight boolean set atomically with refcount_inc_not_zero() under ls_lock, and cleared under ls_lock before the final nfs4_put_stid() on the dispose path; the retry path intentionally retains it. Remove the self-rearm mod_delayed_work() at the top of the worker.
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/nfs4layouts.c
fs/nfsd/state.h
- What this is
- KERNEL NFSD, SUNRPC, AND LOCKD SERVERSMAINTAINERS section matched on fs/nfsd/ · linux-nfs@vger.kernel.org · Supported
- Kernel module
- nfsd
Check it on the machine
grep -wE 'CONFIG_NFSD|CONFIG_NFSD_PNFS' /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.
lsmod | grep -w nfsdPrints a line if that module is loaded right now, and nothing if it is not. One that is not loaded can still be loaded later by anything that needs it.
Show whether Debian builds this, per release
Debian builds this as a loadable module. Every release below compiles it as a module on every flavour, so whether the code is in the running kernel depends on whether that module is loaded.
- bookworm, trixie, forky, sidm
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- Build switches
- CONFIG_NFSD, CONFIG_NFSD_PNFS
- 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
- 9.8 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.55%, higher than 45.0% 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 |
|---|---|---|
| 7.1 | 7.2.4 | a278d361e0e8 |
| 7.1 | 7.3 | 858057122745 |
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:NAttack vector, network
- The flaw is in nfsd pNFS SCSI layout fencing (nfsd4_layout_lm_breaker_timedout → nfsd4_layout_fence_worker in fs/nfsd/nfs4layouts.c), reached over NFSv4.1 COMPOUND on TCP/2049: LAYOUTGET installs an FL_LAYOUT lease, then a conflicting OPEN/WRITE hits nfsd_open_break_lease → time_out_leases after recall timeout.
- AC:LAttack complexity, low
- The attacker controls both sides of the delayed_work_pending race: one client holds a SCSI layout and ignores CB_LAYOUTRECALL so the fence worker runs, while other attacker connections spam OPEN/WRITE so concurrent nfsd threads call lm_breaker_timedout while WORK_STRUCT_PENDING is already clear; the sequence is retryable.
- PR:NPrivileges required, none
- Typical nfsd exports accept AUTH_SYS/AUTH_NULL with no cryptographic RPC authentication; EXCHANGE_ID, CREATE_SESSION, OPEN, and LAYOUTGET succeed for any host permitted by the export, so a remote NFSv4.1 client needs no kernel-verified credentials or local account on the server.
- UI:NUser interaction, none
- The attacker is the NFS client and sends LAYOUTGET plus conflicting OPEN/WRITE compounds against an already-running pNFS SCSI export; no victim user must mount a filesystem, open a file, or take any other action on the host.
- S:UScope, unchanged
- Impact is kernel nfsd layout-stateid and nfs4_client lifetime on the NFS server host and does not cross a VM, hypervisor, sandbox, or IOMMU security boundary.
- C:HConfidentiality impact, high
- The unmatched sc_count leaves the nfs4_layout_stateid on nfs4_file.fi_lo_states without pinning nfs4_client; after that client is expired, a later LAYOUTGET walks the list and nfsd4_run_cb dereferences freed cb_clp, a use-after-free that can be reused for arbitrary kernel reads.
- I:HIntegrity impact, high
- The same use-after-free of nfs4_client (cl_cb_session, cl_callback_wq, and nfsd4_callback_ops) from nfsd4_recall_file_layout → nfsd4_run_cb enables heap spraying and function-pointer hijack of the CB_LAYOUTRECALL work path.
- A:HAvailability impact, high
- Each won race permanently leaks a layout stateid (and can pin nfs4_client via cl_lo_states); repeating LAYOUTGET/fence races exhausts nfsd state and kernel memory, and the later nfs4_client use-after-free oopses or panics the nfsd thread.