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CVE-2026-89688
nfsd: drop the stateid, not the stateowner, on seqid_op replay 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-2026-89688 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 | fixedDSA-6528-1 · 2026-09-29 · text | 6.12.111-1 | Update the kernel and reboot. DSA-6528-1 shipped the fix on 2026-09-29, in 6.12.111-1. A running kernel keeps the old code until the machine restarts. |
| 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. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4817-1 · 2026-10-04 · text | 6.12.111-1~deb12u1 | Update the kernel and reboot. DLA-4817-1 shipped the fix on 2026-10-04, in 6.12.111-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
In nfs4_preprocess_seqid_op() the stateid is obtained from nfsd4_lookup_stateid(), which holds a reference on the nfs4_stid (sc_count) but takes no reference on the stateowner. openlockstateid() merely casts that stid and likewise takes no reference. When nfsd4_cstate_assign_replay() returns -EAGAIN (the replay owner is being torn down, RP_UNHASHED) it has not taken a stateowner reference on that path. The error handling nevertheless called nfs4_put_stateowner(stp->st_stateowner), dropping an so_count reference the function never acquired -- risking a stateowner refcount underflow and use-after-free -- while leaking the sc_count reference held on the stid. The leaked stid reference can also stall a concurrent nfsd4_close_open_stateid() waiting for sc_count to drop. Drop the reference actually held -- the stid -- before retrying. The stateowner stays alive through the reference held by the stid. This mirrors the open path in nfsd4_process_open1(), where the put balances a reference that path explicitly holds on the stateowner.
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/nfs4state.c
- 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_V4' /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, sid, bookworm · linux-6.12m
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- Build switches
- CONFIG_NFSD, CONFIG_NFSD_V4
- 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
- 0.74%, higher than 53.6% 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 |
|---|---|---|
| 6.10 | 6.12.109 | 69ed78b6b947 |
| 6.10 | 6.18.50 | 00843074d9b8 |
| 6.10 | 7.2.4 | f7cb90ddc021 |
| 6.10 | 7.3 | 5e4627d3513e |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4817-1 | 2026-10-04 | bookworm 6.12.111-1~deb12u1 | tracker |
| DSA-6528-1 | 2026-09-29 | trixie 6.12.111-1 | tracker |
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
- nfsd processes NFSv4 COMPOUND ops (CLOSE, OPEN_DOWNGRADE, LOCK, OPEN_CONFIRM) in the in-kernel NFS server on TCP/UDP 2049; those handlers call nfs4_preprocess_seqid_op(), so a remote peer reaches the bug with no local access.
- AC:LAttack complexity, low
- A remote NFSv4.0 client drives both sides by sending concurrent CLOSE and another seqid-mutating op on the same openowner; CLOSE's move_to_close_lru() sets RP_UNHASHED and waits on sc_count, so the window is attacker-controlled and retryable.
- PR:NPrivileges required, none
- Typical nfsd exports use AUTH_SYS/AUTH_NULL, so RPC uids are self-asserted and not cryptographically verified; any host allowed by the export list can SETCLIENTID, OPEN, then CLOSE/OPEN_DOWNGRADE with no local Linux account.
- UI:NUser interaction, none
- The attacker issues the NFSv4 COMPOUND requests itself; no victim on the server must mount a filesystem, open a file, or take any other action.
- S:UScope, unchanged
- The underflow frees an nfsd nfs4_stateowner in the NFS server kernel; impact stays in that kernel and does not cross a VM, IOMMU, or sandbox boundary.
- C:HConfidentiality impact, high
- nfs4_put_stateowner() drops an so_count the seqid path never took, freeing the stateowner while the stid and retry loop still use it; that UAF enables slab reclaim and kernel memory disclosure.
- I:HIntegrity impact, high
- The same UAF exposes so_ops->so_unhash/so_free function pointers and list writes under cl_lock, yielding a kernel write and control-flow hijack primitive.
- A:HAvailability impact, high
- The UAF oopses or panics nfsd; the leaked stid sc_count also leaves move_to_close_lru() waiting forever for sc_count==2, hanging an nfsd thread, and the -EAGAIN retry can spin while RP_UNHASHED stays set.