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Subject[PATCH v6.1 2/2] x86/sev: Split up runtime #VC handler for correct state tracking
From: Joerg Roedel <jroedel@suse.de>

Split up the #VC handler code into a from-user and a from-kernel part.
This allows clean and correct state tracking, as the #VC handler needs
to enter NMI-state when raised from kernel mode and plain IRQ state when
raised from user-mode.

Fixes: 62441a1fb532 ("x86/sev-es: Correctly track IRQ states in runtime #VC handler")
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
arch/x86/entry/entry_64.S | 4 +-
arch/x86/include/asm/idtentry.h | 29 +++----
arch/x86/kernel/sev.c | 148 +++++++++++++++++---------------
3 files changed, 91 insertions(+), 90 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index a16a5294d55f..1886aaf19914 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -506,7 +506,7 @@ SYM_CODE_START(\asmsym)

movq %rsp, %rdi /* pt_regs pointer */

- call \cfunc
+ call kernel_\cfunc

/*
* No need to switch back to the IST stack. The current stack is either
@@ -517,7 +517,7 @@ SYM_CODE_START(\asmsym)

/* Switch to the regular task stack */
.Lfrom_usermode_switch_stack_\@:
- idtentry_body safe_stack_\cfunc, has_error_code=1
+ idtentry_body user_\cfunc, has_error_code=1

_ASM_NOKPROBE(\asmsym)
SYM_CODE_END(\asmsym)
diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index 73d45b0dfff2..cd9f3e304944 100644
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -312,8 +312,8 @@ static __always_inline void __##func(struct pt_regs *regs)
*/
#define DECLARE_IDTENTRY_VC(vector, func) \
DECLARE_IDTENTRY_RAW_ERRORCODE(vector, func); \
- __visible noinstr void ist_##func(struct pt_regs *regs, unsigned long error_code); \
- __visible noinstr void safe_stack_##func(struct pt_regs *regs, unsigned long error_code)
+ __visible noinstr void kernel_##func(struct pt_regs *regs, unsigned long error_code); \
+ __visible noinstr void user_##func(struct pt_regs *regs, unsigned long error_code)

/**
* DEFINE_IDTENTRY_IST - Emit code for IST entry points
@@ -355,33 +355,24 @@ static __always_inline void __##func(struct pt_regs *regs)
DEFINE_IDTENTRY_RAW_ERRORCODE(func)

/**
- * DEFINE_IDTENTRY_VC_SAFE_STACK - Emit code for VMM communication handler
- which runs on a safe stack.
+ * DEFINE_IDTENTRY_VC_KERNEL - Emit code for VMM communication handler
+ when raised from kernel mode
* @func: Function name of the entry point
*
* Maps to DEFINE_IDTENTRY_RAW_ERRORCODE
*/
-#define DEFINE_IDTENTRY_VC_SAFE_STACK(func) \
- DEFINE_IDTENTRY_RAW_ERRORCODE(safe_stack_##func)
+#define DEFINE_IDTENTRY_VC_KERNEL(func) \
+ DEFINE_IDTENTRY_RAW_ERRORCODE(kernel_##func)

/**
- * DEFINE_IDTENTRY_VC_IST - Emit code for VMM communication handler
- which runs on the VC fall-back stack
+ * DEFINE_IDTENTRY_VC_USER - Emit code for VMM communication handler
+ when raised from user mode
* @func: Function name of the entry point
*
* Maps to DEFINE_IDTENTRY_RAW_ERRORCODE
*/
-#define DEFINE_IDTENTRY_VC_IST(func) \
- DEFINE_IDTENTRY_RAW_ERRORCODE(ist_##func)
-
-/**
- * DEFINE_IDTENTRY_VC - Emit code for VMM communication handler
- * @func: Function name of the entry point
- *
- * Maps to DEFINE_IDTENTRY_RAW_ERRORCODE
- */
-#define DEFINE_IDTENTRY_VC(func) \
- DEFINE_IDTENTRY_RAW_ERRORCODE(func)
+#define DEFINE_IDTENTRY_VC_USER(func) \
+ DEFINE_IDTENTRY_RAW_ERRORCODE(user_##func)

#else /* CONFIG_X86_64 */

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index e0d12728bcb7..fe98cec2973e 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -796,7 +796,7 @@ void __init sev_es_init_vc_handling(void)
sev_es_setup_play_dead();

/* Secondary CPUs use the runtime #VC handler */
- initial_vc_handler = (unsigned long)safe_stack_exc_vmm_communication;
+ initial_vc_handler = (unsigned long)kernel_exc_vmm_communication;
}

static void __init vc_early_forward_exception(struct es_em_ctxt *ctxt)
@@ -1234,14 +1234,6 @@ static enum es_result vc_handle_trap_ac(struct ghcb *ghcb,
return ES_EXCEPTION;
}

-static __always_inline void vc_handle_trap_db(struct pt_regs *regs)
-{
- if (user_mode(regs))
- noist_exc_debug(regs);
- else
- exc_debug(regs);
-}
-
static enum es_result vc_handle_exitcode(struct es_em_ctxt *ctxt,
struct ghcb *ghcb,
unsigned long exit_code)
@@ -1337,41 +1329,13 @@ static __always_inline bool on_vc_fallback_stack(struct pt_regs *regs)
return (sp >= __this_cpu_ist_bottom_va(VC2) && sp < __this_cpu_ist_top_va(VC2));
}

