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Date:   Fri, 1 Mar 2019 15:05:29 +0530
From:   Amit Daniel Kachhap <amit.kachhap@....com>
To:     Dave Martin <Dave.Martin@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Kristina Martsenko <kristina.martsenko@....com>,
        kvmarm@...ts.cs.columbia.edu,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 3/6] arm64/kvm: context-switch ptrauth registers

Hi,

On 2/21/19 9:23 PM, Dave Martin wrote:
> On Tue, Feb 19, 2019 at 02:54:28PM +0530, Amit Daniel Kachhap wrote:
>> From: Mark Rutland <mark.rutland@....com>
>>
>> When pointer authentication is supported, a guest may wish to use it.
>> This patch adds the necessary KVM infrastructure for this to work, with
>> a semi-lazy context switch of the pointer auth state.
>>
>> Pointer authentication feature is only enabled when VHE is built
>> in the kernel and present into CPU implementation so only VHE code
>> paths are modified.
>>
>> When we schedule a vcpu, we disable guest usage of pointer
>> authentication instructions and accesses to the keys. While these are
>> disabled, we avoid context-switching the keys. When we trap the guest
>> trying to use pointer authentication functionality, we change to eagerly
>> context-switching the keys, and enable the feature. The next time the
>> vcpu is scheduled out/in, we start again. However the host key registers
>> are saved in vcpu load stage as they remain constant for each vcpu
>> schedule.
>>
>> Pointer authentication consists of address authentication and generic
>> authentication, and CPUs in a system might have varied support for
>> either. Where support for either feature is not uniform, it is hidden
>> from guests via ID register emulation, as a result of the cpufeature
>> framework in the host.
>>
>> Unfortunately, address authentication and generic authentication cannot
>> be trapped separately, as the architecture provides a single EL2 trap
>> covering both. If we wish to expose one without the other, we cannot
>> prevent a (badly-written) guest from intermittently using a feature
>> which is not uniformly supported (when scheduled on a physical CPU which
>> supports the relevant feature). Hence, this patch expects both type of
>> authentication to be present in a cpu.
>>
>> Signed-off-by: Mark Rutland <mark.rutland@....com>
>> [Only VHE, key switch from from assembly, kvm_supports_ptrauth
>> checks, save host key in vcpu_load]
>> Signed-off-by: Amit Daniel Kachhap <amit.kachhap@....com>
>> Reviewed-by: Julien Thierry <julien.thierry@....com>
>> Cc: Marc Zyngier <marc.zyngier@....com>
>> Cc: Christoffer Dall <christoffer.dall@....com>
>> Cc: kvmarm@...ts.cs.columbia.edu
>> ---
>>   arch/arm/include/asm/kvm_host.h   |   1 +
>>   arch/arm64/include/asm/kvm_host.h |  23 +++++++++
>>   arch/arm64/include/asm/kvm_hyp.h  |   7 +++
>>   arch/arm64/kernel/traps.c         |   1 +
>>   arch/arm64/kvm/handle_exit.c      |  21 +++++---
>>   arch/arm64/kvm/hyp/Makefile       |   1 +
>>   arch/arm64/kvm/hyp/entry.S        |  17 +++++++
>>   arch/arm64/kvm/hyp/ptrauth-sr.c   | 101 ++++++++++++++++++++++++++++++++++++++
>>   arch/arm64/kvm/sys_regs.c         |  37 +++++++++++++-
>>   virt/kvm/arm/arm.c                |   2 +
>>   10 files changed, 201 insertions(+), 10 deletions(-)
>>   create mode 100644 arch/arm64/kvm/hyp/ptrauth-sr.c
> 
> [...]
> 
>> diff --git a/arch/arm64/kvm/hyp/ptrauth-sr.c b/arch/arm64/kvm/hyp/ptrauth-sr.c
>> new file mode 100644
>> index 0000000..528ee6e
>> --- /dev/null
>> +++ b/arch/arm64/kvm/hyp/ptrauth-sr.c
>> @@ -0,0 +1,101 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * arch/arm64/kvm/hyp/ptrauth-sr.c: Guest/host ptrauth save/restore
>> + *
>> + * Copyright 2018 Arm Limited
>> + * Author: Mark Rutland <mark.rutland@....com>
>> + *         Amit Daniel Kachhap <amit.kachhap@....com>
>> + */
>> +#include <linux/compiler.h>
>> +#include <linux/kvm_host.h>
>> +
>> +#include <asm/cpucaps.h>
>> +#include <asm/cpufeature.h>
>> +#include <asm/kvm_asm.h>
>> +#include <asm/kvm_hyp.h>
>> +#include <asm/pointer_auth.h>
>> +
>> +static __always_inline bool __ptrauth_is_enabled(struct kvm_vcpu *vcpu)
>> +{
>> +	return IS_ENABLED(CONFIG_ARM64_PTR_AUTH) &&
>> +			vcpu->arch.ctxt.hcr_el2 & (HCR_API | HCR_APK);
>> +}
>> +
>> +#define __ptrauth_save_key(regs, key)						\
>> +({										\
>> +	regs[key ## KEYLO_EL1] = read_sysreg_s(SYS_ ## key ## KEYLO_EL1);	\
>> +	regs[key ## KEYHI_EL1] = read_sysreg_s(SYS_ ## key ## KEYHI_EL1);	\
>> +})
>> +
>> +static __always_inline void __ptrauth_save_state(struct kvm_cpu_context *ctxt)
> 
> Why __always_inline?
> 
>> +{
>> +	__ptrauth_save_key(ctxt->sys_regs, APIA);
>> +	__ptrauth_save_key(ctxt->sys_regs, APIB);
>> +	__ptrauth_save_key(ctxt->sys_regs, APDA);
>> +	__ptrauth_save_key(ctxt->sys_regs, APDB);
>> +	__ptrauth_save_key(ctxt->sys_regs, APGA);
>> +}
>> +
>> +#define __ptrauth_restore_key(regs, key) 					\
>> +({										\
>> +	write_sysreg_s(regs[key ## KEYLO_EL1], SYS_ ## key ## KEYLO_EL1);	\
>> +	write_sysreg_s(regs[key ## KEYHI_EL1], SYS_ ## key ## KEYHI_EL1);	\
>> +})
>> +
>> +static __always_inline void __ptrauth_restore_state(struct kvm_cpu_context *ctxt)
> 
> Same here.  I would hope these just need to be marked with the correct
> function attribute to disable ptrauth by the compiler.  I don't see why
> it makes a difference whether it's inline or not.
> 
> If the compiler semantics are not sufficiently clear, make it a macro.
ok.
> 
> (Bikeshedding here, so it you feel this has already been discussed to
> death I'm happy for this to stay as-is.)
> 
>> +{
>> +	__ptrauth_restore_key(ctxt->sys_regs, APIA);
>> +	__ptrauth_restore_key(ctxt->sys_regs, APIB);
>> +	__ptrauth_restore_key(ctxt->sys_regs, APDA);
>> +	__ptrauth_restore_key(ctxt->sys_regs, APDB);
>> +	__ptrauth_restore_key(ctxt->sys_regs, APGA);
>> +}
>> +
>> +/**
>> + * This function changes the key so assign Pointer Authentication safe
>> + * GCC attribute if protected by it.
>> + */
> 
> (I'd have preferred to keep __noptrauth here and define it do nothing for
> now.  But I'll defer to others on that, since this has already been
> discussed...)

