Re: [PATCH] arm64: KVM: Use SMCCC_ARCH_WORKAROUND_1 for Falkor BP hardening

[Will Deacon]

On Mon, Mar 05, 2018 at 10:57:58AM -0600, Shanker Donthineni wrote:
> Hi Will,
> 
> On 03/05/2018 09:56 AM, Will Deacon wrote:
> > Hi Shanker,
> > 
> > On Fri, Mar 02, 2018 at 03:50:18PM -0600, Shanker Donthineni wrote:
> >> The function SMCCC_ARCH_WORKAROUND_1 was introduced as part of SMC
> >> V1.1 Calling Convention to mitigate CVE-2017-5715. This patch uses
> >> the standard call SMCCC_ARCH_WORKAROUND_1 for Falkor chips instead
> >> of Silicon provider service ID 0xC2001700.
> >>
> >> Signed-off-by: Shanker Donthineni <shankerd@xxxxxxxxxxxxxx>
> >> ---
> >>  arch/arm64/include/asm/cpucaps.h |  2 +-
> >>  arch/arm64/include/asm/kvm_asm.h |  2 --
> >>  arch/arm64/kernel/bpi.S          |  8 ------
> >>  arch/arm64/kernel/cpu_errata.c   | 55 ++++++++++++++--------------------------
> >>  arch/arm64/kvm/hyp/entry.S       | 12 ---------
> >>  arch/arm64/kvm/hyp/switch.c      | 10 --------
> >>  6 files changed, 20 insertions(+), 69 deletions(-)
> > 
> > I'm happy to take this via arm64 if I get an ack from Marc/Christoffer.
> > 
> >> diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
> >> index bb26382..6ecc249 100644
> >> --- a/arch/arm64/include/asm/cpucaps.h
> >> +++ b/arch/arm64/include/asm/cpucaps.h
> >> @@ -43,7 +43,7 @@
> >>  #define ARM64_SVE				22
> >>  #define ARM64_UNMAP_KERNEL_AT_EL0		23
> >>  #define ARM64_HARDEN_BRANCH_PREDICTOR		24
> >> -#define ARM64_HARDEN_BP_POST_GUEST_EXIT		25
> >> +/* #define ARM64_UNALLOCATED_ENTRY			25 */
> >>  #define ARM64_HAS_RAS_EXTN			26
> >>  
> >>  #define ARM64_NCAPS				27
> > 
> > These aren't ABI, so I think you can just drop
> > ARM64_HARDEN_BP_POST_GUEST_EXIT and repack the others accordingly.
> > 
> Sure, I'll remove it completely in v2 patch.
>  
> >> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> >> index 24961b7..ab4d0a9 100644
> >> --- a/arch/arm64/include/asm/kvm_asm.h
> >> +++ b/arch/arm64/include/asm/kvm_asm.h
> >> @@ -68,8 +68,6 @@
> >>  
> >>  extern u32 __init_stage2_translation(void);
> >>  
> >> -extern void __qcom_hyp_sanitize_btac_predictors(void);
> >> -
> >>  #endif
> >>  
> >>  #endif /* __ARM_KVM_ASM_H__ */
> >> diff --git a/arch/arm64/kernel/bpi.S b/arch/arm64/kernel/bpi.S
> >> index e5de335..dc4eb15 100644
> >> --- a/arch/arm64/kernel/bpi.S
> >> +++ b/arch/arm64/kernel/bpi.S
> >> @@ -55,14 +55,6 @@ ENTRY(__bp_harden_hyp_vecs_start)
> >>  	.endr
> >>  ENTRY(__bp_harden_hyp_vecs_end)
> >>  
> >> -ENTRY(__qcom_hyp_sanitize_link_stack_start)
> >> -	stp     x29, x30, [sp, #-16]!
> >> -	.rept	16
> >> -	bl	. + 4
> >> -	.endr
> >> -	ldp	x29, x30, [sp], #16
> >> -ENTRY(__qcom_hyp_sanitize_link_stack_end)
> >> -
> >>  .macro smccc_workaround_1 inst
