Re: [PATCH v3] KVM: arm64: ARM_SMCCC_ARCH_WORKAROUND_1 doesn't return SMCCC_RET_NOT_REQUIRED

On 2020-10-26 13:25, Will Deacon wrote:

On Fri, Oct 23, 2020 at 08:47:50AM -0700, Stephen Boyd wrote:

...

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

Cc: Andre Przywara andre.przywara@arm.com Cc: Steven Price steven.price@arm.com Cc: Marc Zyngier maz@kernel.org Cc: stable@vger.kernel.org Link: https://developer.arm.com/documentation/den0028/latest [1] Fixes: c118bbb52743 ("arm64: KVM: Propagate full Spectre v2 workaround state to KVM guests") Signed-off-by: Stephen Boyd swboyd@chromium.org

---

I see that before commit c118bbb52743 ("arm64: KVM: Propagate full Spectre v2 workaround state to KVM guests") we had this mapping:

0 is SPECTRE_MITIGATED SMCCC_RET_NOT_SUPPORTED is SPECTRE_VULNERABLE

so the return value '1' wasn't there then. Once the commit was merged we introduced the notion of NOT_REQUIRED here when it shouldn't have been introduced.

Changes from v2:

• Moved define to header file and used it

Changes from v1:

• Way longer commit text, more background (sorry)
• Dropped proton-pack part because it was wrong
• Rebased onto other patch accepted upstream

arch/arm64/kernel/proton-pack.c | 2 -- arch/arm64/kvm/hypercalls.c | 2 +- include/linux/arm-smccc.h | 2 ++ 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/proton-pack.c b/arch/arm64/kernel/proton-pack.c index 25f3c80b5ffe..c18eb7d41274 100644 --- a/arch/arm64/kernel/proton-pack.c +++ b/arch/arm64/kernel/proton-pack.c @@ -135,8 +135,6 @@ static enum mitigation_state spectre_v2_get_cpu_hw_mitigation_state(void) return SPECTRE_VULNERABLE; }

-#define SMCCC_ARCH_WORKAROUND_RET_UNAFFECTED (1)

static enum mitigation_state spectre_v2_get_cpu_fw_mitigation_state(void) { int ret; diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c index 9824025ccc5c..25ea4ecb6449 100644 --- a/arch/arm64/kvm/hypercalls.c +++ b/arch/arm64/kvm/hypercalls.c @@ -31,7 +31,7 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu) val = SMCCC_RET_SUCCESS; break; case SPECTRE_UNAFFECTED:

• 		val = SMCCC_RET_NOT_REQUIRED;
  

• 		val = SMCCC_ARCH_WORKAROUND_RET_UNAFFECTED;
  	break;
  }
  break;
  

diff --git a/include/linux/arm-smccc.h b/include/linux/arm-smccc.h index 15c706fb0a37..0e50ba3e88d7 100644 --- 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

I thought we'd stick this in asm/spectre.h, but here is also good:

Acked-by: Will Deacon will@kernel.org

Acked-by: Marc Zyngier maz@kernel.org

Will, if you're about to send fixes to Linus, can you please pick this one up?

M.