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SubjectRe: [PATCH v6 3/6] arm64/kvm: context-switch ptrauth registers
On Tue, Feb 19, 2019 at 02:54:28PM +0530, Amit Daniel Kachhap wrote:
> From: Mark Rutland <mark.rutland@arm.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@arm.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@arm.com>
> Reviewed-by: Julien Thierry <julien.thierry@arm.com>
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> Cc: Christoffer Dall <christoffer.dall@arm.com>
> Cc: kvmarm@lists.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@arm.com>
> + * Amit Daniel Kachhap <amit.kachhap@arm.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.

(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...)

> +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...)

> +{
> + 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.)

> +}
> +
> +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?

(I'm now wondering how this works for SVE.)

> +}
> +
> +#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.

> 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@lists.cs.columbia.edu
> https://lists.cs.columbia.edu/mailman/listinfo/kvmarm

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