> >>  	sub	sp, sp, #(8 * 4)
> >>  	stp	x2, x3, [sp, #(8 * 0)]
> >> diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
> >> index 52f15cd..d779ffd4 100644
> >> --- a/arch/arm64/kernel/cpu_errata.c
> >> +++ b/arch/arm64/kernel/cpu_errata.c
> >> @@ -67,8 +67,6 @@ static int cpu_enable_trap_ctr_access(void *__unused)
> >>  DEFINE_PER_CPU_READ_MOSTLY(struct bp_hardening_data, bp_hardening_data);
> >>  
> >>  #ifdef CONFIG_KVM
> >> -extern char __qcom_hyp_sanitize_link_stack_start[];
> >> -extern char __qcom_hyp_sanitize_link_stack_end[];
> >>  extern char __smccc_workaround_1_smc_start[];
> >>  extern char __smccc_workaround_1_smc_end[];
> >>  extern char __smccc_workaround_1_hvc_start[];
> >> @@ -115,8 +113,6 @@ static void __install_bp_hardening_cb(bp_hardening_cb_t fn,
> >>  	spin_unlock(&bp_lock);
> >>  }
> >>  #else
> >> -#define __qcom_hyp_sanitize_link_stack_start	NULL
> >> -#define __qcom_hyp_sanitize_link_stack_end	NULL
> >>  #define __smccc_workaround_1_smc_start		NULL
> >>  #define __smccc_workaround_1_smc_end		NULL
> >>  #define __smccc_workaround_1_hvc_start		NULL
> >> @@ -161,12 +157,25 @@ static void call_hvc_arch_workaround_1(void)
> >>  	arm_smccc_1_1_hvc(ARM_SMCCC_ARCH_WORKAROUND_1, NULL);
> >>  }
> >>  
> >> +static void qcom_link_stack_sanitization(void)
> >> +{
> >> +	u64 tmp;
> >> +
> >> +	asm volatile("mov	%0, x30		\n"
> >> +		     ".rept	16		\n"
> >> +		     "bl	. + 4		\n"
> >> +		     ".endr			\n"
> >> +		     "mov	x30, %0		\n"
> >> +		     : "=&r" (tmp));
> >> +}
> >> +
> >>  static int enable_smccc_arch_workaround_1(void *data)
> >>  {
> >>  	const struct arm64_cpu_capabilities *entry = data;
> >>  	bp_hardening_cb_t cb;
> >>  	void *smccc_start, *smccc_end;
> >>  	struct arm_smccc_res res;
> >> +	u32 midr = read_cpuid_id();
> >>  
> >>  	if (!entry->matches(entry, SCOPE_LOCAL_CPU))
> >>  		return 0;
> >> @@ -199,33 +208,15 @@ static int enable_smccc_arch_workaround_1(void *data)
> >>  		return 0;
> >>  	}
> >>  
> >> +	if (((midr & MIDR_CPU_MODEL_MASK) == MIDR_QCOM_FALKOR) ||
> >> +	    ((midr & MIDR_CPU_MODEL_MASK) == MIDR_QCOM_FALKOR_V1))
> >> +		cb = qcom_link_stack_sanitization;
> > 
> > Is this just a performance thing? Do you actually see an advantage over
> > always making the firmware call? We've seen minimal impact in our testing.
> > 
> 
> Yes, we've couple of advantages using the standard SMCCC_ARCH_WOKAROUND_1 framework. 
>   - Improves the code readability.
>   - Avoid the unnecessary MIDR checks on each vCPU exit.
>   - Validates ID_AA64PFR0_CVS2 feature for Falkor chips.
>   - Avoids the 2nd link stack sanitization workaround in firmware.

What I mean is, can we drop qcom_link_stack_sanitization altogether and
use the SMCCC interface for everything?

Will