linux-kernel - Re: [PATCHv3 03/11] arm64/kvm: hide ptrauth from guests

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Message-ID: <20180427095158.GI13249@C02W217FHV2R.local>
Date:   Fri, 27 Apr 2018 11:51:58 +0200
From:   Christoffer Dall <christoffer.dall@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        arnd@...db.de, marc.zyngier@....com, catalin.marinas@....com,
        awallis@...eaurora.org, kernel-hardening@...ts.openwall.com,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        ramana.radhakrishnan@....com, kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCHv3 03/11] arm64/kvm: hide ptrauth from guests

On Tue, Apr 17, 2018 at 07:37:27PM +0100, Mark Rutland wrote:
> In subsequent patches we're going to expose ptrauth to the host kernel
> and userspace, but things are a bit trickier for guest kernels. For the
> time being, let's hide ptrauth from KVM guests.
> 
> Regardless of how well-behaved the guest kernel is, guest userspace
> could attempt to use ptrauth instructions, triggering a trap to EL2,
> resulting in noise from kvm_handle_unknown_ec(). So let's write up a
> handler for the PAC trap, which silently injects an UNDEF into the
> guest, as if the feature were really missing.
> 
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Cc: Christoffer Dall <cdall@...nel.org>
> Cc: Marc Zyngier <marc.zyngier@....com>
> Cc: kvmarm@...ts.cs.columbia.edu
> ---
>  arch/arm64/kvm/handle_exit.c | 18 ++++++++++++++++++
>  arch/arm64/kvm/sys_regs.c    |  9 +++++++++
>  2 files changed, 27 insertions(+)
> 
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index e5e741bfffe1..5114ad691eae 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -173,6 +173,23 @@ static int handle_sve(struct kvm_vcpu *vcpu, struct kvm_run *run)
>  	return 1;
>  }
>  
> +/*
> + * Guest usage of a ptrauth instruction (which the guest EL1 did not turn into
> + * a NOP), or guest EL1 access to a ptrauth register.
> + */
> +static int kvm_handle_ptrauth(struct kvm_vcpu *vcpu, struct kvm_run *run)
> +{
> +	/*
> +	 * We don't currently suport ptrauth in a guest, and we mask the ID
> +	 * registers to prevent well-behaved guests from trying to make use of
> +	 * it.
> +	 *
> +	 * Inject an UNDEF, as if the feature really isn't present.
> +	 */
> +	kvm_inject_undefined(vcpu);
> +	return 1;
> +}
> +
>  static exit_handle_fn arm_exit_handlers[] = {
>  	[0 ... ESR_ELx_EC_MAX]	= kvm_handle_unknown_ec,
>  	[ESR_ELx_EC_WFx]	= kvm_handle_wfx,
> @@ -195,6 +212,7 @@ static exit_handle_fn arm_exit_handlers[] = {
>  	[ESR_ELx_EC_BKPT32]	= kvm_handle_guest_debug,
>  	[ESR_ELx_EC_BRK64]	= kvm_handle_guest_debug,
>  	[ESR_ELx_EC_FP_ASIMD]	= handle_no_fpsimd,
> +	[ESR_ELx_EC_PAC]	= kvm_handle_ptrauth,
>  };
>  
>  static exit_handle_fn kvm_get_exit_handler(struct kvm_vcpu *vcpu)
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 806b0b126a64..eee399c35e84 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1000,6 +1000,15 @@ static u64 read_id_reg(struct sys_reg_desc const *r, bool raz)
>  				    task_pid_nr(current));
>  
>  		val &= ~(0xfUL << ID_AA64PFR0_SVE_SHIFT);
> +	} else if (id == SYS_ID_AA64ISAR1_EL1) {
> +		const u64 ptrauth_mask = (0xfUL << ID_AA64ISAR1_APA_SHIFT) |
> +					 (0xfUL << ID_AA64ISAR1_API_SHIFT) |
> +					 (0xfUL << ID_AA64ISAR1_GPA_SHIFT) |
> +					 (0xfUL << ID_AA64ISAR1_GPI_SHIFT);
> +		if (val & ptrauth_mask)
> +			pr_err_once("kvm [%i]: ptrauth unsupported for guests, suppressing\n",
> +					task_pid_nr(current));
> +		val &= ~ptrauth_mask;
>  	} else if (id == SYS_ID_AA64MMFR1_EL1) {
>  		if (val & (0xfUL << ID_AA64MMFR1_LOR_SHIFT))
>  			pr_err_once("kvm [%i]: LORegions unsupported for guests, suppressing\n",
> -- 
> 2.11.0
> 

With the change to the debugging print:

Reviewed-by: Christoffer Dall <christoffer.dall@....com>