Re: [PATCH v7 11/27] KVM: arm64: Support runtime sysreg visibility filtering

Andrew Jones
2019-04-03T19:17:18Z

On Fri, Mar 29, 2019 at 01:00:36PM +0000, Dave Martin wrote:
> Some optional features of the Arm architecture add new system
> registers that are not present in the base architecture.
>
> Where these features are optional for the guest, the visibility of
> these registers may need to depend on some runtime configuration,
> such as a flag passed to KVM_ARM_VCPU_INIT.
>
> For example, ZCR_EL1 and ID_AA64ZFR0_EL1 need to be hidden if SVE
> is not enabled for the guest, even though these registers may be
> present in the hardware and visible to the host at EL2.
>
> Adding special-case checks all over the place for individual
> registers is going to get messy as the number of conditionally-
> visible registers grows.
>
> In order to help solve this problem, this patch adds a new sysreg
> method visibility() that can be used to hook in any needed runtime
> visibility checks. This method can currently return
> REG_HIDDEN_USER to inhibit enumeration and ioctl access to the
> register for userspace, and REG_HIDDEN_GUEST to inhibit runtime
> access by the guest using MSR/MRS. Wrappers are added to allow
> these flags to be conveniently queried.
>
> This approach allows a conditionally modified view of individual
> system registers such as the CPU ID registers, in addition to
> completely hiding register where appropriate.
>
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> Tested-by: zhang.lei <zhang.lei@jp.fujitsu.com>
>
> ---
>
> Changes since v5:
>
> * Rename the visibility override flags, add some comments, and rename/
> introduce helpers to make the purpose of this code clearer.
> ---
> arch/arm64/kvm/sys_regs.c | 24 +++++++++++++++++++++---
> arch/arm64/kvm/sys_regs.h | 25 +++++++++++++++++++++++++
> 2 files changed, 46 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index a5d14b5..c86a7b0 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1927,6 +1927,12 @@ static void perform_access(struct kvm_vcpu *vcpu,
> {
> trace_kvm_sys_access(*vcpu_pc(vcpu), params, r);
>
> + /* Check for regs disabled by runtime config */
> + if (sysreg_hidden_from_guest(vcpu, r)) {
> + kvm_inject_undefined(vcpu);
> + return;
> + }
> +
> /*
> * Not having an accessor means that we have configured a trap
> * that we don't know how to handle. This certainly qualifies
> @@ -2438,6 +2444,10 @@ int kvm_arm_sys_reg_get_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg
> if (!r)
> return get_invariant_sys_reg(reg->id, uaddr);
>
> + /* Check for regs disabled by runtime config */
> + if (sysreg_hidden_from_user(vcpu, r))
> + return -ENOENT;
> +
> if (r->get_user)
> return (r->get_user)(vcpu, r, reg, uaddr);
>
> @@ -2459,6 +2469,10 @@ int kvm_arm_sys_reg_set_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg
> if (!r)
> return set_invariant_sys_reg(reg->id, uaddr);
>
> + /* Check for regs disabled by runtime config */
> + if (sysreg_hidden_from_user(vcpu, r))
> + return -ENOENT;
> +
> if (r->set_user)
> return (r->set_user)(vcpu, r, reg, uaddr);
>
> @@ -2515,7 +2529,8 @@ static bool copy_reg_to_user(const struct sys_reg_desc *reg, u64 __user **uind)
> return true;
> }
>
> -static int walk_one_sys_reg(const struct sys_reg_desc *rd,
> +static int walk_one_sys_reg(const struct kvm_vcpu *vcpu,
> + const struct sys_reg_desc *rd,
> u64 __user **uind,
> unsigned int *total)
> {
> @@ -2526,6 +2541,9 @@ static int walk_one_sys_reg(const struct sys_reg_desc *rd,
> if (!(rd->reg || rd->get_user))
> return 0;
>
> + if (sysreg_hidden_from_user(vcpu, rd))
> + return 0;
> +
> if (!copy_reg_to_user(rd, uind))
> return -EFAULT;
>
> @@ -2554,9 +2572,9 @@ static int walk_sys_regs(struct kvm_vcpu *vcpu, u64 __user *uind)
> int cmp = cmp_sys_reg(i1, i2);
> /* target-specific overrides generic entry. */
> if (cmp <= 0)
> - err = walk_one_sys_reg(i1, &uind, &total);
> + err = walk_one_sys_reg(vcpu, i1, &uind, &total);
> else
> - err = walk_one_sys_reg(i2, &uind, &total);
> + err = walk_one_sys_reg(vcpu, i2, &uind, &total);
>
> if (err)
> return err;
> diff --git a/arch/arm64/kvm/sys_regs.h b/arch/arm64/kvm/sys_regs.h
> index 3b1bc7f..2be9950 100644
> --- a/arch/arm64/kvm/sys_regs.h
> +++ b/arch/arm64/kvm/sys_regs.h
> @@ -64,8 +64,15 @@ struct sys_reg_desc {
> const struct kvm_one_reg *reg, void __user *uaddr);
> int (*set_user)(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> const struct kvm_one_reg *reg, void __user *uaddr);
> +
> + /* Return mask of REG_* runtime visibility overrides */
> + unsigned int (*visibility)(const struct kvm_vcpu *vcpu,
> + const struct sys_reg_desc *rd);
> };
>
> +#define REG_HIDDEN_USER (1 << 0) /* hidden from userspace ioctls */
> +#define REG_HIDDEN_GUEST (1 << 1) /* hidden from guest */
> +
> static inline void print_sys_reg_instr(const struct sys_reg_params *p)
> {
> /* Look, we even formatted it for you to paste into the table! */
> @@ -102,6 +109,24 @@ static inline void reset_val(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r
> __vcpu_sys_reg(vcpu, r->reg) = r->val;
> }
>
> +static inline bool sysreg_hidden_from_guest(const struct kvm_vcpu *vcpu,
> + const struct sys_reg_desc *r)
> +{
> + if (likely(!r->visibility))
> + return false;
> +
> + return r->visibility(vcpu, r) & REG_HIDDEN_GUEST;
> +}
> +
> +static inline bool sysreg_hidden_from_user(const struct kvm_vcpu *vcpu,
> + const struct sys_reg_desc *r)
> +{
> + if (likely(!r->visibility))
> + return false;
> +
> + return r->visibility(vcpu, r) & REG_HIDDEN_USER;
> +}
> +
> static inline int cmp_sys_reg(const struct sys_reg_desc *i1,
> const struct sys_reg_desc *i2)
> {
> --
> 2.1.4
>

Reviewed-by: Andrew Jones <drjones@redhat.com>