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KVM: arm64: nv: Don't erroneously claim FEAT_DoubleLock for NV VMs

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ID_AA64DFR0_EL1.DoubleLock is one of those annoying signed feature
fields where a non-negative value implies that a feature is implemented
and a negative value implies that it is not. While the intention of
masking this field was likely to hide the feature, KVM actually
advertises it, even on unsupporting hardware.

Remove FEAT_DoubleLock from the mask, making the NI value visible to the
VM. Take care to accept the old, incorrect values for this field as
we've lied to userspace.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
This commit is contained in:
Oliver Upton 2025-09-12 14:22:49 -07:00 committed by Marc Zyngier
parent d3c35b7c57
commit 49da9872a6
2 changed files with 25 additions and 1 deletions
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1
arch/arm64/kvm/nested.c
View File

@ -1582,7 +1582,6 @@ u64 limit_nv_id_reg(struct kvm *kvm, u32 reg, u64 val)
ID_AA64DFR0_EL1_MTPMU |
ID_AA64DFR0_EL1_TraceBuffer |
ID_AA64DFR0_EL1_TraceFilt |
ID_AA64DFR0_EL1_DoubleLock |
ID_AA64DFR0_EL1_PMSVer |
ID_AA64DFR0_EL1_CTX_CMPs |
ID_AA64DFR0_EL1_SEBEP |

25
arch/arm64/kvm/sys_regs.c
View File

@ -1997,6 +1997,26 @@ static u64 sanitise_id_aa64dfr0_el1(const struct kvm_vcpu *vcpu, u64 val)
return val;
}
/*
* Older versions of KVM erroneously claim support for FEAT_DoubleLock with
* NV-enabled VMs on unsupporting hardware. Silently ignore the incorrect
* value if it is consistent with the bug.
*/
static bool ignore_feat_doublelock(struct kvm_vcpu *vcpu, u64 val)
{
u8 host, user;
if (!vcpu_has_nv(vcpu))
return false;
host = SYS_FIELD_GET(ID_AA64DFR0_EL1, DoubleLock,
read_sanitised_ftr_reg(SYS_ID_AA64DFR0_EL1));
user = SYS_FIELD_GET(ID_AA64DFR0_EL1, DoubleLock, val);
return host == ID_AA64DFR0_EL1_DoubleLock_NI &&
user == ID_AA64DFR0_EL1_DoubleLock_IMP;
}
static int set_id_aa64dfr0_el1(struct kvm_vcpu *vcpu,
const struct sys_reg_desc *rd,
u64 val)
@ -2028,6 +2048,11 @@ static int set_id_aa64dfr0_el1(struct kvm_vcpu *vcpu,
if (debugver < ID_AA64DFR0_EL1_DebugVer_IMP)
return -EINVAL;
if (ignore_feat_doublelock(vcpu, val)) {
val &= ~ID_AA64DFR0_EL1_DoubleLock;
val |= SYS_FIELD_PREP_ENUM(ID_AA64DFR0_EL1, DoubleLock, NI);
}
return set_id_reg(vcpu, rd, val);
}