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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>
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