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

Joerg Roedel
2020-08-24T09:07:38Z

From: Joerg Roedel <jroedel@suse.de>

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@suse.de>
Link: https://lore.kernel.org/r/20200724160336.5435-46-joro@8bytes.org
---
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