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CVE-2026-89682
nfsd: fix fcache_disposal UAF by inlining dispose state into nfsd_net
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-2026-89682 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 | vulnerable | 6.1.190-1 | No fix published anywhere yet, neither in Debian nor upstream for the series bookworm tracks. Watch the Debian tracker page for this CVE. Advisories for bookworm have historically landed a median of 23 days after publication. |
| Debian 13trixie | vulnerable | 6.12.113-1 | No fix published anywhere yet, neither in Debian nor upstream for the series trixie tracks. Watch the Debian tracker page for this CVE. Advisories for trixie have historically landed a median of 11 days after publication. |
| 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_dispose_list_delayed() defers fput() to nfsd service threads via a per-net freeme queue, preventing the shrinker and GC worker from bearing the cost of closing files (see ffb402596147). However, the queue lives in a separately-allocated struct nfsd_fcache_disposal that is freed by nfsd_free_fcache_disposal_net() during per-net teardown. The global shrinker, laundrette, and fsnotify callbacks can still be inside nfsd_file_dispose_list_delayed() dereferencing that pointer, causing a use-after-free. Inline the spinlock and freeme list directly into struct nfsd_net (as fcache_dispose_lock and fcache_dispose_list), eliminating the separately allocated struct nfsd_fcache_disposal entirely. These fields now have the same lifetime as the net namespace itself, so there is no dangling pointer to chase. nfsd_file_cache_start_net() now just initializes the inline fields and cannot fail due to allocation. nfsd_file_cache_shutdown_net() drains the inline list directly instead of freeing a separate struct. The alloc/free helpers are removed.
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/filecache.c
fs/nfsd/netns.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 -w CONFIG_NFSD /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 switch
- CONFIG_NFSD
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
- 8.1 high
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.53%, higher than 43.9% 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 |
|---|---|---|
| 5.17 | 6.18.51 | ba0ee9e04b7a |
| 5.17 | 7.2.4 | cadc9036d5a8 |
| 5.17 | 7.3 | bbf13732f743 |
| 5.10.220 | 0 | 0 |
| 5.15.154 | 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:NAttack vector, network
- nfsd_file_dispose_list_delayed() runs in the in-kernel NFS server filecache on nfsd_files cached from remote OPEN/READ/WRITE over TCP/UDP 2049; a network NFS client populates that cache and can drive delayed dispose via the global GC laundrette, shrinker, or fsnotify from REMOVE, matching kernel guidance that nfsd is Network.
- AC:HAttack complexity, high
- The UAF requires nfsd_free_fcache_disposal_net() to kfree the separately allocated per-net nfsd_fcache_disposal during nfsd_file_cache_shutdown_net() while the global shrinker, laundrette, or fsnotify still dereference it—a victim-side nfsd restart, shutdown, or netns teardown the remote attacker cannot cause, only race against.
- PR:NPrivileges required, none
- Typical nfsd deployments use AUTH_SYS/AUTH_NULL with IP-based exports, so a remote peer that can reach the export self-asserts credentials with no cryptographic authentication and can OPEN/READ files into the filecache; no local Linux account or capability is required.
- UI:NUser interaction, none
- The attacker issues NFS operations that populate and reclaim the filecache themselves; nfsd per-net teardown is an environmental race counted under Attack Complexity, not an interactive victim action such as mounting a filesystem or opening a file.
- S:UScope, unchanged
- The use-after-free corrupts host-kernel slab (struct nfsd_fcache_disposal) within the same OS security authority as nfsd and does not cross a VM, IOMMU, or sandbox boundary.
- C:HConfidentiality impact, high
- This is a slab use-after-free of the kmalloc'd nfsd_fcache_disposal object; reclaim or spray of the freed object lets dispose_list_delayed read attacker-controlled lock and list_head fields, enabling arbitrary kernel disclosure per UAF guidance.
- I:HIntegrity impact, high
- spin_lock and list_move_tail on the freed disposal object write into reclaimed slab, corrupting list nodes and lock state and yielding a groomable write or control-flow hijack primitive characteristic of exploitable kernel UAFs.
- A:HAvailability impact, high
- Use-after-free of the disposal spinlock and list from shrinker, GC worker, or fsnotify context causes kernel oops, panic, or list-corruption hangs, fully disrupting availability of the NFS server host.