Re: [PATCH v4 03/12] arm64: Remove the ability to build a kernel without ssbd

[Andre Przywara]

On Fri, 25 Jan 2019 12:07:02 -0600
Jeremy Linton <jeremy.linton@xxxxxxx> wrote:

Hi,

> Buried behind EXPERT is the ability to build a kernel without
> SSBD, this needlessly clutters up the code as well as creates
> the opportunity for bugs. It also removes the kernel's ability
> to determine if the machine its running on is vulnerable.

I don't know the original motivation for this config option, typically
they are not around for no reason.
I see the benefit of dropping those config options, but we want to make
sure that people don't start hacking around to remove them again.

> Since its also possible to disable it at boot time, lets remove
> the config option.

Given the level of optimisation a compiler can do with the state being
known at compile time, I would imagine that it's not the same (though
probably very close).

But that's not my call, it would be good to hear some maintainer's
opinion on this.

Apart from the nit mentioned below, the technical part looks correct to
me (also compile tested).

> Signed-off-by: Jeremy Linton <jeremy.linton@xxxxxxx>
> Cc: Christoffer Dall <christoffer.dall@xxxxxxx>
> Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx
> ---
>  arch/arm64/Kconfig                  | 9 ---------
>  arch/arm64/include/asm/cpufeature.h | 8 --------
>  arch/arm64/include/asm/kvm_mmu.h    | 7 -------
>  arch/arm64/kernel/Makefile          | 3 +--
>  arch/arm64/kernel/cpu_errata.c      | 4 ----
>  arch/arm64/kernel/cpufeature.c      | 4 ----
>  arch/arm64/kernel/entry.S           | 2 --
>  arch/arm64/kvm/hyp/hyp-entry.S      | 2 --
>  arch/arm64/kvm/hyp/switch.c         | 4 ----
>  9 files changed, 1 insertion(+), 42 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index a4168d366127..0baa632bf0a8 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1038,15 +1038,6 @@ config HARDEN_EL2_VECTORS
>  
>  	  If unsure, say Y.
>  
> -config ARM64_SSBD
> -	bool "Speculative Store Bypass Disable" if EXPERT
> -	default y
> -	help
> -	  This enables mitigation of the bypassing of previous stores
> -	  by speculative loads.
> -
> -	  If unsure, say Y.
> -
>  config RODATA_FULL_DEFAULT_ENABLED
>  	bool "Apply r/o permissions of VM areas also to their linear
> aliases" default y
> diff --git a/arch/arm64/include/asm/cpufeature.h
> b/arch/arm64/include/asm/cpufeature.h index
> dfcfba725d72..bbed2067a1a4 100644 ---
> a/arch/arm64/include/asm/cpufeature.h +++
> b/arch/arm64/include/asm/cpufeature.h @@ -620,19 +620,11 @@ static
> inline bool system_supports_generic_auth(void) 
>  static inline int arm64_get_ssbd_state(void)
>  {
> -#ifdef CONFIG_ARM64_SSBD
>  	extern int ssbd_state;

Wouldn't this be a good opportunity to move this declaration outside of
this function, so that it looks less awkward?

Cheers,
Andre.

