Re: [PATCH kvmtool v3 7/9] arm64: Make ptrauth enable/disable diagnostics more user-friendly

Andre Przywara
2019-05-31T17:05:11Z

On Thu, 30 May 2019 16:13:12 +0100
Dave Martin <Dave.Martin@arm.com> wrote:

> To help the user understand what is going on, amend ptrauth
> configuration diagnostic messages to refer to command line options
> by the exact name used on the command line.
>
> Also, provide a clean diagnostic when ptrauth is requested, but not
> availble. The generic "Unable to initialise vcpu" message is
> rather cryptic for this case.

Again I don't see much value in having this as a separate patch, as it
basically just touches code introduced two patches earlier. I think it
should be merged into 5/9.

> Since we now don't attempt to enable ptrauth at all unless KVM
> reports the relevant capabilities, remove the error message for
> that case too: in any case, we can't diagnose precisely why
> KVM_ARM_VCPU_INIT failed, so the message may be misleading.

So this leaves the only point where we use .enable_ptrauth to that error
message about the host not supporting it. Not sure if that's worth this
separate option?

Cheers,
Andre.

> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> ---
> arm/aarch64/include/kvm/kvm-config-arch.h | 4 ++--
> arm/aarch64/kvm-cpu.c | 15 +++++++++++----
> arm/kvm-cpu.c | 8 ++------
> 3 files changed, 15 insertions(+), 12 deletions(-)
>
> diff --git a/arm/aarch64/include/kvm/kvm-config-arch.h b/arm/aarch64/include/kvm/kvm-config-arch.h
> index 0279b13..fe1699d 100644
> --- a/arm/aarch64/include/kvm/kvm-config-arch.h
> +++ b/arm/aarch64/include/kvm/kvm-config-arch.h
> @@ -10,9 +10,9 @@
> "Specify random seed for Kernel Address Space " \
> "Layout Randomization (KASLR)"), \
> OPT_BOOLEAN('\0', "enable-ptrauth", &(cfg)->enable_ptrauth, \
> - "Enables pointer authentication"), \
> + "Enable pointer authentication for the guest"), \
> OPT_BOOLEAN('\0', "disable-ptrauth", &(cfg)->disable_ptrauth, \
> - "Disables pointer authentication"),
> + "Disable pointer authentication for the guest"),
>
> #include "arm-common/kvm-config-arch.h"
>
> diff --git a/arm/aarch64/kvm-cpu.c b/arm/aarch64/kvm-cpu.c
> index d3c32e0..08e4fd5 100644
> --- a/arm/aarch64/kvm-cpu.c
> +++ b/arm/aarch64/kvm-cpu.c
> @@ -130,16 +130,23 @@ static void reset_vcpu_aarch64(struct kvm_cpu *vcpu)
>
> static void select_ptrauth_feature(struct kvm *kvm, struct kvm_vcpu_init *init)
> {
> + bool supported;
> +
> /* Check Pointer Authentication command line arguments. */
> if (kvm->cfg.arch.enable_ptrauth && kvm->cfg.arch.disable_ptrauth)
> - die("Both enable-ptrauth and disable-ptrauth option cannot be present");
> + die("--enable-ptrauth conflicts with --disable-ptrauth");
> +
> + supported = kvm__supports_extension(kvm, KVM_CAP_ARM_PTRAUTH_ADDRESS) &&
> + kvm__supports_extension(kvm, KVM_CAP_ARM_PTRAUTH_GENERIC);
> +
> + if (kvm->cfg.arch.enable_ptrauth && !supported)
> + die("--enable-ptrauth not supported on this host");
> +
> /*
> * Always enable Pointer Authentication if system supports
> * this extension unless disable-ptrauth option is present.
> */
> - if (kvm__supports_extension(kvm, KVM_CAP_ARM_PTRAUTH_ADDRESS) &&
> - kvm__supports_extension(kvm, KVM_CAP_ARM_PTRAUTH_GENERIC) &&
> - !kvm->cfg.arch.disable_ptrauth) {
> + if (supported && !kvm->cfg.arch.disable_ptrauth) {
> init->features[0] |= 1UL << KVM_ARM_VCPU_PTRAUTH_ADDRESS;
> init->features[0] |= 1UL << KVM_ARM_VCPU_PTRAUTH_GENERIC;
> }
> diff --git a/arm/kvm-cpu.c b/arm/kvm-cpu.c
> index 764fb05..1652f6f 100644
> --- a/arm/kvm-cpu.c
> +++ b/arm/kvm-cpu.c
> @@ -108,12 +108,8 @@ struct kvm_cpu *kvm_cpu__arch_init(struct kvm *kvm, unsigned long cpu_id)
> die("Unable to find matching target");
> }
>
> - if (err || target->init(vcpu)) {
> - if (kvm->cfg.arch.enable_ptrauth)
> - die("Unable to initialise vcpu with pointer authentication feature");
> - else
> - die("Unable to initialise vcpu");
> - }
> + if (err || target->init(vcpu))
> + die("Unable to initialise vcpu");
>
> coalesced_offset = ioctl(kvm->sys_fd, KVM_CHECK_EXTENSION,
> KVM_CAP_COALESCED_MMIO);