Merge tag 'x86-urgent-2026-05-31' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 fixes from Ingo Molnar:

 - Make the clearcpuid= boot parameter less prominent
   and warn about its dangers & caveats (Borislav Petkov)

 - Do not access the (new) PLATFORM_ID MSR when running
   as a guest (Borislav Petkov)

 - x86 ftrace: Relocate %rip-relative percpu refs in dynamic
   trampolines, to fix crash when using such trampolines
   (Alexis Lothoré)

 - Fix x86-64 CFI build error (Peter Zijlstra)

 - Revert FPU signal return magic number check optimization,
   because it broke CRIU and gVisor in certain FPU configurations
   (Andrei Vagin)

* tag 'x86-urgent-2026-05-31' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  Revert "x86/fpu: Refine and simplify the magic number check during signal return"
  x86/kvm/vmx: Fix x86_64 CFI build
  x86/ftrace: Relocate %rip-relative percpu refs in dynamic trampolines
  x86/microcode: Do not access MSR_IA32_PLATFORM_ID when running as a guest
  Documentation/arch/x86: Hide clearcpuid=

Author: torvalds

Documentation/admin-guide/kernel-parameters.txt

@@ -789,24 +789,6 @@ Kernel parameters
 	cio_ignore=	[S390]
 			See Documentation/arch/s390/common_io.rst for details.
 
-	clearcpuid=X[,X...] [X86]
-			Disable CPUID feature X for the kernel. See
-			arch/x86/include/asm/cpufeatures.h for the valid bit
-			numbers X. Note the Linux-specific bits are not necessarily
-			stable over kernel options, but the vendor-specific
-			ones should be.
-			X can also be a string as appearing in the flags: line
-			in /proc/cpuinfo which does not have the above
-			instability issue. However, not all features have names
-			in /proc/cpuinfo.
-			Note that using this option will taint your kernel.
-			Also note that user programs calling CPUID directly
-			or using the feature without checking anything
-			will still see it. This just prevents it from
-			being used by the kernel or shown in /proc/cpuinfo.
-			Also note the kernel might malfunction if you disable
-			some critical bits.
-
 	clk_ignore_unused
 			[CLK]
 			Prevents the clock framework from automatically gating

Documentation/arch/x86/cpuinfo.rst

@@ -187,6 +187,10 @@ to disable features using the feature number as defined in
 Protection can be disabled using clearcpuid=514. The number 514 is calculated
 from #define X86_FEATURE_UMIP (16*32 + 2).
 
+DO NOT USE this cmdline option in production - it is meant to be used only as
+a quick'n'dirty debugging aid to rule out a feature-enabling code is the
+culprit. If you use it, it'll taint the kernel.
+
 In addition, there exists a variety of custom command-line parameters that
 disable specific features. The list of parameters includes, but is not limited
 to, nofsgsbase, nosgx, noxsave, etc. 5-level paging can also be disabled using

arch/x86/entry/common.c

@@ -55,7 +55,7 @@ noinstr void x86_entry_from_kvm(unsigned int event_type, unsigned int vector)
 	 * The FRED NMI context is significantly different and will not work
 	 * right (specifically FRED fixed the NMI recursion issue).
 	 */
-	idt_entry_from_kvm(vector);
+	idt_do_nmi_irqoff();
 }
 EXPORT_SYMBOL_FOR_KVM(x86_entry_from_kvm);
 #endif

arch/x86/entry/entry.S

@@ -109,11 +109,13 @@ EXPORT_SYMBOL(__ref_stack_chk_guard);
 	RET
 .endm
 
+#ifndef CONFIG_X86_64
 .pushsection .text, "ax"
 SYM_FUNC_START(idt_do_interrupt_irqoff)
 	IDT_DO_EVENT_IRQOFF CALL_NOSPEC _ASM_ARG1
 SYM_FUNC_END(idt_do_interrupt_irqoff)
 .popsection
+#endif
 
 .pushsection .noinstr.text, "ax"
 SYM_FUNC_START(idt_do_nmi_irqoff)

arch/x86/include/asm/processor.h

@@ -733,6 +733,7 @@ bool xen_set_default_idle(void);
 #endif
 
 void __noreturn stop_this_cpu(void *dummy);
+extern bool x86_hypervisor_present;
 void microcode_check(struct cpuinfo_x86 *prev_info);
 void store_cpu_caps(struct cpuinfo_x86 *info);
 

arch/x86/kernel/cpu/microcode/amd.c

@@ -322,7 +322,7 @@ static u32 get_patch_level(void)
 {
 	u32 rev, dummy __always_unused;
 
-	if (IS_ENABLED(CONFIG_MICROCODE_DBG) && hypervisor_present) {
+	if (IS_ENABLED(CONFIG_MICROCODE_DBG) && x86_hypervisor_present) {
 		int cpu = smp_processor_id();
 
 		if (!microcode_rev[cpu]) {
@@ -714,7 +714,7 @@ static bool __apply_microcode_amd(struct microcode_amd *mc, u32 *cur_rev,
 			invlpg(p_addr_end);
 	}
 
