Subject: [PATCH 3/3] KVM: arm64: Enforce PtrAuth being disabled if not advertized

From: Marc Zyngier <[email-masked]>
Date: Thu, 4 Jun 2020 14:33:54 +0100

Even if we don't expose PtrAuth to a guest, the guest can still
write to its SCTIRLE_1 register and set the En{I,D}{A,B} bits
and execute PtrAuth instructions from the NOP space. This has
the effect of trapping to EL2, and we currently inject an UNDEF.
This is definitely the wrong thing to do, as the architecture says
that these instructions should behave as NOPs.

Instead, we can simply reset the offending SCTLR_EL1 bits to
zero, and resume the guest. It can still observe the SCTLR bits
being set and then being cleared by magic, but that's much better
than delivering an unexpected extension.

Signed-off-by: Marc Zyngier <[email-masked]>
---
 arch/arm64/kvm/handle_exit.c | 12 ------------
 arch/arm64/kvm/hyp/switch.c  | 18 ++++++++++++++++--
 2 files changed, 16 insertions(+), 14 deletions(-)

diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
index 5a02d4c90559..98d8adf6f865 100644
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c
@@ -162,17 +162,6 @@ 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). If we get here, it is that we didn't fixup ptrauth on exit, and all
- * that we can do is give the guest an UNDEF.
- */
-static int kvm_handle_ptrauth(struct kvm_vcpu *vcpu, struct kvm_run *run)
-{
-	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,
@@ -195,7 +184,6 @@ 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/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
index 2a50b3771c3b..fc09c3dfa466 100644
--- a/arch/arm64/kvm/hyp/switch.c
+++ b/arch/arm64/kvm/hyp/switch.c
@@ -503,8 +503,22 @@ static bool __hyp_text __hyp_handle_ptrauth(struct kvm_vcpu *vcpu)
 	struct kvm_cpu_context *ctxt;
 	u64 val;
 
-	if (!vcpu_has_ptrauth(vcpu))
-		return false;
+	if (!vcpu_has_ptrauth(vcpu)) {
+		if (ec != ESR_ELx_EC_PAC)
+			return false;
+
+		/*
+		 * Interesting situation: the guest has enabled PtrAuth,
+		 * despite KVM not advertising it. Fix SCTLR_El1 on behalf
+		 * of the guest (the bits should behave as RES0 anyway).
+		 */
+		val = read_sysreg_el1(SYS_SCTLR);
+		val &= ~(SCTLR_ELx_ENIA | SCTLR_ELx_ENIB |
+			 SCTLR_ELx_ENDA | SCTLR_ELx_ENDB);
+		write_sysreg_el1(val, SYS_SCTLR);
+
+		return true;
+	}
 
 	switch (ec) {
 	case ESR_ELx_EC_PAC:
-- 
2.26.2

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