linux-kernel - [PATCH 5.9 391/391] KVM: arm64: ARM_SMCCC_ARCH_WORKAROUND_1 doesnt return SMCCC_RET_NOT

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Message-Id: <20201103203413.547890667@linuxfoundation.org>
Date:   Tue,  3 Nov 2020 21:37:22 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Stephen Boyd <swboyd@...omium.org>,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        Andre Przywara <andre.przywara@....com>,
        Steven Price <steven.price@....com>
Subject: [PATCH 5.9 391/391] KVM: arm64: ARM_SMCCC_ARCH_WORKAROUND_1 doesnt return SMCCC_RET_NOT_REQUIRED

From: Stephen Boyd <swboyd@...omium.org>

commit 1de111b51b829bcf01d2e57971f8fd07a665fa3f upstream.

According to the SMCCC spec[1](7.5.2 Discovery) the
ARM_SMCCC_ARCH_WORKAROUND_1 function id only returns 0, 1, and
SMCCC_RET_NOT_SUPPORTED.

 0 is "workaround required and safe to call this function"
 1 is "workaround not required but safe to call this function"
 SMCCC_RET_NOT_SUPPORTED is "might be vulnerable or might not be, who knows, I give up!"

SMCCC_RET_NOT_SUPPORTED might as well mean "workaround required, except
calling this function may not work because it isn't implemented in some
cases". Wonderful. We map this SMC call to

 0 is SPECTRE_MITIGATED
 1 is SPECTRE_UNAFFECTED
 SMCCC_RET_NOT_SUPPORTED is SPECTRE_VULNERABLE

For KVM hypercalls (hvc), we've implemented this function id to return
SMCCC_RET_NOT_SUPPORTED, 0, and SMCCC_RET_NOT_REQUIRED. One of those
isn't supposed to be there. Per the code we call
arm64_get_spectre_v2_state() to figure out what to return for this
feature discovery call.

 0 is SPECTRE_MITIGATED
 SMCCC_RET_NOT_REQUIRED is SPECTRE_UNAFFECTED
 SMCCC_RET_NOT_SUPPORTED is SPECTRE_VULNERABLE

Let's clean this up so that KVM tells the guest this mapping:

 0 is SPECTRE_MITIGATED
 1 is SPECTRE_UNAFFECTED
 SMCCC_RET_NOT_SUPPORTED is SPECTRE_VULNERABLE

Note: SMCCC_RET_NOT_AFFECTED is 1 but isn't part of the SMCCC spec

Fixes: c118bbb52743 ("arm64: KVM: Propagate full Spectre v2 workaround state to KVM guests")
Signed-off-by: Stephen Boyd <swboyd@...omium.org>
Acked-by: Marc Zyngier <maz@...nel.org>
Acked-by: Will Deacon <will@...nel.org>
Cc: Andre Przywara <andre.przywara@....com>
Cc: Steven Price <steven.price@....com>
Cc: Marc Zyngier <maz@...nel.org>
Cc: stable@...r.kernel.org
Link: https://developer.arm.com/documentation/den0028/latest [1]
Link: https://lore.kernel.org/r/20201023154751.1973872-1-swboyd@chromium.org
Signed-off-by: Will Deacon <will@...nel.org>
Signed-off-by: Stephen Boyd <swboyd@...omium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/arm64/kvm/hypercalls.c |    2 +-
 include/linux/arm-smccc.h   |    2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)

--- a/arch/arm64/kvm/hypercalls.c
+++ b/arch/arm64/kvm/hypercalls.c
@@ -31,7 +31,7 @@ int kvm_hvc_call_handler(struct kvm_vcpu
 				val = SMCCC_RET_SUCCESS;
 				break;
 			case KVM_BP_HARDEN_NOT_REQUIRED:
-				val = SMCCC_RET_NOT_REQUIRED;
+				val = SMCCC_ARCH_WORKAROUND_RET_UNAFFECTED;
 				break;
 			}
 			break;
--- a/include/linux/arm-smccc.h
+++ b/include/linux/arm-smccc.h
@@ -86,6 +86,8 @@
 			   ARM_SMCCC_SMC_32,				\
 			   0, 0x7fff)
 
+#define SMCCC_ARCH_WORKAROUND_RET_UNAFFECTED	1
+
 /* Paravirtualised time calls (defined by ARM DEN0057A) */
 #define ARM_SMCCC_HV_PV_TIME_FEATURES				\
 	ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL,			\