ok __noptrauth annotation will make it clear. I will add it for all C 
error prone functions in the next iteration.
> 
>> +void __ptrauth_switch_to_guest(struct kvm_vcpu *vcpu,
>> +				  struct kvm_cpu_context *host_ctxt,
>> +				  struct kvm_cpu_context *guest_ctxt)
>> +{
>> +	if (!__ptrauth_is_enabled(vcpu))
>> +		return;
>> +
>> +	__ptrauth_restore_state(guest_ctxt);
>> +}
>> +
>> +/**
>> + * This function changes the key so assign Pointer Authentication safe
>> + * GCC attribute if protected by it.
>> + */
>> +void __ptrauth_switch_to_host(struct kvm_vcpu *vcpu,
>> +				 struct kvm_cpu_context *guest_ctxt,
>> +				 struct kvm_cpu_context *host_ctxt)
>> +{
>> +	if (!__ptrauth_is_enabled(vcpu))
>> +		return;
>> +
>> +	__ptrauth_save_state(guest_ctxt);
>> +	__ptrauth_restore_state(host_ctxt);
>> +}
>> +
>> +/**
>> + * kvm_arm_vcpu_ptrauth_reset - resets ptrauth for vcpu schedule
>> + *
>> + * @vcpu: The VCPU pointer
>> + *
>> + * This function may be used to disable ptrauth and use it in a lazy context
>> + * via traps. However host key registers are saved here as they dont change
>> + * during host/guest switch.
>> + */
>> +void kvm_arm_vcpu_ptrauth_reset(struct kvm_vcpu *vcpu)
> 
> I feel this is not a good name.  It sounds too much like it resets the
> registers as part of vcpu reset, whereas really it's doing something
> completely different.
> 
> (Do you reset the regs anywhere btw?  I may have missed it...)
No there is not reset of registers. May be name like 
kvm_arm_vcpu_ptrauth_setup_lazy would be better.
> 
>> +{
>> +	struct kvm_cpu_context *host_ctxt;
>> +
>> +	if (kvm_supports_ptrauth()) {
>> +		kvm_arm_vcpu_ptrauth_disable(vcpu);
>> +		host_ctxt = vcpu->arch.host_cpu_context;
>> +		__ptrauth_save_state(host_ctxt);
>> +	}
>> +}
>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>> index a6c9381..12529df 100644
>> --- a/arch/arm64/kvm/sys_regs.c
>> +++ b/arch/arm64/kvm/sys_regs.c
>> @@ -986,6 +986,32 @@ static bool access_pmuserenr(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
>>   	{ SYS_DESC(SYS_PMEVTYPERn_EL0(n)),					\
>>   	  access_pmu_evtyper, reset_unknown, (PMEVTYPER0_EL0 + n), }
>>   
>> +
>> +void kvm_arm_vcpu_ptrauth_enable(struct kvm_vcpu *vcpu)
>> +{
>> +	vcpu->arch.ctxt.hcr_el2 |= (HCR_API | HCR_APK);
> 
> Pedantic nit: surplus ().
> 
> (Although opinions differ, and keeping them looks more symmetric with
> kvm_arm_vcpu_ptrauth_disable() -- either way, the code can stay as-is if
> you prefer.)
ok.
> 
>> +}
>> +
>> +void kvm_arm_vcpu_ptrauth_disable(struct kvm_vcpu *vcpu)
>> +{
>> +	vcpu->arch.ctxt.hcr_el2 &= ~(HCR_API | HCR_APK);
>> +}
>> +
>> +static bool trap_ptrauth(struct kvm_vcpu *vcpu,
>> +			 struct sys_reg_params *p,
>> +			 const struct sys_reg_desc *rd)
>> +{
>> +	kvm_arm_vcpu_ptrauth_trap(vcpu);
>> +	return false;
> 
> Can we ever get here?  Won't PAC traps always be handled via
> handle_exit()?
> 
> Or can we also take sysreg access traps when the guest tries to access
> the ptrauth key registers?

