linux-kernel - [PATCH v4 2/2] The ccp driver can be overloaded even with guest request rate limits. The return value of -EBUSY means that there is no firmware error to report back to user space, so the guest VM would see this as exitinfo2 = 0. The false success can trick the guest to update its message sequence number when it shouldn't have.

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Message-ID: <20250514184136.238446-4-dionnaglaze@google.com>
Date: Wed, 14 May 2025 18:41:36 +0000
From: Dionna Glaze <dionnaglaze@...gle.com>
To: kvm@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev, 
	Dionna Glaze <dionnaglaze@...gle.com>, Thomas Lendacky <Thomas.Lendacky@....com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Joerg Roedel <jroedel@...e.de>, Peter Gonda <pgonda@...gle.com>, 
	Borislav Petkov <bp@...en8.de>, Sean Christopherson <seanjc@...gle.com>
Subject: [PATCH v4 2/2] The ccp driver can be overloaded even with guest
 request rate limits. The return value of -EBUSY means that there is no
 firmware error to report back to user space, so the guest VM would see this
 as exitinfo2 = 0. The false success can trick the guest to update its message
 sequence number when it shouldn't have.

Instead, when ccp returns -EBUSY, that is reported to userspace as the
throttling return value.

Cc: Thomas Lendacky <Thomas.Lendacky@....com>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Joerg Roedel <jroedel@...e.de>
Cc: Peter Gonda <pgonda@...gle.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Sean Christopherson <seanjc@...gle.com>

Signed-off-by: Dionna Glaze <dionnaglaze@...gle.com>
---
 arch/x86/kvm/svm/sev.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index ddbfdce9dc18..5901a7f59f88 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -4065,6 +4065,11 @@ static int snp_handle_guest_req(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t resp_
 	 * the PSP is dead and commands are timing out.
 	 */
 	ret = sev_issue_cmd(kvm, SEV_CMD_SNP_GUEST_REQUEST, &data, &fw_err);
+	if (ret == -EBUSY) {
+		svm_vmgexit_no_action(svm, SNP_GUEST_ERR(SNP_GUEST_VMM_ERR_BUSY, fw_err));
+		ret = 1;
+		goto out_unlock;
+	}
 	if (ret && !fw_err)
 		goto out_unlock;
 
-- 
2.49.0.1045.g170613ef41-goog