KVM: x86: Revert MSR_IA32_FLUSH_CMD.FLUSH_L1D enabling

Revert the recently added virtualizing of MSR_IA32_FLUSH_CMD, as both
the VMX and SVM are fatally buggy to guests that use MSR_IA32_FLUSH_CMD or
MSR_IA32_PRED_CMD, and because the entire foundation of the logic is
flawed.

The most immediate problem is an inverted check on @cmd that results in
rejecting legal values.  SVM doubles down on bugs and drops the error,
i.e. silently breaks all guest mitigations based on the command MSRs.

The next issue is that neither VMX nor SVM was updated to mark
MSR_IA32_FLUSH_CMD as being a possible passthrough MSR,
which isn't hugely problematic, but does break MSR filtering and triggers
a WARN on VMX designed to catch this exact bug.

The foundational issues stem from the MSR_IA32_FLUSH_CMD code reusing
logic from MSR_IA32_PRED_CMD, which in turn was likely copied from KVM's
support for MSR_IA32_SPEC_CTRL.  The copy+paste from MSR_IA32_SPEC_CTRL
was misguided as MSR_IA32_PRED_CMD (and MSR_IA32_FLUSH_CMD) is a
write-only MSR, i.e. doesn't need the same "deferred passthrough"
shenanigans as MSR_IA32_SPEC_CTRL.

Revert all MSR_IA32_FLUSH_CMD enabling in one fell swoop so that there is
no point where KVM advertises, but does not support, L1D_FLUSH.

This reverts commits 45cf86f26148e549c5ba4a8ab32a390e4bde216e,
723d5fb0ffe4c02bd4edf47ea02c02e454719f28, and
a807b78ad04b2eaa348f52f5cc7702385b6de1ee.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lkml.kernel.org/r/20230317190432.GA863767%40dev-arch.thelio-3990X
Cc: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Cc: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc: Jim Mattson <jmattson@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Message-Id: <20230322011440.2195485-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 25 insertions, 46 deletions

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index f63b28f46a71..1bc2b80273c9 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -654,9 +654,6 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
 	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
 					 MSR_IA32_PRED_CMD, MSR_TYPE_W);
 
-	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
-					 MSR_IA32_FLUSH_CMD, MSR_TYPE_W);
-
 	kvm_vcpu_unmap(vcpu, &vmx->nested.msr_bitmap_map, false);
 
 	vmx->nested.force_msr_bitmap_recalc = false;
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index d7bf14abdba1..f777509ecf17 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2133,39 +2133,6 @@ static u64 vmx_get_supported_debugctl(struct kvm_vcpu *vcpu, bool host_initiated
 	return debugctl;
 }
 
-static int vmx_set_msr_ia32_cmd(struct kvm_vcpu *vcpu,
-				struct msr_data *msr_info,
-				bool guest_has_feat, u64 cmd,
-				int x86_feature_bit)
-{
-	if (!msr_info->host_initiated && !guest_has_feat)
-		return 1;
-
-	if (!(msr_info->data & ~cmd))
-		return 1;
-	if (!boot_cpu_has(x86_feature_bit))
-		return 1;
-	if (!msr_info->data)
-		return 0;
-
-	wrmsrl(msr_info->index, cmd);
-
-	/*
-	 * For non-nested:
-	 * When it's written (to non-zero) for the first time, pass
-	 * it through.
-	 *
-	 * For nested:
-	 * The handling of the MSR bitmap for L2 guests is done in
-	 * nested_vmx_prepare_msr_bitmap. We should not touch the
-	 * vmcs02.msr_bitmap here since it gets completely overwritten
-	 * in the merging.
-	 */
-	vmx_disable_intercept_for_msr(vcpu, msr_info->index, MSR_TYPE_W);
-
-	return 0;
-}
-
 /*
  * Writes msr value into the appropriate "register".
  * Returns 0 on success, non-0 otherwise.
@@ -2319,16 +2286,31 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 			return 1;
 		goto find_uret_msr;
 	case MSR_IA32_PRED_CMD:
-		ret = vmx_set_msr_ia32_cmd(vcpu, msr_info,
-					   guest_has_pred_cmd_msr(vcpu),
-					   PRED_CMD_IBPB,
-					   X86_FEATURE_IBPB);
-		break;
-	case MSR_IA32_FLUSH_CMD:
-		ret = vmx_set_msr_ia32_cmd(vcpu, msr_info,
-					   guest_cpuid_has(vcpu, X86_FEATURE_FLUSH_L1D),
-					   L1D_FLUSH,
-					   X86_FEATURE_FLUSH_L1D);
+		if (!msr_info->host_initiated &&
+		    !guest_has_pred_cmd_msr(vcpu))
+			return 1;
+
+		if (data & ~PRED_CMD_IBPB)
+			return 1;
+		if (!boot_cpu_has(X86_FEATURE_IBPB))
+			return 1;
+		if (!data)
+			break;
+
+		wrmsrl(MSR_IA32_PRED_CMD, PRED_CMD_IBPB);
+
+		/*
+		 * For non-nested:
+		 * When it's written (to non-zero) for the first time, pass
+		 * it through.
+		 *
+		 * For nested:
+		 * The handling of the MSR bitmap for L2 guests is done in
+		 * nested_vmx_prepare_msr_bitmap. We should not touch the
+		 * vmcs02.msr_bitmap here since it gets completely overwritten
+		 * in the merging.
+		 */
+		vmx_disable_intercept_for_msr(vcpu, MSR_IA32_PRED_CMD, MSR_TYPE_W);
 		break;
 	case MSR_IA32_CR_PAT:
 		if (!kvm_pat_valid(data))