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

Joerg Roedel
2020-08-24T09:07:10Z

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

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200724160336.5435-71-joro@8bytes.org
---
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");