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CVE-2026-89712
NFSD: restart ssc_expire_umount walk after dropping nfsd_ssc_lock
Published 2026-09-11 (37 days ago) by the Linux kernel CNA. Subsystem fs/nfsd. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-89712 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 | fixedDLA-4841-1 · 2026-10-17 · text | 6.1.190-1 | Update the kernel and reboot. DLA-4841-1 shipped the fix on 2026-10-17, in 6.1.190-1. A running kernel keeps the old code until the machine restarts. |
| 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
nfsd4_ssc_expire_umount() walks nn->nfsd_ssc_mount_list with list_for_each_entry_safe(ni, tmp, ...). For each expired entry it sets nsui_busy = true, drops nfsd_ssc_lock to run mntput() on the source vfsmount, then reacquires the lock to list_del + kfree the entry and continue iterating via the macro's saved tmp pointer. The nsui_busy flag protects the current ni from concurrent nfsd4_ssc_setup_dul() finders during the lock-drop window, but it does not pin tmp. Another nfsd RPC thread that fails its source- server mount and reaches nfsd4_ssc_cancel_dul() will, during that same window, take nfsd_ssc_lock, list_del + kfree its own ssc_umount item, and release the lock. If that item is the saved tmp of the expire walk, the next iteration dereferences a freed nfsd4_ssc_umount_item. Restart the walk from the head after the mntput() unlock window so no saved next pointer survives the lock-drop. The list is bounded by the number of active inter-server source mounts (typically small) and the expire delayed-work runs periodically rather than per-IO, so the restart is cheap.
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
- 0.71%, higher than 52.8% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- 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 |
|---|---|---|
| 5.10.220 | 5.10.270 | 2b59029b8f24 |
| 5.14 | 5.15.221 | d9e151fea5ed |
| 5.14 | 6.1.188 | 659ee3da0731 |
| 5.14 | 6.6.157 | 60680ae7243b |
| 5.14 | 6.12.109 | 77de363d9a1c |
| 5.14 | 6.18.50 | 4ed8d2317aef |
| 5.14 | 7.2.4 | 7377fa964b8a |
| 5.14 | 7.3 | 036c1b182f4d |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4841-1 | 2026-10-17 | bookworm 6.1.190-1 | tracker |
| 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
- The UAF is between nfsd4_ssc_expire_umount() and nfsd4_ssc_cancel_dul(); cancel_dul runs from nfsd4_copy() on NFSv4.2 OP_COPY, reached by a COMPOUND over TCP/2049 to in-kernel nfsd. A remote client populates nfsd_ssc_mount_list and frees the saved iterator with no local access.
- AC:LAttack complexity, low
- The attacker drives both race sides with concurrent async inter-server COPY requests: completed copies leave expired nfsd4_ssc_umount_item entries for the laundromat, while a COPY to another source IP fails vfs_kern_mount() into cancel_dul() during mntput(). An attacker-controlled source NFS server can stall unmount, so the window is attacker-created and retryable.
- PR:NPrivileges required, none
- Under AUTH_SYS (default for typical NFS deployments) the server verifies no secret; EXCHANGE_ID, CREATE_SESSION, and OPEN succeed for any host allowed by the export, so no server-verified privileges are required to issue the COPY compounds that hit setup_dul/cancel_dul.
- UI:NUser interaction, none
- The attacker’s own NFS client issues the COPY compounds and source-server traffic that create, expire, and cancel ssc_umount items; no administrator or other user action on the victim server is required.
- S:UScope, unchanged
- The use-after-free is of a kmalloc’d nfsd4_ssc_umount_item in the nfsd host kernel; impact stays within the same kernel security authority and does not cross a VM, IOMMU, or sandbox boundary.
- C:HConfidentiality impact, high
- After cancel_dul() kfree()s the saved tmp, the expire walk continues from a freed nfsd4_ssc_umount_item whose vfsmount, list links, and metadata the attacker can replace via slab reuse, giving a UAF read primitive over kernel heap.
- I:HIntegrity impact, high
- The expire path then writes nsui_busy, calls mntput() on attacker-controlled nsui_vfsmount, and runs list_del()/kfree() on poisoned pointers, yielding arbitrary-write and control-flow hijack primitives from the same UAF.
- A:HAvailability impact, high
- Dereferencing the freed list node in nfsd4_ssc_expire_umount() oopses or panics the laundromat worker even without a full exploit, taking down the in-kernel NFS server.