[PATCH v4 2/2] arm64: Don't read GMID_EL1 when MTE is disabled

[Fuad Tabba <fuad.tabba@linux.dev>]

__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 __read_sysreg_by_encoding(), which applies the cmdline
override, and on CONFIG_ARM64_MTE, which no register reflects.

Fixes: f35abcbb8a084 ("KVM: arm64: Trap MTE access and discovery when MTE is disabled")
Cc: stable@vger.kernel.org # needs "arm64: Apply overrides to CPU local capabilities"
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
 arch/arm64/include/asm/cpufeature.h | 9 +++++++++
 arch/arm64/kernel/cpufeature.c      | 6 ++----
 arch/arm64/kernel/cpuinfo.c         | 2 +-
 3 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index a57870fa96db5..0b374e938f708 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -1085,6 +1085,15 @@ static inline bool cpu_has_lpa2(void)
 #endif
 }
 
+/* No ID register reflects CONFIG_ARM64_MTE. */
+static inline bool gmid_el1_accessible(void)
+{
+	if (!IS_ENABLED(CONFIG_ARM64_MTE))
+		return false;
+
+	return id_aa64pfr1_mte(__read_sysreg_by_encoding(SYS_ID_AA64PFR1_EL1));
+}
+
 #endif /* __ASSEMBLER__ */
 
 #endif
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 88b15b5ef2f7f..23f174b3c4e26 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -1228,7 +1228,7 @@ void __init init_cpu_features(struct cpuinfo_arm64 *info)
 		init_cpu_ftr_reg(SYS_MPAMIDR_EL1, info->reg_mpamidr);
 	}
 
-	if (id_aa64pfr1_mte(info->reg_id_aa64pfr1))
+	if (gmid_el1_accessible())
 		init_cpu_ftr_reg(SYS_GMID_EL1, info->reg_gmid);
 }
 
@@ -1523,11 +1523,9 @@ void update_cpu_features(int cpu,
 	 * they read/write depends on the GMID_EL1.BS field. Check that the
 	 * value is the same on all CPUs.
 	 */
-	if (IS_ENABLED(CONFIG_ARM64_MTE) &&
-	    id_aa64pfr1_mte(info->reg_id_aa64pfr1)) {
+	if (gmid_el1_accessible())
 		taint |= check_update_ftr_reg(SYS_GMID_EL1, cpu,
 					      info->reg_gmid, boot->reg_gmid);
-	}
 
 	/*
 	 * If we don't have AArch32 at all then skip the checks entirely
diff --git a/arch/arm64/kernel/cpuinfo.c b/arch/arm64/kernel/cpuinfo.c
index d50e2a9b066b3..389fca84f106b 100644
--- a/arch/arm64/kernel/cpuinfo.c
+++ b/arch/arm64/kernel/cpuinfo.c
@@ -502,7 +502,7 @@ static void __cpuinfo_store_cpu(struct cpuinfo_arm64 *info)
 	info->reg_id_aa64smfr0 = read_cpuid(ID_AA64SMFR0_EL1);
 	info->reg_id_aa64fpfr0 = read_cpuid(ID_AA64FPFR0_EL1);
 
-	if (id_aa64pfr1_mte(info->reg_id_aa64pfr1))
+	if (gmid_el1_accessible())
 		info->reg_gmid = read_cpuid(GMID_EL1);
 
 	if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0))
-- 
2.39.5