[PATCH v3] 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 on MTE hardware even when the
kernel has disabled MTE (CONFIG_ARM64_MTE=n or arm64.nomte). KVM sets
HCR_EL2.TID5 in that case, trapping the read to EL2, where pKVM injects
an UNDEF the host cannot handle:

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

Only pKVM is affected, and only on a CPU that is offlined and brought
back online: a first bring-up either precedes KVM's initcall or is
refused by pKVM's CPU_ON relay. That relay sets the host HCR before the
CPU enters EL1, whereas plain nVHE sets it at CPUHP_AP_KVM_ONLINE,
after cpuinfo_store_cpu().

Gate the read on ID_AA64PFR1_EL1 with the cmdline override applied, and
on CONFIG_ARM64_MTE, which no register value reflects. That leaves the
SYS_GMID_EL1 feature register uninitialised when MTE is off, and it has
no readers.

Fixes: f35abcbb8a084 ("KVM: arm64: Trap MTE access and discovery when MTE is disabled")
Cc: stable@vger.kernel.org
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---

Notes:
    Changes since v2:
    - Keep the GMID_EL1 read in __cpuinfo_store_cpu() and gate it on the
      local ID_AA64PFR1_EL1 with the cmdline override applied, rather than
      deferring the read to {init,update}_cpu_features() (Will). This also
      keeps reg_gmid set in cpu_data[0], which v2 dropped (Catalin).
    
    Changes since v1:
    - Clarified that the trap only fires on a CPU that is offlined and
      brought back online, not a late first boot (Marc).
    
    Tested on QEMU with -machine virt,mte=on, under pKVM.
    
    Offline/online CPU1 with arm64.nomte on the host cmdline: unpatched
    panics in __cpuinfo_store_cpu(), patched does not.

 arch/arm64/include/asm/cpufeature.h | 15 +++++++++++++++
 arch/arm64/kernel/cpufeature.c      |  6 ++----
 arch/arm64/kernel/cpuinfo.c         |  2 +-
 3 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index a57870fa96db5..5bb242721e4b9 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -1085,6 +1085,21 @@ static inline bool cpu_has_lpa2(void)
 #endif
 }
 
+/*
+ * Reading GMID_EL1 when the kernel has disabled MTE traps to EL2, and the raw
+ * ID_AA64PFR1_EL1 reflects neither CONFIG_ARM64_MTE nor the cmdline override.
+ */
+static inline bool gmid_el1_accessible(u64 pfr1)
+{
+	if (!IS_ENABLED(CONFIG_ARM64_MTE))
+		return false;
+
+	pfr1 &= ~id_aa64pfr1_override.mask;
+	pfr1 |= id_aa64pfr1_override.val;
+
+	return id_aa64pfr1_mte(pfr1);
+}
+
 #endif /* __ASSEMBLER__ */
 
 #endif
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 9a22df0c5120f..4b339346f6c78 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(info->reg_id_aa64pfr1))
 		init_cpu_ftr_reg(SYS_GMID_EL1, info->reg_gmid);
 }
 
@@ -1490,11 +1490,9 @@ 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(info->reg_id_aa64pfr1))
 		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..52a161fc38250 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_id_aa64pfr1))
 		info->reg_gmid = read_cpuid(GMID_EL1);
 
 	if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0))
-- 
2.39.5