>  	return ssbd_state;
> -#else
> -	return ARM64_SSBD_UNKNOWN;
> -#endif
>  }
>  
> -#ifdef CONFIG_ARM64_SSBD
>  void arm64_set_ssbd_mitigation(bool state);
> -#else
> -static inline void arm64_set_ssbd_mitigation(bool state) {}
> -#endif
>  
>  extern int do_emulate_mrs(struct pt_regs *regs, u32 sys_reg, u32 rt);
>  
> diff --git a/arch/arm64/include/asm/kvm_mmu.h
> b/arch/arm64/include/asm/kvm_mmu.h index 8af4b1befa42..a5c152d79820
> 100644 --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -541,7 +541,6 @@ static inline int kvm_map_vectors(void)
>  }
>  #endif
>  
> -#ifdef CONFIG_ARM64_SSBD
>  DECLARE_PER_CPU_READ_MOSTLY(u64, arm64_ssbd_callback_required);
>  
>  static inline int hyp_map_aux_data(void)
> @@ -558,12 +557,6 @@ static inline int hyp_map_aux_data(void)
>  	}
>  	return 0;
>  }
> -#else
> -static inline int hyp_map_aux_data(void)
> -{
> -	return 0;
> -}
> -#endif
>  
>  #define kvm_phys_to_vttbr(addr)		phys_to_ttbr(addr)
>  
> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
> index cd434d0719c1..306336a2fa34 100644
> --- a/arch/arm64/kernel/Makefile
> +++ b/arch/arm64/kernel/Makefile
> @@ -19,7 +19,7 @@ obj-y			:= debug-monitors.o
> entry.o irq.o fpsimd.o		\ return_address.o cpuinfo.o
> cpu_errata.o		\ cpufeature.o alternative.o
> cacheinfo.o		\ smp.o smp_spin_table.o topology.o
> smccc-call.o	\
> -			   syscall.o
> +			   syscall.o ssbd.o
>  
>  extra-$(CONFIG_EFI)			:= efi-entry.o
>  
> @@ -57,7 +57,6 @@ arm64-reloc-test-y := reloc_test_core.o
> reloc_test_syms.o obj-$(CONFIG_CRASH_DUMP)		+=
> crash_dump.o obj-$(CONFIG_CRASH_CORE)		+= crash_core.o
>  obj-$(CONFIG_ARM_SDE_INTERFACE)		+= sdei.o
> -obj-$(CONFIG_ARM64_SSBD)		+= ssbd.o
>  obj-$(CONFIG_ARM64_PTR_AUTH)		+= pointer_auth.o
>  
>  obj-y					+= vdso/ probes/
> diff --git a/arch/arm64/kernel/cpu_errata.c
> b/arch/arm64/kernel/cpu_errata.c index 9a7b5fca51a0..934d50788ca3
> 100644 --- a/arch/arm64/kernel/cpu_errata.c
> +++ b/arch/arm64/kernel/cpu_errata.c
> @@ -281,7 +281,6 @@ enable_smccc_arch_workaround_1(const struct
> arm64_cpu_capabilities *entry) }
>  #endif	/* CONFIG_HARDEN_BRANCH_PREDICTOR */
>  
> -#ifdef CONFIG_ARM64_SSBD
>  DEFINE_PER_CPU_READ_MOSTLY(u64, arm64_ssbd_callback_required);
>  
>  int ssbd_state __read_mostly = ARM64_SSBD_KERNEL;
> @@ -473,7 +472,6 @@ static bool has_ssbd_mitigation(const struct
> arm64_cpu_capabilities *entry, 
>  	return required;
>  }
> -#endif	/* CONFIG_ARM64_SSBD */
>  
>  static void __maybe_unused
>  cpu_enable_cache_maint_trap(const struct arm64_cpu_capabilities
> *__unused) @@ -726,14 +724,12 @@ const struct arm64_cpu_capabilities
> arm64_errata[] = { ERRATA_MIDR_RANGE_LIST(arm64_harden_el2_vectors),
>  	},
>  #endif
> -#ifdef CONFIG_ARM64_SSBD
>  	{
>  		.desc = "Speculative Store Bypass Disable",
>  		.capability = ARM64_SSBD,
>  		.type = ARM64_CPUCAP_LOCAL_CPU_ERRATUM,
>  		.matches = has_ssbd_mitigation,
>  	},
> -#endif
>  #ifdef CONFIG_ARM64_ERRATUM_1188873
>  	{
>  		/* Cortex-A76 r0p0 to r2p0 */
> diff --git a/arch/arm64/kernel/cpufeature.c
> b/arch/arm64/kernel/cpufeature.c index f6d84e2c92fe..d1a7fd7972f9
> 100644 --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1131,7 +1131,6 @@ static void cpu_has_fwb(const struct
> arm64_cpu_capabilities *__unused) WARN_ON(val & (7 << 27 | 7 << 21));
>  }
>  
