linux-kernel - [PATCH] KVM: nVMX: Document that ignoring memory failures for VMCLEAR is deliberate

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Message-ID: <20221220154224.526568-1-seanjc@google.com>
Date:   Tue, 20 Dec 2022 15:42:24 +0000
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,
        coverity-bot <keescook+coverity-bot@...omium.org>,
        Jim Mattson <jmattson@...gle.com>
Subject: [PATCH] KVM: nVMX: Document that ignoring memory failures for VMCLEAR
 is deliberate

Explicitly drop the result of kvm_vcpu_write_guest() when writing the
"launch state" as part of VMCLEAR emulation, and add a comment to call
out that KVM's behavior is architecturally valid.  Intel's pseudocode
effectively says that VMCLEAR is a nop if the target VMCS address isn't
in memory, e.g. if the address points at MMIO.

Add a FIXME to call out that suppressing failures on __copy_to_user() is
wrong, as memory (a memslot) does exist in that case.  Punt the issue to
the future as open coding kvm_vcpu_write_guest() just to make sure the
guest dies with -EFAULT isn't worth the extra complexity.  The flaw will
need to be addressed if KVM ever does something intelligent on uaccess
failures, e.g. to support post-copy demand paging, but in that case KVM
will need a more thorough overhaul, i.e. VMCLEAR shouldn't need to open
code a core KVM helper.

No functional change intended.

Reported-by: coverity-bot <keescook+coverity-bot@...omium.org>
Addresses-Coverity-ID: 1527765 ("Error handling issues")
Fixes: 587d7e72aedc ("kvm: nVMX: VMCLEAR should not cause the vCPU to shut down")
Cc: Jim Mattson <jmattson@...gle.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/nested.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index b6f4411b613e..f18f3a9f0943 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -5296,10 +5296,19 @@ static int handle_vmclear(struct kvm_vcpu *vcpu)
 		if (vmptr == vmx->nested.current_vmptr)
 			nested_release_vmcs12(vcpu);
 
-		kvm_vcpu_write_guest(vcpu,
-				     vmptr + offsetof(struct vmcs12,
-						      launch_state),
-				     &zero, sizeof(zero));
+		/*
+		 * Silently ignore memory errors on VMCLEAR, Intel's pseudocode
+		 * for VMCLEAR includes a "ensure that data for VMCS referenced
+		 * by the operand is in memory" clause that guards writes to
+		 * memory, i.e. doing nothing for I/O is architecturally valid.
+		 *
+		 * FIXME: Suppress failures if and only if no memslot is found,
+		 * i.e. exit to userspace if __copy_to_user() fails.
+		 */
+		(void)kvm_vcpu_write_guest(vcpu,
+					   vmptr + offsetof(struct vmcs12,
+							    launch_state),
+					   &zero, sizeof(zero));
 	} else if (vmx->nested.hv_evmcs && vmptr == vmx->nested.hv_evmcs_vmptr) {
 		nested_release_evmcs(vcpu);
 	}

base-commit: 9d75a3251adfbcf444681474511b58042a364863
-- 
2.39.0.314.g84b9a713c41-goog