CVE-2026-64294: mm: do file ownership checks with the proper mount idmap

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

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

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

mm: do file ownership checks with the proper mount idmap

Ever since idmapped mounts were introduced, inode ownership checks (for
side-channel protection) in mincore() and madvise(MADV_PAGEOUT) were done
against the nop_mnt_idmap, which completely ignores the file's mount's
idmap.  This results in odd edgecases like:

1) mount/bind-mount with an idmap userA:userB:1
2) userB runs an owner_or_capable() check on file that is owned by userA
on-disk/in-memory, but owned by userB after idmap translation
3) owner_or_capable() mysteriously fails as the correct idmap wasn't supplied

In the case of mincore/madvise MADV_PAGEOUT, this is usually benign,
because file_permission(file, MAY_WRITE) will probably succeed, as it uses
the proper idmap internally, but it does not need to be the case on e.g a
0444 file where even the owner itself doesn't have permissions to write to
it.

Since this is clearly not trivial to get right, introduce a
file_owner_or_capable() that can carry the correct semantics, and switch
the various users in mm to it.

The issue was found by manual code inspection & an off-list discussion
with Jan Kara.

The Linux kernel CVE team has assigned CVE-2026-64294 to this issue.


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

	Issue introduced in 5.12 with commit 9caccd41541a6f7d6279928d9f971f6642c361af and fixed in 6.6.145 with commit 744b23aa430d52f5c8e4dbff7d71496d6643bed2
	Issue introduced in 5.12 with commit 9caccd41541a6f7d6279928d9f971f6642c361af and fixed in 6.12.96 with commit 8344bdf0629457e532797b42d9d2bbf2a2900bbf
	Issue introduced in 5.12 with commit 9caccd41541a6f7d6279928d9f971f6642c361af and fixed in 6.18.39 with commit 5c942ad7df75925ee166e7f0fb36892d8dde376b
	Issue introduced in 5.12 with commit 9caccd41541a6f7d6279928d9f971f6642c361af and fixed in 7.1.4 with commit 04ba248d02d9eaa3d9077b00a6134caa75fa3e90
	Issue introduced in 5.12 with commit 9caccd41541a6f7d6279928d9f971f6642c361af and fixed in 7.2-rc3 with commit e187bc02f8fa4226d62814592cf064ee4557c470

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-2026-64294
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:
	include/linux/fs.h
	mm/filemap.c
	mm/madvise.c
	mm/mincore.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/744b23aa430d52f5c8e4dbff7d71496d6643bed2
	https://git.kernel.org/stable/c/8344bdf0629457e532797b42d9d2bbf2a2900bbf
	https://git.kernel.org/stable/c/5c942ad7df75925ee166e7f0fb36892d8dde376b
	https://git.kernel.org/stable/c/04ba248d02d9eaa3d9077b00a6134caa75fa3e90
	https://git.kernel.org/stable/c/e187bc02f8fa4226d62814592cf064ee4557c470