[RFC PATCH V3 12/16] x86/sev: Add a #HV exception handler

From: Tianyu Lan
Date: Sat Jan 21 2023 - 21:47:32 EST

From: Tianyu Lan <tiala@xxxxxxxxxxxxx>

Add a #HV exception handler that uses IST stack.

Signed-off-by: Tianyu Lan <tiala@xxxxxxxxxxxxx>
---
Change since RFC V2:
* Remove unnecessary line in the change log.
---
arch/x86/entry/entry_64.S | 58 +++++++++++++++++++++++++++
arch/x86/include/asm/cpu_entry_area.h | 6 +++
arch/x86/include/asm/idtentry.h | 39 +++++++++++++++++-
arch/x86/include/asm/page_64_types.h | 1 +
arch/x86/include/asm/trapnr.h | 1 +
arch/x86/include/asm/traps.h | 1 +
arch/x86/kernel/cpu/common.c | 1 +
arch/x86/kernel/dumpstack_64.c | 9 ++++-
arch/x86/kernel/idt.c | 1 +
arch/x86/kernel/sev.c | 53 ++++++++++++++++++++++++
arch/x86/kernel/traps.c | 40 ++++++++++++++++++
arch/x86/mm/cpu_entry_area.c | 2 +
12 files changed, 209 insertions(+), 3 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 15739a2c0983..6baec7653f19 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -563,6 +563,64 @@ SYM_CODE_START(\asmsym)
.Lfrom_usermode_switch_stack_\@:
idtentry_body user_\cfunc, has_error_code=1

+_ASM_NOKPROBE(\asmsym)
+SYM_CODE_END(\asmsym)
+.endm
+/*
+ * idtentry_hv - Macro to generate entry stub for #HV
+ * @vector: Vector number
+ * @asmsym: ASM symbol for the entry point
+ * @cfunc: C function to be called
+ *
+ * The macro emits code to set up the kernel context for #HV. The #HV handler
+ * runs on an IST stack and needs to be able to support nested #HV exceptions.
+ *
+ * To make this work the #HV entry code tries its best to pretend it doesn't use
+ * an IST stack by switching to the task stack if coming from user-space (which
+ * includes early SYSCALL entry path) or back to the stack in the IRET frame if
+ * entered from kernel-mode.
+ *
+ * If entered from kernel-mode the return stack is validated first, and if it is
+ * not safe to use (e.g. because it points to the entry stack) the #HV handler
+ * will switch to a fall-back stack (HV2) and call a special handler function.
+ *
+ * The macro is only used for one vector, but it is planned to be extended in
+ * the future for the #HV exception.
+ */
+.macro idtentry_hv vector asmsym cfunc
+SYM_CODE_START(\asmsym)
+ UNWIND_HINT_IRET_REGS
+ ASM_CLAC
+ pushq $-1 /* ORIG_RAX: no syscall to restart */
+
+ testb $3, CS-ORIG_RAX(%rsp)
+ jnz .Lfrom_usermode_switch_stack_\@
+
+ call paranoid_entry
+
+ UNWIND_HINT_REGS
+
+ /*
+ * Switch off the IST stack to make it free for nested exceptions.
+ */
+ movq %rsp, %rdi /* pt_regs pointer */
+ call hv_switch_off_ist
+ movq %rax, %rsp /* Switch to new stack */
+
+ UNWIND_HINT_REGS
+
+ /* Update pt_regs */
+ movq ORIG_RAX(%rsp), %rsi /* get error code into 2nd argument*/
+ movq $-1, ORIG_RAX(%rsp) /* no syscall to restart */
+
+ movq %rsp, %rdi /* pt_regs pointer */
+ call kernel_\cfunc
+
+ jmp paranoid_exit
+
+.Lfrom_usermode_switch_stack_\@:
+ idtentry_body user_\cfunc, has_error_code=1
+
_ASM_NOKPROBE(\asmsym)
SYM_CODE_END(\asmsym)
.endm
diff --git a/arch/x86/include/asm/cpu_entry_area.h b/arch/x86/include/asm/cpu_entry_area.h
index 462fc34f1317..2186ed601b4a 100644
--- a/arch/x86/include/asm/cpu_entry_area.h
+++ b/arch/x86/include/asm/cpu_entry_area.h
@@ -30,6 +30,10 @@
char VC_stack[optional_stack_size]; \
char VC2_stack_guard[guardsize]; \
char VC2_stack[optional_stack_size]; \
+ char HV_stack_guard[guardsize]; \
+ char HV_stack[optional_stack_size]; \
+ char HV2_stack_guard[guardsize]; \
+ char HV2_stack[optional_stack_size]; \
char IST_top_guard[guardsize]; \

