[PATCH v6 71/76] x86/head/64: Don't call verify_cpu() on starting APs

[Joerg Roedel]

From: Joerg Roedel <jroedel@xxxxxxx>

The APs are not ready to handle exceptions when verify_cpu() is called
in secondary_startup_64.

Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20200724160336.5435-71-joro@xxxxxxxxxx
---
 arch/x86/include/asm/realmode.h |  1 +
 arch/x86/kernel/head_64.S       | 12 ++++++++++++
 arch/x86/realmode/init.c        |  6 ++++++
 3 files changed, 19 insertions(+)

diff --git a/arch/x86/include/asm/realmode.h b/arch/x86/include/asm/realmode.h
index 6590394af309..5c97807c38a4 100644
--- a/arch/x86/include/asm/realmode.h
+++ b/arch/x86/include/asm/realmode.h
@@ -69,6 +69,7 @@ extern unsigned char startup_32_smp[];
 extern unsigned char boot_gdt[];
 #else
 extern unsigned char secondary_startup_64[];
+extern unsigned char secondary_startup_64_no_verify[];
 #endif
 
 static inline size_t real_mode_size_needed(void)
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 8d5bf7c568a4..a708107688a2 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -140,6 +140,18 @@ SYM_CODE_START(secondary_startup_64)
 	/* Sanitize CPU configuration */
 	call verify_cpu
 
+	/*
+	 * The secondary_startup_64_no_verify entry point is only used by
+	 * SEV-ES guests. In those guests the call to verify_cpu() would cause
+	 * #VC exceptions which can not be handled at this stage of secondary
+	 * CPU bringup.
+	 *
+	 * All non SEV-ES systems, especially Intel systems, need to execute
+	 * verify_cpu() above to make sure NX is enabled.
+	 */
+SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
+	UNWIND_HINT_EMPTY
+
 	/*
 	 * Retrieve the modifier (SME encryption mask if SME is active) to be
 	 * added to the initial pgdir entry that will be programmed into CR3.
diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
index 61a52b925d15..df701f87ddef 100644
--- a/arch/x86/realmode/init.c
+++ b/arch/x86/realmode/init.c
@@ -46,6 +46,12 @@ static void sme_sev_setup_real_mode(struct trampoline_header *th)
 		th->flags |= TH_FLAGS_SME_ACTIVE;
 
 	if (sev_es_active()) {
+		/*
+		 * Skip the call to verify_cpu() in secondary_startup_64 as it
+		 * will cause #VC exceptions when the AP can't handle them yet.
+		 */
+		th->start = (u64) secondary_startup_64_no_verify;
+
 		if (sev_es_setup_ap_jump_table(real_mode_header))
 			panic("Failed to update SEV-ES AP Jump Table");
 	}
-- 
2.28.0