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

From: Andre Przywara <andre.przywara@arm.com>
To: Dave Martin <Dave.Martin@arm.com>
Cc: Christoffer Dall <cdall@kernel.org>,
	Marc Zyngier <marc.zyngier@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Kristina Martsenko <kristina.martsenko@arm.com>,
	Zhang Lei <zhang.lei@jp.fujitsu.com>,
	Amit Daniel Kachhap <amit.kachhap@arm.com>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH kvmtool v3 7/9] arm64: Make ptrauth enable/disable diagnostics more user-friendly
Date: Fri, 31 May 2019 18:05:01 +0100	[thread overview]
Message-ID: <20190531180501.51459aef@donnerap.cambridge.arm.com> (raw)
In-Reply-To: <1559229194-3036-8-git-send-email-Dave.Martin@arm.com>

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);

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