/* The exception stacks' physical storage. No guard pages required */
@@ -52,6 +56,8 @@ enum exception_stack_ordering {
ESTACK_MCE,
ESTACK_VC,
ESTACK_VC2,
+ ESTACK_HV,
+ ESTACK_HV2,
N_EXCEPTION_STACKS
};

diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index 72184b0b2219..652fea10d377 100644
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -317,6 +317,19 @@ static __always_inline void __##func(struct pt_regs *regs)
__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)

+
+/**
+ * DECLARE_IDTENTRY_HV - Declare functions for the HV entry point
+ * @vector: Vector number (ignored for C)
+ * @func: Function name of the entry point
+ *
+ * Maps to DECLARE_IDTENTRY_RAW, but declares also the user C handler.
+ */
+#define DECLARE_IDTENTRY_HV(vector, func) \
+ DECLARE_IDTENTRY_RAW_ERRORCODE(vector, func); \
+ __visible noinstr void kernel_##func(struct pt_regs *regs); \
+ __visible noinstr void user_##func(struct pt_regs *regs)
+
/**
* DEFINE_IDTENTRY_IST - Emit code for IST entry points
* @func: Function name of the entry point
@@ -376,6 +389,26 @@ static __always_inline void __##func(struct pt_regs *regs)
#define DEFINE_IDTENTRY_VC_USER(func) \
DEFINE_IDTENTRY_RAW_ERRORCODE(user_##func)

+/**
+ * DEFINE_IDTENTRY_HV_KERNEL - Emit code for HV injection handler
+ * when raised from kernel mode
+ * @func: Function name of the entry point
+ *
+ * Maps to DEFINE_IDTENTRY_RAW
+ */
+#define DEFINE_IDTENTRY_HV_KERNEL(func) \
+ DEFINE_IDTENTRY_RAW(kernel_##func)
+
+/**
+ * DEFINE_IDTENTRY_HV_USER - Emit code for HV injection handler
+ * when raised from user mode
+ * @func: Function name of the entry point
+ *
+ * Maps to DEFINE_IDTENTRY_RAW
+ */
+#define DEFINE_IDTENTRY_HV_USER(func) \
+ DEFINE_IDTENTRY_RAW(user_##func)
+
#else /* CONFIG_X86_64 */

/**
@@ -465,6 +498,9 @@ __visible noinstr void func(struct pt_regs *regs, \
# define DECLARE_IDTENTRY_VC(vector, func) \
idtentry_vc vector asm_##func func

+# define DECLARE_IDTENTRY_HV(vector, func) \
+ idtentry_hv vector asm_##func func
+
#else
# define DECLARE_IDTENTRY_MCE(vector, func) \
DECLARE_IDTENTRY(vector, func)
@@ -622,9 +658,10 @@ DECLARE_IDTENTRY_RAW_ERRORCODE(X86_TRAP_DF, xenpv_exc_double_fault);
DECLARE_IDTENTRY_ERRORCODE(X86_TRAP_CP, exc_control_protection);
#endif

-/* #VC */
+/* #VC & #HV */
#ifdef CONFIG_AMD_MEM_ENCRYPT
DECLARE_IDTENTRY_VC(X86_TRAP_VC, exc_vmm_communication);
+DECLARE_IDTENTRY_HV(X86_TRAP_HV, exc_hv_injection);
#endif

#ifdef CONFIG_XEN_PV
diff --git a/arch/x86/include/asm/page_64_types.h b/arch/x86/include/asm/page_64_types.h
index e9e2c3ba5923..0bd7dab676c5 100644
--- a/arch/x86/include/asm/page_64_types.h
+++ b/arch/x86/include/asm/page_64_types.h
@@ -29,6 +29,7 @@
#define IST_INDEX_DB 2
#define IST_INDEX_MCE 3
#define IST_INDEX_VC 4
+#define IST_INDEX_HV 5

