[PATCH] KVM: SEV: Return INVALID_INPUT on SNP req/resp buffer access failure

[Jacky Li <jackyli@google.com>]

Currently, snp_handle_(ext_)guest_req() returns -EIO when
kvm_{read/write/clear}_guest() fails while accessing guest-provided
buffers. Returning -EIO causes KVM_RUN to exit to userspace, likely
killing the VM.

Fix this by returning GHCB_HV_RESP_MALFORMED_INPUT with sub-error code
GHCB_ERR_INVALID_INPUT to the guest and resuming the vCPU.

Per the GHCB specification, guest-provided GPA buffers that cannot
be accessed by the hypervisor (e.g. private pages) should be treated
as guest input errors. Because kvm_{read/write/clear}_guest() only
returns -EFAULT on failure, treating this failure as an invalid input
aligns with the definition of -EFAULT ("Bad address").

Returning GHCB_ERR_INVALID_INPUT also matches existing SNP handling
in KVM, which already returns this error code for unaligned or
overlapping buffers. It also aligns with other hypercall implementations
in KVM (e.g. Hyper-V returning INVALID_HYPERCALL_INPUT on
kvm_read_guest() failures in kvm_hv_flush_tlb()).

Performing upfront validation (e.g. via kvm_mem_is_private()) is
avoided because it is prone to TOCTOU races with concurrent Page State
Changes.

Fixes: 88caf544c930 ("KVM: SEV: Provide support for SNP_GUEST_REQUEST NAE event")
Fixes: 74458e4859d8 ("KVM: SEV: Provide support for SNP_EXTENDED_GUEST_REQUEST NAE event")
Signed-off-by: Jacky Li <jackyli@google.com>
---
 arch/x86/kvm/svm/sev.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 5705723f1f41..d07562310519 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -4228,8 +4228,10 @@ static int snp_handle_guest_req(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t resp_
 
 	guard(mutex)(&sev->guest_req_mutex);
 
-	if (kvm_read_guest(kvm, req_gpa, sev->guest_req_buf, PAGE_SIZE))
-		return -EIO;
+	if (kvm_read_guest(kvm, req_gpa, sev->guest_req_buf, PAGE_SIZE)) {
+		svm_vmgexit_bad_input(svm, GHCB_ERR_INVALID_INPUT);
+		return 1;
+	}
 
 	data.gctx_paddr = __psp_pa(sev->snp_context);
 	data.req_paddr = __psp_pa(sev->guest_req_buf);
@@ -4244,8 +4246,10 @@ static int snp_handle_guest_req(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t resp_
 	if (ret && !fw_err)
 		return ret;
 
-	if (kvm_write_guest(kvm, resp_gpa, sev->guest_resp_buf, PAGE_SIZE))
-		return -EIO;
+	if (kvm_write_guest(kvm, resp_gpa, sev->guest_resp_buf, PAGE_SIZE)) {
+		svm_vmgexit_bad_input(svm, GHCB_ERR_INVALID_INPUT);
+		return 1;
+	}
 
 	/* No action is requested *from KVM* if there was a firmware error. */
 	svm_vmgexit_no_action(svm, SNP_GUEST_ERR(0, fw_err));
@@ -4296,7 +4300,7 @@ static int snp_handle_ext_guest_req(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t r
 
 	if (kvm_read_guest(kvm, req_gpa + offsetof(struct snp_guest_msg_hdr, msg_type),
 			   &msg_type, 1))
-		return -EIO;
+		goto request_invalid;
 
 	/*
 	 * As per GHCB spec, requests of type MSG_REPORT_REQ also allow for
@@ -4336,7 +4340,7 @@ static int snp_handle_ext_guest_req(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t r
 		 * certificate table is terminated by 24-bytes of zeroes.
 		 */
 		if (data_npages && kvm_clear_guest(kvm, data_gpa, 24))
-			return -EIO;
+			goto request_invalid;
 	}
 
 	return snp_handle_guest_req(svm, req_gpa, resp_gpa);

---
base-commit: 50d05c7c76c96b90462f24debacca971d2e86713
change-id: 20260910-snp-invalid-input-5dbb6a408e7f

Best regards,
-- 
Jacky Li <jackyli@google.com>