When Guest kernel forks thread then the key registers are accessed to 
fill them with ptrauth keys and hcr_el2(APK bit) is not appropriately 
set at that moment. This causes trap to EL2 and the above function is 
invoked.
> 
> (I'm now wondering how this works for SVE.)
Not sure. Need to check.
> 
>> +}
>> +
>> +#define __PTRAUTH_KEY(k)						\
>> +	{ SYS_DESC(SYS_## k), trap_ptrauth, reset_unknown, k }
>> +
>> +#define PTRAUTH_KEY(k)							\
>> +	__PTRAUTH_KEY(k ## KEYLO_EL1),					\
>> +	__PTRAUTH_KEY(k ## KEYHI_EL1)
>> +
>>   static bool access_cntp_tval(struct kvm_vcpu *vcpu,
>>   		struct sys_reg_params *p,
>>   		const struct sys_reg_desc *r)
>> @@ -1045,9 +1071,10 @@ static u64 read_id_reg(struct sys_reg_desc const *r, bool raz)
>>   					 (0xfUL << ID_AA64ISAR1_API_SHIFT) |
>>   					 (0xfUL << ID_AA64ISAR1_GPA_SHIFT) |
>>   					 (0xfUL << ID_AA64ISAR1_GPI_SHIFT);
>> -		if (val & ptrauth_mask)
>> +		if (!kvm_supports_ptrauth()) {
> 
> Don't we now always print this when ptrauth is not supported?
> 
> Previously we only printed a message in the interesting case, i.e.,
> where the host supports ptrauch but we cannot offer it to the guest.
Yes agreed. I will add proper checks here to skip prints for non ptrauth 
hosts.

Thanks,
Amit D
> 
>>   			kvm_debug("ptrauth unsupported for guests, suppressing\n");
>> -		val &= ~ptrauth_mask;
>> +			val &= ~ptrauth_mask;
>> +		}
>>   	} else if (id == SYS_ID_AA64MMFR1_EL1) {
>>   		if (val & (0xfUL << ID_AA64MMFR1_LOR_SHIFT))
>>   			kvm_debug("LORegions unsupported for guests, suppressing\n");
>> @@ -1316,6 +1343,12 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>>   	{ SYS_DESC(SYS_TTBR1_EL1), access_vm_reg, reset_unknown, TTBR1_EL1 },
>>   	{ SYS_DESC(SYS_TCR_EL1), access_vm_reg, reset_val, TCR_EL1, 0 },
>>   
>> +	PTRAUTH_KEY(APIA),
>> +	PTRAUTH_KEY(APIB),
>> +	PTRAUTH_KEY(APDA),
>> +	PTRAUTH_KEY(APDB),
>> +	PTRAUTH_KEY(APGA),
>> +
>>   	{ SYS_DESC(SYS_AFSR0_EL1), access_vm_reg, reset_unknown, AFSR0_EL1 },
>>   	{ SYS_DESC(SYS_AFSR1_EL1), access_vm_reg, reset_unknown, AFSR1_EL1 },
>>   	{ SYS_DESC(SYS_ESR_EL1), access_vm_reg, reset_unknown, ESR_EL1 },
>> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
>> index 2032a66..d7e003f 100644
>> --- a/virt/kvm/arm/arm.c
>> +++ b/virt/kvm/arm/arm.c
>> @@ -388,6 +388,8 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>>   		vcpu_clear_wfe_traps(vcpu);
>>   	else
>>   		vcpu_set_wfe_traps(vcpu);
>> +
>> +	kvm_arm_vcpu_ptrauth_reset(vcpu);
>>   }
>>   
>>   void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>> -- 
>> 2.7.4
>>
>> _______________________________________________
>> kvmarm mailing list
>> kvmarm@...ts.cs.columbia.edu
>> https://lists.cs.columbia.edu/mailman/listinfo/kvmarm

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