/*
* Set __PAGE_OFFSET to the most negative possible address +
diff --git a/arch/x86/include/asm/trapnr.h b/arch/x86/include/asm/trapnr.h
index f5d2325aa0b7..c6583631cecb 100644
--- a/arch/x86/include/asm/trapnr.h
+++ b/arch/x86/include/asm/trapnr.h
@@ -26,6 +26,7 @@
#define X86_TRAP_XF 19 /* SIMD Floating-Point Exception */
#define X86_TRAP_VE 20 /* Virtualization Exception */
#define X86_TRAP_CP 21 /* Control Protection Exception */
+#define X86_TRAP_HV 28 /* HV injected exception in SNP restricted mode */
#define X86_TRAP_VC 29 /* VMM Communication Exception */
#define X86_TRAP_IRET 32 /* IRET Exception */

diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
index 47ecfff2c83d..6795d3e517d6 100644
--- a/arch/x86/include/asm/traps.h
+++ b/arch/x86/include/asm/traps.h
@@ -16,6 +16,7 @@ asmlinkage __visible notrace
struct pt_regs *fixup_bad_iret(struct pt_regs *bad_regs);
void __init trap_init(void);
asmlinkage __visible noinstr struct pt_regs *vc_switch_off_ist(struct pt_regs *eregs);
+asmlinkage __visible noinstr struct pt_regs *hv_switch_off_ist(struct pt_regs *eregs);
#endif

extern bool ibt_selftest(void);
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 9cfca3d7d0e2..e48a489777ec 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2162,6 +2162,7 @@ static inline void tss_setup_ist(struct tss_struct *tss)
tss->x86_tss.ist[IST_INDEX_MCE] = __this_cpu_ist_top_va(MCE);
/* Only mapped when SEV-ES is active */
tss->x86_tss.ist[IST_INDEX_VC] = __this_cpu_ist_top_va(VC);
+ tss->x86_tss.ist[IST_INDEX_HV] = __this_cpu_ist_top_va(HV);
}

#else /* CONFIG_X86_64 */
diff --git a/arch/x86/kernel/dumpstack_64.c b/arch/x86/kernel/dumpstack_64.c
index f05339fee778..6d8f8864810c 100644
--- a/arch/x86/kernel/dumpstack_64.c
+++ b/arch/x86/kernel/dumpstack_64.c
@@ -26,11 +26,14 @@ static const char * const exception_stack_names[] = {
[ ESTACK_MCE ] = "#MC",
[ ESTACK_VC ] = "#VC",
[ ESTACK_VC2 ] = "#VC2",
+ [ ESTACK_HV ] = "#HV",
+ [ ESTACK_HV2 ] = "#HV2",
+
};

const char *stack_type_name(enum stack_type type)
{
- BUILD_BUG_ON(N_EXCEPTION_STACKS != 6);
+ BUILD_BUG_ON(N_EXCEPTION_STACKS != 8);

if (type == STACK_TYPE_TASK)
return "TASK";
@@ -89,6 +92,8 @@ struct estack_pages estack_pages[CEA_ESTACK_PAGES] ____cacheline_aligned = {
EPAGERANGE(MCE),
EPAGERANGE(VC),
EPAGERANGE(VC2),
+ EPAGERANGE(HV),
+ EPAGERANGE(HV2),
};

static __always_inline bool in_exception_stack(unsigned long *stack, struct stack_info *info)
@@ -98,7 +103,7 @@ static __always_inline bool in_exception_stack(unsigned long *stack, struct stac
struct pt_regs *regs;
unsigned int k;

- BUILD_BUG_ON(N_EXCEPTION_STACKS != 6);
+ BUILD_BUG_ON(N_EXCEPTION_STACKS != 8);

begin = (unsigned long)__this_cpu_read(cea_exception_stacks);
/*
diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
index a58c6bc1cd68..48c0a7e1dbcb 100644
--- a/arch/x86/kernel/idt.c
+++ b/arch/x86/kernel/idt.c
@@ -113,6 +113,7 @@ static const __initconst struct idt_data def_idts[] = {

#ifdef CONFIG_AMD_MEM_ENCRYPT
ISTG(X86_TRAP_VC, asm_exc_vmm_communication, IST_INDEX_VC),
+ ISTG(X86_TRAP_HV, asm_exc_hv_injection, IST_INDEX_HV),
#endif

