CVE-2026-68166: userfaultfd: prevent registration of special VMAs

CVE-2026-68166: userfaultfd: prevent registration of special VMAs

[Greg Kroah-Hartman]

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

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

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

userfaultfd: prevent registration of special VMAs

Vova Tokarev says:

  userfaultfd allows registration on shadow stack VMAs.  With userfaultfd
  access, you can register on the shadow stack, discard a page ... and
  inject a page with chosen return addresses via UFFDIO_COPY.

Update vma_can_userfault() to reject VM_SHADOW_STACK.

While on it, also reject VM_SPECIAL so that if a driver would implement
vm_uffd_ops, it wouldn't be possible to register special VMAs with
userfaultfd.

Since VM_SPECIAL includes VM_DONTEXPAND which is set but hugetlb, exclude
hugetlb VMAs from the check for VM_SPECIAL.

The Linux kernel CVE team has assigned CVE-2026-68166 to this issue.


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

	Issue introduced in 6.6 with commit 54007f818206dc27309ca423df4c87dd160a7208 and fixed in 6.18.44 with commit 165613191ad9d034bf17c00e3a142f9561597ec5
	Issue introduced in 6.6 with commit 54007f818206dc27309ca423df4c87dd160a7208 and fixed in 7.1.6 with commit 0c26202b157f1efc3cd2f26f5c30f59b508a6a5d
	Issue introduced in 6.6 with commit 54007f818206dc27309ca423df4c87dd160a7208 and fixed in 7.2-rc5 with commit 3c58f641e813c3c71039f8fd4d4e2a3aab713288

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-2026-68166
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:
	mm/userfaultfd.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/165613191ad9d034bf17c00e3a142f9561597ec5
	https://git.kernel.org/stable/c/0c26202b157f1efc3cd2f26f5c30f59b508a6a5d
	https://git.kernel.org/stable/c/3c58f641e813c3c71039f8fd4d4e2a3aab713288