CVE-2026-89640: cifs: fix loff_t underflow in cifs_remap_file_range() when len == 0

CVE-2026-89640: cifs: fix loff_t underflow in cifs_remap_file_range() when len == 0

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

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().

The Linux kernel CVE team has assigned CVE-2026-89640 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 4.5 with commit 04b38d601239b4d9be641b412cf4b7456a041c67 and fixed in 6.12.109 with commit b098f5e5858797827666e6cd73033f52fc39b5f6
	Issue introduced in 4.5 with commit 04b38d601239b4d9be641b412cf4b7456a041c67 and fixed in 6.18.50 with commit c2a0dcb5a7a1516aa6eb6d5cedca6a8e76527028
	Issue introduced in 4.5 with commit 04b38d601239b4d9be641b412cf4b7456a041c67 and fixed in 7.2.4 with commit b057ca17b656345d04669cb87f2aff9b31d873db
	Issue introduced in 4.5 with commit 04b38d601239b4d9be641b412cf4b7456a041c67 and fixed in 7.3-rc1 with commit 6c322f5cf7476ded7a9a20f7be72462065a03c68

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-89640
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	fs/smb/client/cifsfs.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/b098f5e5858797827666e6cd73033f52fc39b5f6
	https://git.kernel.org/stable/c/c2a0dcb5a7a1516aa6eb6d5cedca6a8e76527028
	https://git.kernel.org/stable/c/b057ca17b656345d04669cb87f2aff9b31d873db
	https://git.kernel.org/stable/c/6c322f5cf7476ded7a9a20f7be72462065a03c68