SYSG(X86_TRAP_OF, asm_exc_overflow),
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 679026a640ef..a8862a2eff67 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -2004,6 +2004,59 @@ DEFINE_IDTENTRY_VC_USER(exc_vmm_communication)
irqentry_exit_to_user_mode(regs);
}

+static bool hv_raw_handle_exception(struct pt_regs *regs)
+{
+ return false;
+}
+
+static __always_inline bool on_hv_fallback_stack(struct pt_regs *regs)
+{
+ unsigned long sp = (unsigned long)regs;
+
+ return (sp >= __this_cpu_ist_bottom_va(HV2) && sp < __this_cpu_ist_top_va(HV2));
+}
+
+DEFINE_IDTENTRY_HV_USER(exc_hv_injection)
+{
+ irqentry_enter_from_user_mode(regs);
+ instrumentation_begin();
+
+ if (!hv_raw_handle_exception(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);
+ }
+
+ instrumentation_end();
+ irqentry_exit_to_user_mode(regs);
+}
+
+DEFINE_IDTENTRY_HV_KERNEL(exc_hv_injection)
+{
+ irqentry_state_t irq_state;
+
+ irq_state = irqentry_nmi_enter(regs);
+ instrumentation_begin();
+
+ if (!hv_raw_handle_exception(regs)) {
+ pr_emerg("PANIC: Unhandled #HV exception in kernel space\n");
+
+ /* Show some debug info */
+ show_regs(regs);
+
+ /* Ask hypervisor to sev_es_terminate */
+ sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SEV_ES_GEN_REQ);
+
+ panic("Returned from Terminate-Request to Hypervisor\n");
+ }
+
+ instrumentation_end();
+ irqentry_nmi_exit(regs, irq_state);
+}
+
bool __init handle_vc_boot_ghcb(struct pt_regs *regs)
{
unsigned long exit_code = regs->orig_ax;
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index d317dc3d06a3..d29debec8134 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -905,6 +905,46 @@ asmlinkage __visible noinstr struct pt_regs *vc_switch_off_ist(struct pt_regs *r

return regs_ret;
}
+
+asmlinkage __visible noinstr struct pt_regs *hv_switch_off_ist(struct pt_regs *regs)
+{
+ unsigned long sp, *stack;
+ struct stack_info info;
+ struct pt_regs *regs_ret;
+
+ /*
+ * In the SYSCALL entry path the RSP value comes from user-space - don't
+ * trust it and switch to the current kernel stack
+ */
+ if (ip_within_syscall_gap(regs)) {
+ sp = this_cpu_read(pcpu_hot.top_of_stack);
+ goto sync;
+ }
+
+ /*
+ * From here on the RSP value is trusted. Now check whether entry
+ * happened from a safe stack. Not safe are the entry or unknown stacks,
+ * use the fall-back stack instead in this case.
+ */
+ sp = regs->sp;
+ stack = (unsigned long *)sp;
+
+ if (!get_stack_info_noinstr(stack, current, &info) || info.type == STACK_TYPE_ENTRY ||
+ info.type > STACK_TYPE_EXCEPTION_LAST)
+ sp = __this_cpu_ist_top_va(HV2);
+sync:
+ /*
+ * Found a safe stack - switch to it as if the entry didn't happen via
+ * IST stack. The code below only copies pt_regs, the real switch happens
+ * in assembly code.
+ */
+ sp = ALIGN_DOWN(sp, 8) - sizeof(*regs_ret);
+
+ regs_ret = (struct pt_regs *)sp;
+ *regs_ret = *regs;
+
+ return regs_ret;
+}
#endif

asmlinkage __visible noinstr struct pt_regs *fixup_bad_iret(struct pt_regs *bad_regs)
diff --git a/arch/x86/mm/cpu_entry_area.c b/arch/x86/mm/cpu_entry_area.c
index 7316a8224259..3ec844cef652 100644
--- a/arch/x86/mm/cpu_entry_area.c
+++ b/arch/x86/mm/cpu_entry_area.c
@@ -153,6 +153,8 @@ static void __init percpu_setup_exception_stacks(unsigned int cpu)
if (cc_platform_has(CC_ATTR_GUEST_STATE_ENCRYPT)) {
cea_map_stack(VC);
cea_map_stack(VC2);
+ cea_map_stack(HV);
+ cea_map_stack(HV2);
}
}
}
--
2.25.1