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CVE-2026-89639
cifs: use cifs_invalidate_cache() in cifs_do_truncate() for O_TRUNC
Published 2026-09-11 (37 days ago) by the Linux kernel CNA. Subsystem fs/smb. Debian urgency: unimportant.
What to do, per Debian release
Debian's own status for CVE-2026-89639 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 | 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
cifs_do_truncate() is invoked from cifs_open() without i_rwsem, so it cannot use cifs_resize_file_locked() to perform a proper fscache cookie resize. Instead, add cifs_invalidate_cache() after cifs_setsize(). cifs_invalidate_cache() calls fscache_invalidate(), which works without holding i_rwsem: it unconditionally increments inval_counter and sets FSCACHE_COOKIE_NO_DATA_TO_READ, ensuring that stale cached data is not served once the cookie is later activated by fscache_use_cookie(). Truncation to zero leaves no valid cached data, making invalidation the correct semantic here.
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/smb/client/file.c
- What this is
- COMMON INTERNET FILE SYSTEM CLIENT (CIFS and SMB3)MAINTAINERS section matched on fs/smb/client/ · linux-cifs@vger.kernel.org · Supported
- Kernel module
- cifs
Check it on the machine
grep -wE 'CONFIG_CIFS|CONFIG_SMBFS' /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 cifsPrints 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_CIFS, CONFIG_SMBFS
- 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
- 7.1 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Nsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.17%, higher than 5.9% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- unimportantsource
- Subsystem
- fs/smb
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 6.18.44 | 6.18.50 | 8b9b10fe5b8b |
| 7.2 | 7.2.4 | 81fc3868a7f7 |
| 7.2 | 7.3 | 364b18323058 |
| 6.12.105 | 0 | 0 |
| 6.18.51 | 0 | 0 |
| 7.1.8 | 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
- cifs_do_truncate() runs from cifs_open() on the SMB client during a local open/openat with O_TRUNC on a mounted CIFS share (fs/smb/client/), not from ksmbd/nfsd or demux-thread packet parsing; a remote SMB peer cannot invoke this path.
- AC:LAttack complexity, low
- A write-capable open with O_TRUNC deterministically skips fscache invalidation, then the same cifs_open() activates the cookie via fscache_use_cookie(); the attacker can also hold another fd so the cookie is already active, with no race or layout outside their control.
- PR:LPrivileges required, low
- O_TRUNC requires only MAY_WRITE on the target inode, which an ordinary local user has on an already-mounted CIFS share; no init-namespace root, CAP_SYS_ADMIN, or user-namespace mount of CIFS is needed.
- UI:NUser interaction, none
- The attacker issues open(O_TRUNC) and follow-on reads/writes themselves; on common fstab/autofs/CIFS-home deployments with the fsc option already set, no other user must mount the share or open the file.
- S:UScope, unchanged
- Stale data stays within the local CIFS client's fscache and the same SMB file's contents; this is not a VM escape, IOMMU bypass, or other cross-authority boundary crossing.
- C:HConfidentiality impact, high
- After O_TRUNC the cookie is activated without FSCACHE_COOKIE_NO_DATA_TO_READ, so later reads (including holes after a size-extending write) can return the full previously truncated file contents that should have been discarded.
- I:HIntegrity impact, high
- The same stale cookie presents discarded bytes as current file data and can persist them to the SMB server via netfs read-modify-write/writeback, fully replacing zeros or newly written contents on that file.
- A:NAvailability impact, none
- The flaw is fscache coherency only; it does not oops, panic, deadlock, or exhaust kernel resources, so host availability is unaffected.