Commit 23711a5e authored by Marc Zyngier's avatar Marc Zyngier
Browse files

KVM: arm64: Allow setting of ID_AA64PFR0_EL1.CSV2 from userspace 



We now expose ID_AA64PFR0_EL1.CSV2=1 to guests running on hosts
that are immune to Spectre-v2, but that don't have this field set,
most likely because they predate the specification.

However, this prevents the migration of guests that have started on
a host the doesn't fake this CSV2 setting to one that does, as KVM
rejects the write to ID_AA64PFR0_EL2 on the grounds that it isn't
what is already there.

In order to fix this, allow userspace to set this field as long as
this doesn't result in a promising more than what is already there
(setting CSV2 to 0 is acceptable, but setting it to 1 when it is
already set to 0 isn't).

Fixes: e1026237 ("KVM: arm64: Set CSV2 for guests on hardware unaffected by Spectre-v2")
Reported-by: default avatarPeng Liang <liangpeng10@huawei.com>
Signed-off-by: default avatarMarc Zyngier <maz@kernel.org>
Acked-by: default avatarWill Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20201110141308.451654-2-maz@kernel.org
parent 4f6b838c

arch/arm64/include/asm/kvm_host.h

+2 −0
Original line number Diff line number Diff line
@@ -118,6 +118,8 @@ struct kvm_arch { 
	 */

	unsigned long *pmu_filter;

	unsigned int pmuver;



	u8 pfr0_csv2;

};



struct kvm_vcpu_fault_info {

arch/arm64/kvm/arm.c

+16 −0
Original line number Diff line number Diff line
@@ -102,6 +102,20 @@ static int kvm_arm_default_max_vcpus(void) 
	return vgic_present ? kvm_vgic_get_max_vcpus() : KVM_MAX_VCPUS;

}



static void set_default_csv2(struct kvm *kvm)

{

	/*

	 * The default is to expose CSV2 == 1 if the HW isn't affected.

	 * Although this is a per-CPU feature, we make it global because

	 * asymmetric systems are just a nuisance.

	 *

	 * Userspace can override this as long as it doesn't promise

	 * the impossible.

	 */

	if (arm64_get_spectre_v2_state() == SPECTRE_UNAFFECTED)

		kvm->arch.pfr0_csv2 = 1;

}



/**

 * kvm_arch_init_vm - initializes a VM data structure

 * @kvm:	pointer to the KVM struct
@@ -127,6 +141,8 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type) 
	/* The maximum number of VCPUs is limited by the host's GIC model */

	kvm->arch.max_vcpus = kvm_arm_default_max_vcpus();



	set_default_csv2(kvm);



	return ret;

out_free_stage2_pgd:

	kvm_free_stage2_pgd(&kvm->arch.mmu);

arch/arm64/kvm/sys_regs.c

+38 −4
Original line number Diff line number Diff line
@@ -1128,9 +1128,8 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu, 
		if (!vcpu_has_sve(vcpu))

			val &= ~(0xfUL << ID_AA64PFR0_SVE_SHIFT);

		val &= ~(0xfUL << ID_AA64PFR0_AMU_SHIFT);

		if (!(val & (0xfUL << ID_AA64PFR0_CSV2_SHIFT)) &&

		    arm64_get_spectre_v2_state() == SPECTRE_UNAFFECTED)

			val |= (1UL << ID_AA64PFR0_CSV2_SHIFT);

		val &= ~(0xfUL << ID_AA64PFR0_CSV2_SHIFT);

		val |= ((u64)vcpu->kvm->arch.pfr0_csv2 << ID_AA64PFR0_CSV2_SHIFT);

	} else if (id == SYS_ID_AA64PFR1_EL1) {

		val &= ~(0xfUL << ID_AA64PFR1_MTE_SHIFT);

	} else if (id == SYS_ID_AA64ISAR1_EL1 && !vcpu_has_ptrauth(vcpu)) {
@@ -1213,6 +1212,40 @@ static unsigned int sve_visibility(const struct kvm_vcpu *vcpu, 
	return REG_HIDDEN;

}



static int set_id_aa64pfr0_el1(struct kvm_vcpu *vcpu,

			       const struct sys_reg_desc *rd,

			       const struct kvm_one_reg *reg, void __user *uaddr)

{

	const u64 id = sys_reg_to_index(rd);

	int err;

	u64 val;

	u8 csv2;



	err = reg_from_user(&val, uaddr, id);

	if (err)

		return err;



	/*

	 * Allow AA64PFR0_EL1.CSV2 to be set from userspace as long as

	 * it doesn't promise more than what is actually provided (the

	 * guest could otherwise be covered in ectoplasmic residue).

	 */

	csv2 = cpuid_feature_extract_unsigned_field(val, ID_AA64PFR0_CSV2_SHIFT);

	if (csv2 > 1 ||

	    (csv2 && arm64_get_spectre_v2_state() != SPECTRE_UNAFFECTED))

		return -EINVAL;



	/* We can only differ with CSV2, and anything else is an error */

	val ^= read_id_reg(vcpu, rd, false);

	val &= ~(0xFUL << ID_AA64PFR0_CSV2_SHIFT);

	if (val)

		return -EINVAL;



	vcpu->kvm->arch.pfr0_csv2 = csv2;



	return 0;

}



/*

 * cpufeature ID register user accessors

 *
@@ -1472,7 +1505,8 @@ static const struct sys_reg_desc sys_reg_descs[] = { 


	/* AArch64 ID registers */

	/* CRm=4 */

	ID_SANITISED(ID_AA64PFR0_EL1),

	{ SYS_DESC(SYS_ID_AA64PFR0_EL1), .access = access_id_reg,

	  .get_user = get_id_reg, .set_user = set_id_aa64pfr0_el1, },

	ID_SANITISED(ID_AA64PFR1_EL1),

	ID_UNALLOCATED(4,2),

	ID_UNALLOCATED(4,3),

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