linux-kernel - [PATCH 0/3] KVM: x86: SGX vs. XCR0 cleanups

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Message-ID: <20230405005911.423699-1-seanjc@google.com>
Date:   Tue,  4 Apr 2023 17:59:08 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kai Huang <kai.huang@...el.com>
Subject: [PATCH 0/3] KVM: x86: SGX vs. XCR0 cleanups

*** WARNING *** ABI breakage.

Stop adjusting the guest's CPUID info for the allowed XFRM (a.k.a. XCR0)
for SGX enclaves.  Past me didn't understand the roles and responsibilities
between userspace and KVM with respect to CPUID leafs, i.e. I thought I was
being helpful by having KVM adjust the entries.

This is clearly an ABI breakage, but QEMU (tries to) do the right thing,
and AFAIK no other VMMs support SGX (yet), so I'm hoping we can excise the
ugly before userspace starts depending on the bad behavior.

Compile tested only (don't have an SGX system these days).

Note, QEMU commit 301e90675c ("target/i386: Enable support for XSAVES
based features") completely broke SGX by using allowed XSS instead of
XCR0, and no one has complained.  That gives me hope that this one will
go through as well.

I belive the QEMU fix is below.  I'll post a patch at some point unless
someone wants to do the dirty work and claim the patch as their own.

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 6576287e5b..f083ff4335 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -5718,8 +5718,8 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
         } else {
             *eax &= env->features[FEAT_SGX_12_1_EAX];
             *ebx &= 0; /* ebx reserve */
-            *ecx &= env->features[FEAT_XSAVE_XSS_LO];
-            *edx &= env->features[FEAT_XSAVE_XSS_HI];
+            *ecx &= env->features[FEAT_XSAVE_XCR0_LO];
+            *edx &= env->features[FEAT_XSAVE_XCR0_HI];

             /* FP and SSE are always allowed regardless of XSAVE/XCR0. */
             *ecx |= XSTATE_FP_MASK | XSTATE_SSE_MASK;

Sean Christopherson (3):
  KVM: VMX: Don't rely _only_ on CPUID to enforce XCR0 restrictions for
    ECREATE
  KVM: x86: Don't adjust guest's CPUID.0x12.1 (allowed SGX enclave XFRM)
  KVM: x86: Open code supported XCR0 calculation in
    kvm_vcpu_after_set_cpuid()

 arch/x86/kvm/cpuid.c   | 43 ++++++++++--------------------------------
 arch/x86/kvm/vmx/sgx.c |  3 ++-
 2 files changed, 12 insertions(+), 34 deletions(-)

base-commit: 27d6845d258b67f4eb3debe062b7dacc67e0c393
-- 
2.40.0.348.gf938b09366-goog