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Subject[tip: x86/seves] x86/sev-es: Adjust #VC IST Stack on entering NMI handler
The following commit has been merged into the x86/seves branch of tip:

Commit-ID: 315562c9af3d583502b35c4b223a08d95ce69864
Gitweb: https://git.kernel.org/tip/315562c9af3d583502b35c4b223a08d95ce69864
Author: Joerg Roedel <jroedel@suse.de>
AuthorDate: Mon, 07 Sep 2020 15:15:44 +02:00
Committer: Borislav Petkov <bp@suse.de>
CommitterDate: Wed, 09 Sep 2020 11:33:19 +02:00

x86/sev-es: Adjust #VC IST Stack on entering NMI handler

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

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

[ bp: Touchups, spelling fixes. ]

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200907131613.12703-44-joro@8bytes.org
---
arch/x86/include/asm/sev-es.h | 19 ++++++++++++-
arch/x86/kernel/nmi.c | 9 ++++++-
arch/x86/kernel/sev-es.c | 53 ++++++++++++++++++++++++++++++++++-
3 files changed, 81 insertions(+)

diff --git a/arch/x86/include/asm/sev-es.h b/arch/x86/include/asm/sev-es.h
index 9fbeeda..59176e8 100644
--- a/arch/x86/include/asm/sev-es.h
+++ b/arch/x86/include/asm/sev-es.h
@@ -78,4 +78,23 @@ extern void vc_no_ghcb(void);
extern void vc_boot_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 4fc9954..4c89c4d 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 +489,12 @@ DEFINE_IDTENTRY_RAW(exc_nmi)
this_cpu_write(nmi_cr2, read_cr2());
nmi_restart:

+ /*
+ * Needs to happen before DR7 is accessed, because the hypervisor can
+ * intercept DR7 reads/writes, turning those into #VC exceptions.
+ */
+ sev_es_ist_enter(regs);
+
this_cpu_write(nmi_dr7, local_db_save());

irq_state = idtentry_enter_nmi(regs);
@@ -501,6 +508,8 @@ nmi_restart:

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 fae8145..39ebb2d 100644
--- a/arch/x86/kernel/sev-es.c
+++ b/arch/x86/kernel/sev-es.c
@@ -51,6 +51,7 @@ struct sev_es_runtime_data {
};

static DEFINE_PER_CPU(struct sev_es_runtime_data*, runtime_data);
+DEFINE_STATIC_KEY_FALSE(sev_es_enable_key);

static void __init setup_vc_stacks(int cpu)
{
@@ -73,6 +74,55 @@ static void __init 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_bottom_va(VC)) && (sp < __this_cpu_ist_top_va(VC)));
+}
+
+/*
+ * This function handles the case when an NMI 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 adjusted back in sev_es_ist_exit(). Otherwise a nested
+ * sev_es_ist_exit() call may adjust back the IST entry too early.
+ */
+void noinstr __sev_es_ist_enter(struct pt_regs *regs)
+{
+ unsigned long old_ist, new_ist;
+
+ /* 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 */
+ *(unsigned long *)new_ist = 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;
+
+ /* 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 */
+ this_cpu_write(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC], *(unsigned long *)ist);
+}
+
/* Needed in vc_early_forward_exception */
void do_early_exception(struct pt_regs *regs, int trapnr);

@@ -277,6 +327,9 @@ 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) {
alloc_runtime_data(cpu);
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