On Thu, Jun 04, 2020 at 02:33:52PM +0100, Marc Zyngier wrote:
> When using the PtrAuth feature in a guest, we need to save the host's
> keys before allowing the guest to program them. For that, we dump
> them in a per-CPU data structure (the so called host context).
>
> But both call sites that do this are in preemptible context,
> which may end up in disaster should the vcpu thread get preempted
> before reentering the guest.
Yuck!
> Instead, save the keys eagerly on each vcpu_load(). This has an
> increased overhead, but is at least safe.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
This looks sound to me given kvm_arch_vcpu_load() is surrounded with
get_cpu() .. put_cpu() and gets called when the thread is preempted.
Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>
Thanks,
Mark.
> ---
> arch/arm64/include/asm/kvm_emulate.h | 6 ------
> arch/arm64/kvm/arm.c | 18 +++++++++++++++++-
> arch/arm64/kvm/handle_exit.c | 19 ++-----------------
> 3 files changed, 19 insertions(+), 24 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> index a30b4eec7cb4..977843e4d5fb 100644
> --- a/arch/arm64/include/asm/kvm_emulate.h
> +++ b/arch/arm64/include/asm/kvm_emulate.h
> @@ -112,12 +112,6 @@ static inline void vcpu_ptrauth_disable(struct kvm_vcpu *vcpu)
> vcpu->arch.hcr_el2 &= ~(HCR_API | HCR_APK);
> }
>
> -static inline void vcpu_ptrauth_setup_lazy(struct kvm_vcpu *vcpu)
> -{
> - if (vcpu_has_ptrauth(vcpu))
> - vcpu_ptrauth_disable(vcpu);
> -}
> -
> static inline unsigned long vcpu_get_vsesr(struct kvm_vcpu *vcpu)
> {
> return vcpu->arch.vsesr_el2;
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index d6988401c22a..152049c5055d 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -337,6 +337,12 @@ void kvm_arch_vcpu_unblocking(struct kvm_vcpu *vcpu)
> preempt_enable();
> }
>
> +#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); \
> +})
> +
> void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> {
> int *last_ran;
> @@ -370,7 +376,17 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> else
> vcpu_set_wfx_traps(vcpu);
>
> - vcpu_ptrauth_setup_lazy(vcpu);
> + if (vcpu_has_ptrauth(vcpu)) {
> + struct kvm_cpu_context *ctxt = vcpu->arch.host_cpu_context;
> +
> + __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);
> +
> + vcpu_ptrauth_disable(vcpu);
> + }
> }
>
> void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index eb194696ef62..065251efa2e6 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -162,31 +162,16 @@ static int handle_sve(struct kvm_vcpu *vcpu, struct kvm_run *run)
> return 1;
> }
>
> -#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); \
> -})
> -
> /*
> * Handle the guest trying to use a ptrauth instruction, or trying to access a
> * ptrauth register.
> */
> void kvm_arm_vcpu_ptrauth_trap(struct kvm_vcpu *vcpu)
> {
> - struct kvm_cpu_context *ctxt;
> -
> - if (vcpu_has_ptrauth(vcpu)) {
> + if (vcpu_has_ptrauth(vcpu))
> vcpu_ptrauth_enable(vcpu);
> - ctxt = vcpu->arch.host_cpu_context;
> - __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);
> - } else {
> + else
> kvm_inject_undefined(vcpu);
> - }
> }
>
> /*
> --
> 2.26.2
>
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