CVE-2026-89640 cifs: fix loff_t underflow in cifs_remap_file_range() when len == 0
An integer underflow in the Linux kernel CIFS module allows corrupting file range requests, potentially leading to data corruption or denial of service.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-89640?
CVE-2026-89640 is a high-severity vulnerability in Linux, published on 11 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
cifs: fix loff_t underflow in cifs_remap_file_range() when len == 0
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().
Which versions are affected by CVE-2026-89640?
How do I fix CVE-2026-89640?
Apply the vendor's patch.
Is CVE-2026-89640 being exploited?
There is no known exploitation of CVE-2026-89640 in the wild and no public exploit code.
Sources
Also known as AZL-100292, BELL-CVE-2026-89640, DEBIAN-CVE-2026-89640, GHSA-hwfc-7pp7-m5j3, UBUNTU-CVE-2026-89640.