-	if (IS_ENABLED(CONFIG_MICROCODE_DBG) && hypervisor_present)
+	if (IS_ENABLED(CONFIG_MICROCODE_DBG) && x86_hypervisor_present)
 		microcode_rev[smp_processor_id()] = mc->hdr.patch_id;
 
 	/* verify patch application was successful */

arch/x86/kernel/cpu/microcode/core.c

@@ -57,7 +57,7 @@ bool force_minrev = IS_ENABLED(CONFIG_MICROCODE_LATE_FORCE_MINREV);
 u32 base_rev;
 u32 microcode_rev[NR_CPUS] = {};
 
-bool hypervisor_present;
+bool __ro_after_init x86_hypervisor_present;
 
 /*
  * Synchronization.
@@ -118,14 +118,9 @@ bool __init microcode_loader_disabled(void)
 	/*
 	 * Disable when:
 	 *
-	 * 1) The CPU does not support CPUID.
-	 */
-	if (!cpuid_feature()) {
-		dis_ucode_ldr = true;
-		return dis_ucode_ldr;
-	}
-
-	/*
+	 * 1) The CPU does not support CPUID, detected below in
+	 *    load_ucode_bsp().
+	 *
 	 * 2) Bit 31 in CPUID[1]:ECX is clear
 	 *    The bit is reserved for hypervisor use. This is still not
 	 *    completely accurate as XEN PV guests don't see that CPUID bit
@@ -135,9 +130,7 @@ bool __init microcode_loader_disabled(void)
 	 * 3) Certain AMD patch levels are not allowed to be
 	 *    overwritten.
 	 */
-	hypervisor_present = native_cpuid_ecx(1) & BIT(31);
-
-	if ((hypervisor_present && !IS_ENABLED(CONFIG_MICROCODE_DBG)) ||
+	if ((x86_hypervisor_present && !IS_ENABLED(CONFIG_MICROCODE_DBG)) ||
 	    amd_check_current_patch_level())
 		dis_ucode_ldr = true;
 
@@ -179,6 +172,11 @@ void __init load_ucode_bsp(void)
 
 	early_parse_cmdline();
 
+	if (!cpuid_feature())
+		dis_ucode_ldr = true;
+	else
+		x86_hypervisor_present = native_cpuid_ecx(1) & BIT(31);
+
 	if (microcode_loader_disabled())
 		return;
 

arch/x86/kernel/cpu/microcode/intel.c

@@ -138,6 +138,9 @@ u32 intel_get_platform_id(void)
 {
 	unsigned int val[2];
 
+	if (x86_hypervisor_present)
+		return 0;
+
 	/*
 	 * This can be called early. Use CPUID directly instead of
 	 * relying on cpuinfo_x86 which may not be fully initialized.

arch/x86/kernel/cpu/microcode/internal.h

@@ -48,7 +48,6 @@ extern struct early_load_data early_data;
 extern struct ucode_cpu_info ucode_cpu_info[];
 extern u32 microcode_rev[NR_CPUS];
 extern u32 base_rev;
-extern bool hypervisor_present;
 
 struct cpio_data find_microcode_in_initrd(const char *path);
 

arch/x86/kernel/fpu/signal.c

@@ -27,14 +27,19 @@
 static inline bool check_xstate_in_sigframe(struct fxregs_state __user *fxbuf,
 					    struct _fpx_sw_bytes *fx_sw)
 {
+	int min_xstate_size = sizeof(struct fxregs_state) +
+			      sizeof(struct xstate_header);
 	void __user *fpstate = fxbuf;
 	unsigned int magic2;
 
 	if (__copy_from_user(fx_sw, &fxbuf->sw_reserved[0], sizeof(*fx_sw)))
 		return false;
 
-	/* Check for the first magic field */
-	if (fx_sw->magic1 != FP_XSTATE_MAGIC1)
+	/* Check for the first magic field and other error scenarios. */
+	if (fx_sw->magic1 != FP_XSTATE_MAGIC1 ||
+	    fx_sw->xstate_size < min_xstate_size ||
+	    fx_sw->xstate_size > x86_task_fpu(current)->fpstate->user_size ||
+	    fx_sw->xstate_size > fx_sw->extended_size)
 		goto setfx;
 
 	/*
@@ -43,7 +48,7 @@ static inline bool check_xstate_in_sigframe(struct fxregs_state __user *fxbuf,
 	 * fpstate layout with out copying the extended state information
 	 * in the memory layout.
 	 */
-	if (__get_user(magic2, (__u32 __user *)(fpstate + x86_task_fpu(current)->fpstate->user_size)))
+	if (__get_user(magic2, (__u32 __user *)(fpstate + fx_sw->xstate_size)))
 		return false;
 
 	if (likely(magic2 == FP_XSTATE_MAGIC2))