Re: [PATCH 06/11] arm64: Don't trap host pointer auth use to EL2

Dave Martin
2017-07-24T10:37:24Z

On Wed, Jul 19, 2017 at 05:01:27PM +0100, Mark Rutland wrote:
> To allow EL0 (and/or EL1) to use pointer authentication functionality,
> we must ensure that pointer authentication instructions and accesses to
> pointer authentication keys are not trapped to EL2 (where we will not be
> able to handle them).
>
> This patch ensures that HCR_EL2 is configured appropriately when the
> kernel is booted at EL2. For non-VHE kernels we set HCR_EL2.{API,APK},
> ensuring that EL1 can access keys and permit EL0 use of instructions.
> For VHE kernels, EL2 access is controlled by EL3, and we need not set
> anything.
>
> This does not enable support for KVM guests, since KVM manages HCR_EL2
> itself.
>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Christoffer Dall <christoffer.dall@linaro.org>
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: kvmarm@lists.cs.columbia.edu
> ---
> arch/arm64/include/asm/kvm_arm.h | 2 ++
> arch/arm64/kernel/head.S | 19 +++++++++++++++++--
> 2 files changed, 19 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
> index 61d694c..c1267e8 100644
> --- a/arch/arm64/include/asm/kvm_arm.h
> +++ b/arch/arm64/include/asm/kvm_arm.h
> @@ -23,6 +23,8 @@
> #include <asm/types.h>
>
> /* Hyp Configuration Register (HCR) bits */
> +#define HCR_API (UL(1) << 41)
> +#define HCR_APK (UL(1) << 40)
> #define HCR_E2H (UL(1) << 34)
> #define HCR_ID (UL(1) << 33)
> #define HCR_CD (UL(1) << 32)
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 973df7d..8b8e8d7 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -412,10 +412,25 @@ CPU_LE( bic x0, x0, #(1 << 25) ) // Clear the EE bit for EL2
>
> /* Hyp configuration. */
> mov x0, #HCR_RW // 64-bit EL1
> - cbz x2, set_hcr
> + cbz x2, 1f

Can we keep the label name here? It still seems appropriate.

> orr x0, x0, #HCR_TGE // Enable Host Extensions
> orr x0, x0, #HCR_E2H
> -set_hcr:
> +1:
> +#ifdef CONFIG_ARM64_POINTER_AUTHENTICATION
> + /*
> + * Disable pointer authentication traps to EL2. The HCR_EL2.{APK,API}
> + * bits exist iff at least one authentication mechanism is implemented.
> + */
> + mrs x1, id_aa64isar1_el1
> + mov_q x3, ((0xf << ID_AA64ISAR1_GPI_SHIFT) | \
> + (0xf << ID_AA64ISAR1_GPA_SHIFT) | \
> + (0xf << ID_AA64ISAR1_API_SHIFT) | \
> + (0xf << ID_AA64ISAR1_APA_SHIFT))

Redundant outer (), I think -- mov_q protects its argument.

> + and x1, x1, x3
> + cbz x1, 1f

tst + b.eq?

> + orr x0, x0, #(HCR_APK | HCR_API)
> +1:
> +#endif
> msr hcr_el2, x0
> isb

Cheers
---Dave