CVE-2026-89981: arm64: Don't read GMID_EL1 when MTE is disabled

CVE-2026-89981: arm64: Don't read GMID_EL1 when MTE is disabled

[Greg Kroah-Hartman]

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

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

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

arm64: Don't read GMID_EL1 when MTE is disabled

__cpuinfo_store_cpu() gates the GMID_EL1 read on the raw
ID_AA64PFR1_EL1, so it reads the register even when the kernel has
disabled MTE (CONFIG_ARM64_MTE=n or arm64.nomte). KVM sets HCR_EL2.TID5
in that case, and pKVM injects an UNDEF the host cannot handle:

  Internal error: Oops - Undefined instruction: 0000000002000000 [#1]  SMP
  pc : __cpuinfo_store_cpu+0xf4/0x264
  Kernel panic - not syncing: Attempted to kill the idle task!

Only pKVM reaches it, and only after a CPU is offlined and brought back
online: its CPU_ON relay sets the host HCR before the CPU enters EL1,
while plain nVHE sets it at CPUHP_AP_KVM_ONLINE.

Gate the read on the CPU's own ID_AA64PFR1_EL1 with the command-line
override applied, and on CONFIG_ARM64_MTE, which no register reflects.
The boot CPU stores its registers before init_cpu_features() strips an
unsafe override, so clamp against the hardware value here too.

The Linux kernel CVE team has assigned CVE-2026-89981 to this issue.


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

	Issue introduced in 7.0 with commit f35abcbb8a084db4c24b66ccc8db0405c08e2f61 and fixed in 7.2.5 with commit d2d22ff377cd43f2e9660426ff22e3cd4c534637
	Issue introduced in 7.0 with commit f35abcbb8a084db4c24b66ccc8db0405c08e2f61 and fixed in 7.3-rc2 with commit 5445d64199626974269fcdf347769ad44b0bb53b

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-2026-89981
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/arm64/include/asm/cpu.h
	arch/arm64/include/asm/cpufeature.h
	arch/arm64/kernel/cpufeature.c
	arch/arm64/kernel/cpuinfo.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/d2d22ff377cd43f2e9660426ff22e3cd4c534637
	https://git.kernel.org/stable/c/5445d64199626974269fcdf347769ad44b0bb53b