Re: [RFC PATCH V5 11/15] x86/sev: Add a #HV exception handler

Gupta, Pankaj
2023-05-05T11:00:01Z

Hi Tianyu,

I tried to understand some details of this patch. Please find below
some comments/questions.

Thanks,

> Add a #HV exception handler that uses IST stack.
>
> Signed-off-by: Tianyu Lan <tiala@microsoft.com>
> ---
> Change since RFC V2:
> * Remove unnecessary line in the change log.
> ---
> arch/x86/entry/entry_64.S | 22 +++++++----
> arch/x86/include/asm/cpu_entry_area.h | 6 +++
> arch/x86/include/asm/idtentry.h | 40 +++++++++++++++++++-
> 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, 165 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index eccc3431e515..653b1f10699b 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -496,7 +496,7 @@ SYM_CODE_END(\asmsym)
>
> #ifdef CONFIG_AMD_MEM_ENCRYPT
> /**
> - * idtentry_vc - Macro to generate entry stub for #VC
> + * idtentry_sev - Macro to generate entry stub for #VC
> * @vector: Vector number
> * @asmsym: ASM symbol for the entry point
> * @cfunc: C function to be called
> @@ -515,14 +515,18 @@ SYM_CODE_END(\asmsym)
> *
> * The macro is only used for one vector, but it is planned to be extended in
> * the future for the #HV exception.
> - */
> -.macro idtentry_vc vector asmsym cfunc
> +*/
> +.macro idtentry_sev vector asmsym cfunc has_error_code:req
> SYM_CODE_START(\asmsym)
> UNWIND_HINT_IRET_REGS
> ENDBR
> ASM_CLAC
> cld
>
> + .if \vector == X86_TRAP_HV
> + pushq $-1 /* ORIG_RAX: no syscall */
> + .endif
> +
> /*
> * If the entry is from userspace, switch stacks and treat it as
> * a normal entry.
> @@ -545,7 +549,12 @@ SYM_CODE_START(\asmsym)
> * stack.
> */
> movq %rsp, %rdi /* pt_regs pointer */
> - call vc_switch_off_ist
> + .if \vector == X86_TRAP_VC
> + call vc_switch_off_ist

I think the stack switching logic is similar for #VC & #HV.
So, we can use common function. Just the corresponding fallback
stack switching is different. Maybe we can pass the hint as an
argument (%rsi?) to something like "sev_switch_off_ist()", and use
the corresponding (#HV or #VC) fallbacks stack?

> + .else
> + call hv_switch_off_ist
> + .endif
> +
> movq %rax, %rsp /* Switch to new stack */
>
> ENCODE_FRAME_POINTER
> @@ -568,10 +577,7 @@ SYM_CODE_START(\asmsym)
>
> /* Switch to the regular task stack */
> .Lfrom_usermode_switch_stack_\@:
> - idtentry_body user_\cfunc, has_error_code=1
> -
> -_ASM_NOKPROBE(\asmsym)
> -SYM_CODE_END(\asmsym)
> + idtentry_body user_\cfunc, \has_error_code
> .endm
> #endif
>
> 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 b241af4ce9b4..b0f3501b2767 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 */
>
> /**
> @@ -463,8 +496,10 @@ __visible noinstr void func(struct pt_regs *regs, \
> DECLARE_IDTENTRY(vector, func)
>
> # define DECLARE_IDTENTRY_VC(vector, func) \
> - idtentry_vc vector asm_##func func
> + idtentry_sev vector asm_##func func has_error_code=1
>
> +# define DECLARE_IDTENTRY_HV(vector, func) \
> + idtentry_sev vector asm_##func func has_error_code=0
> #else
> # define DECLARE_IDTENTRY_MCE(vector, func) \
> DECLARE_IDTENTRY(vector, func)
> @@ -618,9 +653,10 @@ 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 8cd4126d8253..5bc44bcf6e48 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -2172,6 +2172,7 @@ 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 20f3fd8ade2f..7b06d7c0914f 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -2006,6 +2006,59 @@ 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)
> +{

Don't see this functioned used yet?

> + 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_enter(regs);

Any reason for not using "irqentry_nmi_enter" here?
We are dispatching both irqs & NMI's right?

Thanks,
Pankaj
> + 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_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 e91500a80963..97554fa0ff30 100644
> --- a/arch/x86/mm/cpu_entry_area.c
> +++ b/arch/x86/mm/cpu_entry_area.c
> @@ -160,6 +160,8 @@ 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);
> }
> }
> }