[PATCH v6 46/76] x86/sev-es: Adjust #VC IST Stack on entering NMI handler

[Joerg Roedel]

From: Joerg Roedel <jroedel@xxxxxxx>

When an NMI hits in the #VC handler entry code before it switched to
another stack, any subsequent #VC exception in the NMI code-path will
overwrite the interrupted #VC handlers stack.

Make sure this doesn't happen by  explicitly adjusting the #VC IST entry
in the NMI handler for the time in can cause #VC exceptions.

Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
Link: https://lore.kernel.org/r/20200724160336.5435-46-joro@xxxxxxxxxx
---
 arch/x86/include/asm/sev-es.h | 19 +++++++++++
 arch/x86/kernel/nmi.c         |  6 ++++
 arch/x86/kernel/sev-es.c      | 59 +++++++++++++++++++++++++++++++++++
 arch/x86/kernel/traps.c       |  2 ++
 4 files changed, 86 insertions(+)

diff --git a/arch/x86/include/asm/sev-es.h b/arch/x86/include/asm/sev-es.h
index 824e9e6b067c..2dd19932a60d 100644
--- a/arch/x86/include/asm/sev-es.h
+++ b/arch/x86/include/asm/sev-es.h
@@ -77,4 +77,23 @@ static inline u64 lower_bits(u64 val, unsigned int bits)
 extern void vc_no_ghcb(void);
 extern bool handle_vc_boot_ghcb(struct pt_regs *regs);
 
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+extern struct static_key_false sev_es_enable_key;
+extern void __sev_es_ist_enter(struct pt_regs *regs);
+extern void __sev_es_ist_exit(void);
+static __always_inline void sev_es_ist_enter(struct pt_regs *regs)
+{
+	if (static_branch_unlikely(&sev_es_enable_key))
+		__sev_es_ist_enter(regs);
+}
+static __always_inline void sev_es_ist_exit(void)
+{
+	if (static_branch_unlikely(&sev_es_enable_key))
+		__sev_es_ist_exit();
+}
+#else
+static inline void sev_es_ist_enter(struct pt_regs *regs) { }
+static inline void sev_es_ist_exit(void) { }
+#endif
+
 #endif
diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index 4fc9954a9560..951f098a4bf5 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -33,6 +33,7 @@
 #include <asm/reboot.h>
 #include <asm/cache.h>
 #include <asm/nospec-branch.h>
+#include <asm/sev-es.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/nmi.h>
@@ -488,6 +489,9 @@ DEFINE_IDTENTRY_RAW(exc_nmi)
 	this_cpu_write(nmi_cr2, read_cr2());
 nmi_restart:
 
+	/* Needs to happen before DR7 is accessed */
+	sev_es_ist_enter(regs);
+
 	this_cpu_write(nmi_dr7, local_db_save());
 
 	irq_state = idtentry_enter_nmi(regs);
@@ -501,6 +505,8 @@ DEFINE_IDTENTRY_RAW(exc_nmi)
 
 	local_db_restore(this_cpu_read(nmi_dr7));
 
+	sev_es_ist_exit();
+
 	if (unlikely(this_cpu_read(nmi_cr2) != read_cr2()))
 		write_cr2(this_cpu_read(nmi_cr2));
 	if (this_cpu_dec_return(nmi_state))
diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
index 64002d86a237..95831d103418 100644
--- a/arch/x86/kernel/sev-es.c
+++ b/arch/x86/kernel/sev-es.c
@@ -52,6 +52,9 @@ struct sev_es_runtime_data {
 
 static DEFINE_PER_CPU(struct sev_es_runtime_data*, runtime_data);
 
+DEFINE_STATIC_KEY_FALSE(sev_es_enable_key);
+EXPORT_SYMBOL_GPL(sev_es_enable_key);
+
 static void __init sev_es_setup_vc_stacks(int cpu)
 {
 	struct sev_es_runtime_data *data;
@@ -73,6 +76,59 @@ static void __init sev_es_setup_vc_stacks(int cpu)
 	cea_set_pte((void *)vaddr, pa, PAGE_KERNEL);
 }
 
+static __always_inline bool on_vc_stack(unsigned long sp)
+{
+	return ((sp >= __this_cpu_ist_bot_va(VC)) && (sp < __this_cpu_ist_top_va(VC)));
+}
+
+/*
+ * This function handles the case when an NM is raised in the #VC exception
+ * handler entry code. In this case the IST entry for VC must be adjusted, so
+ * that any subsequent VC exception will not overwrite the stack contents of the
+ * interrupted VC handler.
+ *
+ * The IST entry is adjusted unconditionally so that it can be also be
+ * unconditionally back-adjusted in sev_es_ist_exit(). Otherwise a nested
+ * sev_es_ist_exit() call may back-adjust the IST entry too early.
+ */
+void noinstr __sev_es_ist_enter(struct pt_regs *regs)
+{
+	unsigned long old_ist, new_ist;
+	unsigned long *p;
+
+	/* Read old IST entry */
+	old_ist = __this_cpu_read(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC]);
+
+	/* Make room on the IST stack */
+	if (on_vc_stack(regs->sp))
+		new_ist = ALIGN_DOWN(regs->sp, 8) - sizeof(old_ist);
+	else
+		new_ist = old_ist - sizeof(old_ist);
+
+	/* Store old IST entry */
+	p       = (unsigned long *)new_ist;
+	*p      = old_ist;
+
+	/* Set new IST entry */
+	this_cpu_write(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC], new_ist);
+}
+
+void noinstr __sev_es_ist_exit(void)
+{
+	unsigned long ist;
+	unsigned long *p;
+
+	/* Read IST entry */
+	ist = __this_cpu_read(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC]);
+
+	if (WARN_ON(ist == __this_cpu_ist_top_va(VC)))
+		return;
+
+	/* Read back old IST entry and write it to the TSS */
+	p = (unsigned long *)ist;
+	this_cpu_write(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC], *p);
+}
+
 /* Needed in vc_early_forward_exception */
 void do_early_exception(struct pt_regs *regs, int trapnr);
 
@@ -277,6 +333,9 @@ void __init sev_es_init_vc_handling(void)
 	if (!sev_es_active())
 		return;
 
+	/* Enable SEV-ES special handling */
+	static_branch_enable(&sev_es_enable_key);
+
 	/* Initialize per-cpu GHCB pages */
 	for_each_possible_cpu(cpu) {
 		sev_es_alloc_runtime_data(cpu);
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 74cfe6eb7ebb..030d882eaad1 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -59,6 +59,7 @@
 #include <asm/umip.h>
 #include <asm/insn.h>
 #include <asm/insn-eval.h>
+#include <asm/sev-es.h>
 
 #ifdef CONFIG_X86_64
 #include <asm/x86_init.h>
@@ -731,6 +732,7 @@ static bool is_sysenter_singlestep(struct pt_regs *regs)
 
 static __always_inline void debug_enter(unsigned long *dr6, unsigned long *dr7)
 {
+
 	/*
 	 * Disable breakpoints during exception handling; recursive exceptions
 	 * are exceedingly 'fun'.
-- 
2.28.0