linux-kernel - [PATCH v4 2/7] KVM: arm64: Don't write to Rt unless sys

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Message-Id: <20220503060205.2823727-3-oupton@google.com>
Date:   Tue,  3 May 2022 06:02:00 +0000
From:   Oliver Upton <oupton@...gle.com>
To:     kvmarm@...ts.cs.columbia.edu
Cc:     kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, maz@...nel.org, james.morse@....com,
        alexandru.elisei@....com, suzuki.poulose@....com,
        reijiw@...gle.com, ricarkol@...gle.com,
        Oliver Upton <oupton@...gle.com>
Subject: [PATCH v4 2/7] KVM: arm64: Don't write to Rt unless sys_reg emulation succeeds

emulate_sys_reg() returns 1 unconditionally, even though a a system
register access can fail. Furthermore, kvm_handle_sys_reg() writes to Rt
for every register read, regardless of if it actually succeeded.

Though this pattern is safe (as params.regval is initialized with the
current value of Rt) it is a bit ugly. Indicate failure if the register
access could not be emulated and only write to Rt on success.

Signed-off-by: Oliver Upton <oupton@...gle.com>
---
 arch/arm64/kvm/sys_regs.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 36895c163eae..f0a076e5cc1c 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -2401,7 +2401,14 @@ static bool is_imp_def_sys_reg(struct sys_reg_params *params)
 	return params->Op0 == 3 && (params->CRn & 0b1011) == 0b1011;
 }
 
-static int emulate_sys_reg(struct kvm_vcpu *vcpu,
+/**
+ * emulate_sys_reg - Emulate a guest access to an AArch64 system register
+ * @vcpu: The VCPU pointer
+ * @params: Decoded system register parameters
+ *
+ * Return: true if the system register access was successful, false otherwise.
+ */
+static bool emulate_sys_reg(struct kvm_vcpu *vcpu,
 			   struct sys_reg_params *params)
 {
 	const struct sys_reg_desc *r;
@@ -2410,7 +2417,10 @@ static int emulate_sys_reg(struct kvm_vcpu *vcpu,
 
 	if (likely(r)) {
 		perform_access(vcpu, params, r);
-	} else if (is_imp_def_sys_reg(params)) {
+		return true;
+	}
+
+	if (is_imp_def_sys_reg(params)) {
 		kvm_inject_undefined(vcpu);
 	} else {
 		print_sys_reg_msg(params,
@@ -2418,7 +2428,7 @@ static int emulate_sys_reg(struct kvm_vcpu *vcpu,
 				  *vcpu_pc(vcpu), *vcpu_cpsr(vcpu));
 		kvm_inject_undefined(vcpu);
 	}
-	return 1;
+	return false;
 }
 
 /**
@@ -2446,18 +2456,18 @@ int kvm_handle_sys_reg(struct kvm_vcpu *vcpu)
 	struct sys_reg_params params;
 	unsigned long esr = kvm_vcpu_get_esr(vcpu);
 	int Rt = kvm_vcpu_sys_get_rt(vcpu);
-	int ret;
 
 	trace_kvm_handle_sys_reg(esr);
 
 	params = esr_sys64_to_params(esr);
 	params.regval = vcpu_get_reg(vcpu, Rt);
 
-	ret = emulate_sys_reg(vcpu, &params);
+	if (!emulate_sys_reg(vcpu, &params))
+		return 1;
 
 	if (!params.is_write)
 		vcpu_set_reg(vcpu, Rt, params.regval);
-	return ret;
+	return 1;
 }
 
 /******************************************************************************
-- 
2.36.0.464.gb9c8b46e94-goog