Merge tag 'x86-urgent-2026-03-29' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 fixes from Ingo Molnar:

 - Fix an early boot crash in AMD SEV-SNP guests, caused by incorrect
   FSGSBASE init ordering (Nikunj A Dadhania)

 - Remove X86_CR4_FRED from the CR4 pinned bits mask, to fix a race
   window during the bootup of SEV-{ES,SNP} or TDX guests, which can
   crash them if they trigger exceptions in that window (Borislav
   Petkov)

 - Fix early boot failures on SEV-ES/SNP guests, due to incorrect early
   GHCB access (Nikunj A Dadhania)

 - Add clarifying comment to the CRn pinning logic, to avoid future
   confusion & bugs (Peter Zijlstra)

* tag 'x86-urgent-2026-03-29' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/cpu: Add comment clarifying CRn pinning
  x86/fred: Fix early boot failures on SEV-ES/SNP guests
  x86/cpu: Remove X86_CR4_FRED from the CR4 pinned bits mask
  x86/cpu: Enable FSGSBASE early in cpu_init_exception_handling()

3 files changed, 46 insertions, 7 deletions

diff --git a/arch/x86/coco/sev/noinstr.c b/arch/x86/coco/sev/noinstr.c
index 9d94aca4a698..5afd663a1c21 100644
--- a/arch/x86/coco/sev/noinstr.c
+++ b/arch/x86/coco/sev/noinstr.c
@@ -121,6 +121,9 @@ noinstr struct ghcb *__sev_get_ghcb(struct ghcb_state *state)
 
 	WARN_ON(!irqs_disabled());
 
+	if (!sev_cfg.ghcbs_initialized)
+		return boot_ghcb;
+
 	data = this_cpu_read(runtime_data);
 	ghcb = &data->ghcb_page;
 
@@ -164,6 +167,9 @@ noinstr void __sev_put_ghcb(struct ghcb_state *state)
 
 	WARN_ON(!irqs_disabled());
 
+	if (!sev_cfg.ghcbs_initialized)
+		return;
+
 	data = this_cpu_read(runtime_data);
 	ghcb = &data->ghcb_page;
 
diff --git a/arch/x86/entry/entry_fred.c b/arch/x86/entry/entry_fred.c
index 88c757ac8ccd..fbe2d10dd737 100644
--- a/arch/x86/entry/entry_fred.c
+++ b/arch/x86/entry/entry_fred.c
@@ -177,6 +177,16 @@ static noinstr void fred_extint(struct pt_regs *regs)
 	}
 }
 
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+noinstr void exc_vmm_communication(struct pt_regs *regs, unsigned long error_code)
+{
+	if (user_mode(regs))
+		return user_exc_vmm_communication(regs, error_code);
+	else
+		return kernel_exc_vmm_communication(regs, error_code);
+}
+#endif
+
 static noinstr void fred_hwexc(struct pt_regs *regs, unsigned long error_code)
 {
 	/* Optimize for #PF. That's the only exception which matters performance wise */
@@ -207,6 +217,10 @@ static noinstr void fred_hwexc(struct pt_regs *regs, unsigned long error_code)
 #ifdef CONFIG_X86_CET
 	case X86_TRAP_CP: return exc_control_protection(regs, error_code);
 #endif
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+	case X86_TRAP_VC: return exc_vmm_communication(regs, error_code);
+#endif
+
 	default: return fred_bad_type(regs, error_code);
 	}
 
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index a8ff4376c286..ec0670114efa 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -433,7 +433,20 @@ static __always_inline void setup_lass(struct cpuinfo_x86 *c)
 
 /* These bits should not change their value after CPU init is finished. */
 static const unsigned long cr4_pinned_mask = X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_UMIP |
-					     X86_CR4_FSGSBASE | X86_CR4_CET | X86_CR4_FRED;
+					     X86_CR4_FSGSBASE | X86_CR4_CET;
+
+/*
+ * The CR pinning protects against ROP on the 'mov %reg, %CRn' instruction(s).
+ * Since you can ROP directly to these instructions (barring shadow stack),
+ * any protection must follow immediately and unconditionally after that.
+ *
+ * Specifically, the CR[04] write functions below will have the value
+ * validation controlled by the @cr_pinning static_branch which is
+ * __ro_after_init, just like the cr4_pinned_bits value.
+ *
+ * Once set, an attacker will have to defeat page-tables to get around these
+ * restrictions. Which is a much bigger ask than 'simple' ROP.
+ */
 static DEFINE_STATIC_KEY_FALSE_RO(cr_pinning);
 static unsigned long cr4_pinned_bits __ro_after_init;
 
@@ -2050,12 +2063,6 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	setup_umip(c);
 	setup_lass(c);
 
-	/* Enable FSGSBASE instructions if available. */
-	if (cpu_has(c, X86_FEATURE_FSGSBASE)) {
-		cr4_set_bits(X86_CR4_FSGSBASE);
-		elf_hwcap2 |= HWCAP2_FSGSBASE;
-	}
-
 	/*
 	 * The vendor-specific functions might have changed features.
 	 * Now we do "generic changes."
@@ -2416,6 +2423,18 @@ void cpu_init_exception_handling(bool boot_cpu)
 	/* GHCB needs to be setup to handle #VC. */
 	setup_ghcb();
 
+	/*
+	 * On CPUs with FSGSBASE support, paranoid_entry() uses
+	 * ALTERNATIVE-patched RDGSBASE/WRGSBASE instructions. Secondary CPUs
+	 * boot after alternatives are patched globally, so early exceptions
+	 * execute patched code that depends on FSGSBASE. Enable the feature
+	 * before any exceptions occur.
+	 */
+	if (cpu_feature_enabled(X86_FEATURE_FSGSBASE)) {
+		cr4_set_bits(X86_CR4_FSGSBASE);
+		elf_hwcap2 |= HWCAP2_FSGSBASE;
+	}
+
 	if (cpu_feature_enabled(X86_FEATURE_FRED)) {
 		/* The boot CPU has enabled FRED during early boot */
 		if (!boot_cpu)