> -#ifdef CONFIG_ARM64_SSBD
>  static int ssbs_emulation_handler(struct pt_regs *regs, u32 instr)
>  {
>  	if (user_mode(regs))
> @@ -1171,7 +1170,6 @@ static void cpu_enable_ssbs(const struct
> arm64_cpu_capabilities *__unused) arm64_set_ssbd_mitigation(true);
>  	}
>  }
> -#endif /* CONFIG_ARM64_SSBD */
>  
>  #ifdef CONFIG_ARM64_PAN
>  static void cpu_enable_pan(const struct arm64_cpu_capabilities
> *__unused) @@ -1400,7 +1398,6 @@ static const struct
> arm64_cpu_capabilities arm64_features[] = { .field_pos =
> ID_AA64ISAR0_CRC32_SHIFT, .min_field_value = 1,
>  	},
> -#ifdef CONFIG_ARM64_SSBD
>  	{
>  		.desc = "Speculative Store Bypassing Safe (SSBS)",
>  		.capability = ARM64_SSBS,
> @@ -1412,7 +1409,6 @@ static const struct arm64_cpu_capabilities
> arm64_features[] = { .min_field_value = ID_AA64PFR1_SSBS_PSTATE_ONLY,
>  		.cpu_enable = cpu_enable_ssbs,
>  	},
> -#endif
>  #ifdef CONFIG_ARM64_CNP
>  	{
>  		.desc = "Common not Private translations",
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 0ec0c46b2c0c..bee54b7d17b9 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -137,7 +137,6 @@ alternative_else_nop_endif
>  	// This macro corrupts x0-x3. It is the caller's duty
>  	// to save/restore them if required.
>  	.macro	apply_ssbd, state, tmp1, tmp2
> -#ifdef CONFIG_ARM64_SSBD
>  alternative_cb	arm64_enable_wa2_handling
>  	b	.L__asm_ssbd_skip\@
>  alternative_cb_end
> @@ -151,7 +150,6 @@ alternative_cb	arm64_update_smccc_conduit
>  	nop					// Patched to
> SMC/HVC #0 alternative_cb_end
>  .L__asm_ssbd_skip\@:
> -#endif
>  	.endm
>  
>  	.macro	kernel_entry, el, regsize = 64
> diff --git a/arch/arm64/kvm/hyp/hyp-entry.S
> b/arch/arm64/kvm/hyp/hyp-entry.S index 73c1b483ec39..53c9344968d4
> 100644 --- a/arch/arm64/kvm/hyp/hyp-entry.S
> +++ b/arch/arm64/kvm/hyp/hyp-entry.S
> @@ -114,7 +114,6 @@ el1_hvc_guest:
>  			  ARM_SMCCC_ARCH_WORKAROUND_2)
>  	cbnz	w1, el1_trap
>  
> -#ifdef CONFIG_ARM64_SSBD
>  alternative_cb	arm64_enable_wa2_handling
>  	b	wa2_end
>  alternative_cb_end
> @@ -141,7 +140,6 @@ alternative_cb_end
>  wa2_end:
>  	mov	x2, xzr
>  	mov	x1, xzr
> -#endif
>  
>  wa_epilogue:
>  	mov	x0, xzr
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index b0b1478094b4..9ce43ae6fc13 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -436,7 +436,6 @@ static inline bool __hyp_text
> __needs_ssbd_off(struct kvm_vcpu *vcpu) 
>  static void __hyp_text __set_guest_arch_workaround_state(struct
> kvm_vcpu *vcpu) {
> -#ifdef CONFIG_ARM64_SSBD
>  	/*
>  	 * The host runs with the workaround always present. If the
>  	 * guest wants it disabled, so be it...
> @@ -444,19 +443,16 @@ static void __hyp_text
> __set_guest_arch_workaround_state(struct kvm_vcpu *vcpu) if
> (__needs_ssbd_off(vcpu) &&
> __hyp_this_cpu_read(arm64_ssbd_callback_required))
> arm_smccc_1_1_smc(ARM_SMCCC_ARCH_WORKAROUND_2, 0, NULL); -#endif
>  }
>  
>  static void __hyp_text __set_host_arch_workaround_state(struct
> kvm_vcpu *vcpu) {
> -#ifdef CONFIG_ARM64_SSBD
>  	/*
>  	 * If the guest has disabled the workaround, bring it back
> on. */
>  	if (__needs_ssbd_off(vcpu) &&
>  	    __hyp_this_cpu_read(arm64_ssbd_callback_required))
>  		arm_smccc_1_1_smc(ARM_SMCCC_ARCH_WORKAROUND_2, 1,
> NULL); -#endif
>  }
>  
>  /* Switch to the guest for VHE systems running in EL2 */