Subject: CVE-2024-35801: x86/fpu: Keep xfd_state in sync with MSR_IA32_XFD

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

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

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

x86/fpu: Keep xfd_state in sync with MSR_IA32_XFD

Commit 672365477ae8 ("x86/fpu: Update XFD state where required") and
commit 8bf26758ca96 ("x86/fpu: Add XFD state to fpstate") introduced a
per CPU variable xfd_state to keep the MSR_IA32_XFD value cached, in
order to avoid unnecessary writes to the MSR.

On CPU hotplug MSR_IA32_XFD is reset to the init_fpstate.xfd, which
wipes out any stale state. But the per CPU cached xfd value is not
reset, which brings them out of sync.

As a consequence a subsequent xfd_update_state() might fail to update
the MSR which in turn can result in XRSTOR raising a #NM in kernel
space, which crashes the kernel.

To fix this, introduce xfd_set_state() to write xfd_state together
with MSR_IA32_XFD, and use it in all places that set MSR_IA32_XFD.

The Linux kernel CVE team has assigned CVE-2024-35801 to this issue.

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

	Issue introduced in 5.16 with commit 672365477ae8 and fixed in 6.1.84 with commit 21c7c00dae55
	Issue introduced in 5.16 with commit 672365477ae8 and fixed in 6.6.24 with commit 1acbca933313
	Issue introduced in 5.16 with commit 672365477ae8 and fixed in 6.7.12 with commit 92b0f04e9376
	Issue introduced in 5.16 with commit 672365477ae8 and fixed in 6.8.3 with commit b61e3b7055ac
	Issue introduced in 5.16 with commit 672365477ae8 and fixed in 6.9 with commit 10e4b5166df9

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-2024-35801
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:
	arch/x86/kernel/fpu/xstate.c
	arch/x86/kernel/fpu/xstate.h

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/21c7c00dae55cb0e3810d5f9506b58f68475d41d
	https://git.kernel.org/stable/c/1acbca933313aa866e39996904c9aca4d435c4cd
	https://git.kernel.org/stable/c/92b0f04e937665bde5768f3fcc622dcce44413d8
	https://git.kernel.org/stable/c/b61e3b7055ac6edee4be071c52f48c26472d2624
	https://git.kernel.org/stable/c/10e4b5166df9ff7a2d5316138ca668b42d004422