Merge tag 'x86_apic_for_v6.1_rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 APIC update from Borislav Petkov:

 - Add support for locking the APIC in X2APIC mode to prevent SGX
   enclave leaks

* tag 'x86_apic_for_v6.1_rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/apic: Don't disable x2APIC if locked

Documentation/admin-guide/kernel-parameters.txt

@@ -3805,6 +3805,10 @@
 
 	nox2apic	[X86-64,APIC] Do not enable x2APIC mode.
 
+			NOTE: this parameter will be ignored on systems with the
+			LEGACY_XAPIC_DISABLED bit set in the
+			IA32_XAPIC_DISABLE_STATUS MSR.
+
 	nps_mtm_hs_ctr=	[KNL,ARC]
 			This parameter sets the maximum duration, in
 			cycles, each HW thread of the CTOP can run

arch/x86/Kconfig

@@ -451,6 +451,11 @@ config X86_X2APIC
 	  This allows 32-bit apic IDs (so it can support very large systems),
 	  and accesses the local apic via MSRs not via mmio.
 
+	  Some Intel systems circa 2022 and later are locked into x2APIC mode
+	  and can not fall back to the legacy APIC modes if SGX or TDX are
+	  enabled in the BIOS.  They will be unable to boot without enabling
+	  this option.
+
 	  If you don't know what to do here, say N.
 
 config X86_MPPARSE
@@ -1922,7 +1927,7 @@ endchoice
 
 config X86_SGX
 	bool "Software Guard eXtensions (SGX)"
-	depends on X86_64 && CPU_SUP_INTEL
+	depends on X86_64 && CPU_SUP_INTEL && X86_X2APIC
 	depends on CRYPTO=y
 	depends on CRYPTO_SHA256=y
 	select SRCU

arch/x86/include/asm/cpu.h

@@ -94,4 +94,6 @@ static inline bool intel_cpu_signatures_match(unsigned int s1, unsigned int p1,
 	return p1 & p2;
 }
 
+extern u64 x86_read_arch_cap_msr(void);
+
 #endif /* _ASM_X86_CPU_H */

arch/x86/include/asm/msr-index.h

@@ -155,6 +155,11 @@
 						 * Return Stack Buffer Predictions.
 						 */
 
+#define ARCH_CAP_XAPIC_DISABLE		BIT(21)	/*
+						 * IA32_XAPIC_DISABLE_STATUS MSR
+						 * supported
+						 */
+
 #define MSR_IA32_FLUSH_CMD		0x0000010b
 #define L1D_FLUSH			BIT(0)	/*
 						 * Writeback and invalidate the
@@ -1054,4 +1059,12 @@
 #define MSR_IA32_HW_FEEDBACK_PTR        0x17d0
 #define MSR_IA32_HW_FEEDBACK_CONFIG     0x17d1
 
+/* x2APIC locked status */
+#define MSR_IA32_XAPIC_DISABLE_STATUS	0xBD
+#define LEGACY_XAPIC_DISABLED		BIT(0) /*
+						* x2APIC mode is locked and
+						* disabling x2APIC will cause
+						* a #GP
+						*/
+
 #endif /* _ASM_X86_MSR_INDEX_H */

arch/x86/kernel/apic/apic.c

@@ -61,6 +61,7 @@
 #include <asm/cpu_device_id.h>
 #include <asm/intel-family.h>
 #include <asm/irq_regs.h>
+#include <asm/cpu.h>
 
 unsigned int num_processors;
 
@@ -1751,11 +1752,26 @@ EXPORT_SYMBOL_GPL(x2apic_mode);
 
 enum {
 	X2APIC_OFF,
-	X2APIC_ON,
 	X2APIC_DISABLED,
+	/* All states below here have X2APIC enabled */
+	X2APIC_ON,
+	X2APIC_ON_LOCKED
 };
 static int x2apic_state;
 
+static bool x2apic_hw_locked(void)
+{
+	u64 ia32_cap;
+	u64 msr;
+
+	ia32_cap = x86_read_arch_cap_msr();
+	if (ia32_cap & ARCH_CAP_XAPIC_DISABLE) {
+		rdmsrl(MSR_IA32_XAPIC_DISABLE_STATUS, msr);
+		return (msr & LEGACY_XAPIC_DISABLED);
+	}
+	return false;
+}
+
 static void __x2apic_disable(void)
 {
 	u64 msr;
@@ -1793,6 +1809,10 @@ static int __init setup_nox2apic(char *str)
 				apicid);
 			return 0;
 		}
+		if (x2apic_hw_locked()) {
+			pr_warn("APIC locked in x2apic mode, can't disable\n");
+			return 0;
+		}
 		pr_warn("x2apic already enabled.\n");
 		__x2apic_disable();
 	}
@@ -1807,10 +1827,18 @@ early_param("nox2apic", setup_nox2apic);
 void x2apic_setup(void)
 {
 	/*
-	 * If x2apic is not in ON state, disable it if already enabled
+	 * Try to make the AP's APIC state match that of the BSP,  but if the
+	 * BSP is unlocked and the AP is locked then there is a state mismatch.
+	 * Warn about the mismatch in case a GP fault occurs due to a locked AP
+	 * trying to be turned off.
+	 */
+	if (x2apic_state != X2APIC_ON_LOCKED && x2apic_hw_locked())
+		pr_warn("x2apic lock mismatch between BSP and AP.\n");
+	/*
+	 * If x2apic is not in ON or LOCKED state, disable it if already enabled
 	 * from BIOS.
 	 */
-	if (x2apic_state != X2APIC_ON) {
+	if (x2apic_state < X2APIC_ON) {
 		__x2apic_disable();
 		return;
 	}
@@ -1831,6 +1859,11 @@ static __init void x2apic_disable(void)
 	if (x2apic_id >= 255)
 		panic("Cannot disable x2apic, id: %08x\n", x2apic_id);
 
+	if (x2apic_hw_locked()) {
+		pr_warn("Cannot disable locked x2apic, id: %08x\n", x2apic_id);
+		return;
+	}
+
 	__x2apic_disable();
 	register_lapic_address(mp_lapic_addr);
 }
@@ -1889,7 +1922,10 @@ void __init check_x2apic(void)
 	if (x2apic_enabled()) {
 		pr_info("x2apic: enabled by BIOS, switching to x2apic ops\n");
 		x2apic_mode = 1;
-		x2apic_state = X2APIC_ON;
+		if (x2apic_hw_locked())
+			x2apic_state = X2APIC_ON_LOCKED;
+		else
+			x2apic_state = X2APIC_ON;
 	} else if (!boot_cpu_has(X86_FEATURE_X2APIC)) {
 		x2apic_state = X2APIC_DISABLED;
 	}