-/*
- * Main #VC exception handler. It is called when the entry code was able to
- * switch off the IST to a safe kernel stack.
- *
- * With the current implementation it is always possible to switch to a safe
- * stack because #VC exceptions only happen at known places, like intercepted
- * instructions or accesses to MMIO areas/IO ports. They can also happen with
- * code instrumentation when the hypervisor intercepts #DB, but the critical
- * paths are forbidden to be instrumented, so #DB exceptions currently also
- * only happen in safe places.
- */
-DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
+static bool vc_raw_handle_exception(struct pt_regs *regs, unsigned long error_code)
{
- irqentry_state_t irq_state;
struct ghcb_state state;
struct es_em_ctxt ctxt;
enum es_result result;
struct ghcb *ghcb;
-
- /*
- * Handle #DB before calling into !noinstr code to avoid recursive #DB.
- */
- if (error_code == SVM_EXIT_EXCP_BASE + X86_TRAP_DB) {
- vc_handle_trap_db(regs);
- return;
- }
-
- irq_state = irqentry_nmi_enter(regs);
- instrumentation_begin();
-
- /*
- * This is invoked through an interrupt gate, so IRQs are disabled. The
- * code below might walk page-tables for user or kernel addresses, so
- * keep the IRQs disabled to protect us against concurrent TLB flushes.
- */
+ bool ret = true;

ghcb = __sev_get_ghcb(&state);

@@ -1391,15 +1355,18 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
case ES_UNSUPPORTED:
pr_err_ratelimited("Unsupported exit-code 0x%02lx in #VC exception (IP: 0x%lx)\n",
error_code, regs->ip);
- goto fail;
+ ret = false;
+ break;
case ES_VMM_ERROR:
pr_err_ratelimited("Failure in communication with VMM (exit-code 0x%02lx IP: 0x%lx)\n",
error_code, regs->ip);
- goto fail;
+ ret = false;
+ break;
case ES_DECODE_FAILED:
pr_err_ratelimited("Failed to decode instruction (exit-code 0x%02lx IP: 0x%lx)\n",
error_code, regs->ip);
- goto fail;
+ ret = false;
+ break;
case ES_EXCEPTION:
vc_forward_exception(&ctxt);
break;
@@ -1415,24 +1382,52 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
BUG();
}

-out:
- instrumentation_end();
- irqentry_nmi_exit(regs, irq_state);
+ return ret;
+}

- return;
+static __always_inline bool vc_is_db(unsigned long error_code)
+{
+ return error_code == SVM_EXIT_EXCP_BASE + X86_TRAP_DB;
+}

-fail:
- if (user_mode(regs)) {
- /*
- * Do not kill the machine if user-space triggered the
- * exception. Send SIGBUS instead and let user-space deal with
- * it.
- */
- force_sig_fault(SIGBUS, BUS_OBJERR, (void __user *)0);
- } else {
- pr_emerg("PANIC: Unhandled #VC exception in kernel space (result=%d)\n",
- result);
+/*
+ * Runtime #VC exception handler when raised from kernel mode. Runs in NMI mode
+ * and will panic when an error happens.
+ */
+DEFINE_IDTENTRY_VC_KERNEL(exc_vmm_communication)
+{
+ irqentry_state_t irq_state;
+
+ /*
+ * With the current implementation it is always possible to switch to a
+ * safe stack because #VC exceptions only happen at known places, like
+ * intercepted instructions or accesses to MMIO areas/IO ports. They can
+ * also happen with code instrumentation when the hypervisor intercepts
+ * #DB, but the critical paths are forbidden to be instrumented, so #DB
+ * exceptions currently also only happen in safe places.
+ *
+ * But keep this here in case the noinstr annotations are violated due
+ * to bug elsewhere.
+ */
+ if (unlikely(on_vc_fallback_stack(regs))) {
+ instrumentation_begin();
+ panic("Can't handle #VC exception from unsupported context\n");
+ instrumentation_end();
+ }
+
+ /*
+ * Handle #DB before calling into !noinstr code to avoid recursive #DB.
+ */
+ if (vc_is_db(error_code)) {
+ exc_debug(regs);
+ return;
+ }
+
+ irq_state = irqentry_nmi_enter(regs);

+ instrumentation_begin();
+
+ if (!vc_raw_handle_exception(regs, error_code)) {
/* Show some debug info */
show_regs(regs);

@@ -1443,23 +1438,38 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
panic("Returned from Terminate-Request to Hypervisor\n");
}

- goto out;
+ instrumentation_end();
+ irqentry_nmi_exit(regs, irq_state);
}

-/* This handler runs on the #VC fall-back stack. It can cause further #VC exceptions */
-DEFINE_IDTENTRY_VC_IST(exc_vmm_communication)
+/*
+ * Runtime #VC exception handler when raised from user mode. Runs in IRQ mode
+ * and will kill the current task with SIGBUS when an error happens.
+ */
+DEFINE_IDTENTRY_VC_USER(exc_vmm_communication)
{
+ /*
+ * Handle #DB before calling into !noinstr code to avoid recursive #DB.
+ */
+ if (vc_is_db(error_code)) {
+ noist_exc_debug(regs);
+ return;
+ }
+
+ irqentry_enter_from_user_mode(regs);
instrumentation_begin();
- panic("Can't handle #VC exception from unsupported context\n");
- instrumentation_end();
-}

-DEFINE_IDTENTRY_VC(exc_vmm_communication)
-{
- if (likely(!on_vc_fallback_stack(regs)))
- safe_stack_exc_vmm_communication(regs, error_code);
- else
- ist_exc_vmm_communication(regs, error_code);
+ if (!vc_raw_handle_exception(regs, error_code)) {
+ /*
+ * Do not kill the machine if user-space triggered the
+ * exception. Send SIGBUS instead and let user-space deal with
+ * it.
+ */
+ force_sig_fault(SIGBUS, BUS_OBJERR, (void __user *)0);
+ }
+
+ instrumentation_end();
+ irqentry_exit_to_user_mode(regs);
}

bool __init handle_vc_boot_ghcb(struct pt_regs *regs)
--
2.31.1
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