Re: [PATCH 2/3] KVM: arm64: Handle PtrAuth traps early
From: Mark Rutland <mark.rutland@arm.com>
Date: 2020-06-04 15:23:43
Also in:
kvm, kvmarm
On Thu, Jun 04, 2020 at 02:33:53PM +0100, Marc Zyngier wrote:
The current way we deal with PtrAuth is a bit heavy handed: - We forcefully save the host's keys on each vcpu_load() - Handling the PtrAuth trap forces us to go all the way back to the exit handling code to just set the HCR bits Overall, this is pretty heavy handed. A better approach would be to handle it the same way we deal with the FPSIMD registers: - On vcpu_load() disable PtrAuth for the guest - On first use, save the host's keys, enable PtrAuth in the guest Crutially, this can happen as a fixup, which is done very early on exit. We can then reenter the guest immediately without leaving the hypervisor role. Another thing is that it simplify the rest of the host handling: exiting all the way to the host means that the only possible outcome for this trap is to inject an UNDEF. Signed-off-by: Marc Zyngier <maz@kernel.org> --- arch/arm64/kvm/arm.c | 17 +---------- arch/arm64/kvm/handle_exit.c | 17 ++--------- arch/arm64/kvm/hyp/switch.c | 59 ++++++++++++++++++++++++++++++++++++ arch/arm64/kvm/sys_regs.c | 13 +++----- 4 files changed, 68 insertions(+), 38 deletions(-)
[...]
+static bool __hyp_text __hyp_handle_ptrauth(struct kvm_vcpu *vcpu)
+{
+ u32 sysreg = esr_sys64_to_sysreg(kvm_vcpu_get_hsr(vcpu));
+ u32 ec = kvm_vcpu_trap_get_class(vcpu);
+ struct kvm_cpu_context *ctxt;
+ u64 val;
+
+ if (!vcpu_has_ptrauth(vcpu))
+ return false;
+
+ switch (ec) {
+ case ESR_ELx_EC_PAC:
+ break;
+ case ESR_ELx_EC_SYS64:
+ switch (sysreg) {
+ case SYS_APIAKEYLO_EL1:
+ case SYS_APIAKEYHI_EL1:
+ case SYS_APIBKEYLO_EL1:
+ case SYS_APIBKEYHI_EL1:
+ case SYS_APDAKEYLO_EL1:
+ case SYS_APDAKEYHI_EL1:
+ case SYS_APDBKEYLO_EL1:
+ case SYS_APDBKEYHI_EL1:
+ case SYS_APGAKEYLO_EL1:
+ case SYS_APGAKEYHI_EL1:
+ break;
+ default:
+ return false;
+ }
+ break;
+ default:
+ return false;
+ }
The ESR triage looks correct, but I think it might be clearer split out
into a helper, since you can avoid the default cases with direct
returns, and you could avoid the nested switch, e.g.
static inline bool __hyp_text esr_is_ptrauth_trap(u32 esr)
{
u32 ec = ESR_ELx_EC(esr);
if (ec == ESR_ELx_EC_PAC)
return true;
if (ec != ESR_ELx_EC_SYS64)
return false;
switch (esr_sys64_to_sysreg(esr)) {
case SYS_APIAKEYLO_EL1:
case SYS_APIAKEYHI_EL1:
case SYS_APIBKEYLO_EL1:
case SYS_APIBKEYHI_EL1:
case SYS_APDAKEYLO_EL1:
case SYS_APDAKEYHI_EL1:
case SYS_APDBKEYLO_EL1:
case SYS_APDBKEYHI_EL1:
case SYS_APGAKEYLO_EL1:
case SYS_APGAKEYHI_EL1:
return true;
}
return false;
}
quoted hunk ↗ jump to hunk
+ + ctxt = kern_hyp_va(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_enable(vcpu); + + val = read_sysreg(hcr_el2); + val |= (HCR_API | HCR_APK); + write_sysreg(val, hcr_el2); + + return true; +} + /* * Return true when we were able to fixup the guest exit and should return to * the guest, false when we should restore the host state and return to the@@ -524,6 +580,9 @@ static bool __hyp_text fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code) if (__hyp_handle_fpsimd(vcpu)) return true; + if (__hyp_handle_ptrauth(vcpu)) + return true; + if (!__populate_fault_info(vcpu)) return true;diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c index ad1d57501d6d..564995084cf8 100644 --- a/arch/arm64/kvm/sys_regs.c +++ b/arch/arm64/kvm/sys_regs.c@@ -1034,16 +1034,13 @@ 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 for both cases as we never skip the trapped - * instruction: - * - * - Either we re-execute the same key register access instruction - * after enabling ptrauth. - * - Or an UNDEF is injected as ptrauth is not supported/enabled. + * If we land here, that is because we didn't fixup the access on exit + * by allowing the PtrAuth sysregs. The only way this happens is when + * the guest does not have PtrAuth support enabled. */ + kvm_inject_undefined(vcpu); + return false; }-- 2.26.2
Regardless of the suggestion above, this looks sound to me. I agree that it's much nicer to handle this in hyp, and AFAICT the context switch should do the right thing, so: Reviewed-by: Mark Rutland <mark.rutland@arm.com> Thanks, Mark. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel