CVE-2026-74439: iommu/vt-d: Clear Present bit before tearing down scalable-mode context entry

CVE-2026-74439: iommu/vt-d: Clear Present bit before tearing down scalable-mode context entry

[Greg Kroah-Hartman]

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

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

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

iommu/vt-d: Clear Present bit before tearing down scalable-mode context entry

device_pasid_table_teardown() zeroes the 128-bit scalable-mode context
entry with context_clear_entry() while the Present bit is still set. This
creates a window where the hardware can fetch a torn entry, with some
fields already zeroed while Present is still set, leading to unpredictable
behavior or spurious faults. The context-cache invalidation is issued only
after the entry has been zeroed, and intel_pasid_free_table() then frees
the PASID directory pages, so the IOMMU can keep walking a stale Present=1
entry that points at freed memory.

While x86 provides strong write ordering, the compiler may reorder the two
64-bit writes to the entry, and the hardware fetch is not guaranteed to be
atomic with respect to multiple CPU writes.

Commit c1e4f1dccbe9d ("iommu/vt-d: Clear Present bit before tearing down
context entry") fixed this exact pattern in domain_context_clear_one() and
the copied-context path, but device_pasid_table_teardown() was not
converted.

Align it with the "Guidance to Software for Invalidations" in the VT-d
spec, Section 6.5.3.3, using the same ownership handshake as the sibling
fix: clear only the Present bit, flush it to the IOMMU, perform the
context-cache invalidation, and only then zero the rest of the entry.

The Linux kernel CVE team has assigned CVE-2026-74439 to this issue.


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

	Issue introduced in 6.9 with commit 81e921fd321614c2ad8ac333b041aae1da7a1c6d and fixed in 6.12.97 with commit e9e83bcfe37dc719182500dd823c03ab57d934f0
	Issue introduced in 6.9 with commit 81e921fd321614c2ad8ac333b041aae1da7a1c6d and fixed in 6.18.40 with commit 588718101e8449605f1c7e858fecb7cfa701cdab
	Issue introduced in 6.9 with commit 81e921fd321614c2ad8ac333b041aae1da7a1c6d and fixed in 7.1.5 with commit 7fd4077dc92b91b1b844333c0a06bb9e286db10a
	Issue introduced in 6.9 with commit 81e921fd321614c2ad8ac333b041aae1da7a1c6d and fixed in 7.2-rc1 with commit f46452c3df7a8d8a5addc0926e76ef19ea7da0a0
	Issue introduced in 6.8.2 with commit 333fe86968482ca701c609af590003bcea450e8f

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-2026-74439
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:
	drivers/iommu/intel/pasid.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/e9e83bcfe37dc719182500dd823c03ab57d934f0
	https://git.kernel.org/stable/c/588718101e8449605f1c7e858fecb7cfa701cdab
	https://git.kernel.org/stable/c/7fd4077dc92b91b1b844333c0a06bb9e286db10a
	https://git.kernel.org/stable/c/f46452c3df7a8d8a5addc0926e76ef19ea7da0a0