CVE-2026-68448: ovl: check access to copy_file_range source with src mounter creds

From: Greg Kroah-Hartman <[email protected]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

ovl: check access to copy_file_range source with src mounter creds

Commit 5dae222a5ff0c ("vfs: allow copy_file_range to copy across devices")
allowed filesystems that implement the copy_file_range() f_op to decide
if they want to access cross-sb copy from/to the same fs type.

The same commit added checks to verify same sb copy for filesystems that
implement ->copy_file_range() and do not support cross-sb copy at the
time, namely, to ceph, fuse and nfs.

The two remaining fs which implement ->copy_file_range(), cifs and
overlayfs started to support cross-sb copy from this time.

While overlayfs does support cross-sb copy when the two underlying files
are on the same base fs, the copy operation on the two real files from
two different overalyfs filesystems is performed with the mounter
creds of the destination overlayfs and the read permission access hook
for the source file was called with the wrong creds.

This could cause either deny of access to copy which would otherwise be
allowed (e.g. with splice) or allow read access to file which would
otherwise be denied.

Fix the latter case by explicitly verifying read access to source file
with the source overlayfs mounter creds.

The former case remains a quirk of cross-sb overlayfs copy, but
userspace could fall back to regular copy so no harm done.

The Linux kernel CVE team has assigned CVE-2026-68448 to this issue.


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

	Issue introduced in 5.3 with commit 5dae222a5ff0c269730393018a5539cc970a4726 and fixed in 6.18.42 with commit 9ec22c8113d8cf72ed7197bb61037dcad09e50d8
	Issue introduced in 5.3 with commit 5dae222a5ff0c269730393018a5539cc970a4726 and fixed in 7.1.6 with commit 1f4a107439d2e43db176e34919933e617cb7f2c5
	Issue introduced in 5.3 with commit 5dae222a5ff0c269730393018a5539cc970a4726 and fixed in 7.2-rc5 with commit a1e0eb8f55cfe09bb31a202a388babc411292656

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-2026-68448
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/overlayfs/file.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/9ec22c8113d8cf72ed7197bb61037dcad09e50d8
	https://git.kernel.org/stable/c/1f4a107439d2e43db176e34919933e617cb7f2c5
	https://git.kernel.org/stable/c/a1e0eb8f55cfe09bb31a202a388babc411292656