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CVE-2026-89640
cifs: fix loff_t underflow in cifs_remap_file_range() when len == 0
Published 2026-09-11 (37 days ago) by the Linux kernel CNA. Subsystem fs/smb. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-89640 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
With len == 0 (clone to EOF), the effective length is computed as: len = src_inode->i_size - off; If off > i_size, this is a negative loff_t, corrupting the ByteCount in the FSCTL_DUPLICATE_EXTENTS_TO_FILE request and inverting the range in filemap_write_and_wait_range(). The existing off >= i_size check fires only after the ioctl has already been sent. Snapshot i_size_read() once for both the bounds check and the length calculation, eliminating the TOCTOU and 32-bit torn-read risk. Reject off > src_size with -EINVAL. Treat off == src_size as a no-op, consistent with __generic_remap_file_range_prep().
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/cifsfs.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, sid, bookworm · linux-6.12m
- 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:N/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.13%, higher than 2.1% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- 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 |
|---|---|---|
| 4.5 | 6.1.188 | f3d1ae1e6bc4 |
| 4.5 | 6.6.157 | 7f62817fe049 |
| 4.5 | 6.12.109 | b098f5e58587 |
| 4.5 | 6.18.50 | c2a0dcb5a7a1 |
| 4.5 | 7.2.4 | b057ca17b656 |
| 4.5 | 7.3 | 6c322f5cf747 |
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:LAttack vector, local
- The bug is in the CIFS client remap method, reached from the local FICLONERANGE/FICLONE ioctls through vfs_clone_file_range() on an already-mounted CIFS share, not from SMB packet parsing in ksmbd or the TCP stack.
- AC:LAttack complexity, low
- An attacker fully controls src_offset and src_length and can set length to 0 with offset past i_size, so the loff_t underflow is deterministic and does not depend on a race or memory layout they cannot influence.
- PR:LPrivileges required, low
- vfs_clone_file_range() only requires a regular source opened for read and a regular destination opened for write; any unprivileged user with those fds on a CIFS mount can issue FICLONERANGE without capabilities or init-namespace root.
- UI:NUser interaction, none
- The attacker opens their own CIFS files and issues the ioctl; no separate victim action such as mounting a share or opening a file is required once a typical enterprise/autofs CIFS mount exists.
- S:UScope, unchanged
- Impact stays inside the host kernel and the CIFS inode/page cache of the same OS security authority; this is not a VM, IOMMU, or sandbox boundary crossing.
- C:NConfidentiality impact, none
- The underflow does not read attacker-inaccessible kernel or file data; it miscomputes a clone length and then truncates destination page cache and sends a bad FSCTL, with no out-of-bounds or UAF read primitive.
- I:HIntegrity impact, high
- Negative len wraps destend to a huge uoff_t so truncate_inode_pages_range() discards the destination file's page cache, including dirty folios not flushed in the inverted range, silently destroying buffered data, and the same value corrupts ByteCount in FSCTL_DUPLICATE_EXTENTS_TO_FILE.
- A:HAvailability impact, high
- The same wrapped range and signed-to-u64 conversion can set a huge/negative EOF via smb2_set_file_size()/cifs_resize_file_locked() on duplicate-extents servers and leave netfs/pagecache state inconsistent, which can oops, hang, or deny access to the affected file and kernel paths.