[PATCH -v2 03/11] virt/coco/sev-guest: Simplify extended guest request handling

[Borislav Petkov <bp@alien8.de>]

From: "Borislav Petkov (AMD)" <bp@alien8.de>

Return a specific error code - -ENOSPC - to signal the too small cert
data buffer instead of checking exit code and exitinfo2.

While at it, hoist the *fw_err assignment in snp_issue_guest_request()
so that a proper error value is returned to the callers.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
---
 arch/x86/kernel/sev.c                   | 11 +++---
 drivers/virt/coco/sev-guest/sev-guest.c | 46 ++++++++++++++-----------
 2 files changed, 31 insertions(+), 26 deletions(-)

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index c644c34372e8..6a3e1425ba17 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -2209,15 +2209,16 @@ int snp_issue_guest_request(u64 exit_code, struct snp_req_data *input, unsigned
 	if (ret)
 		goto e_put;
 
+	*fw_err = ghcb->save.sw_exit_info_2;
 	if (ghcb->save.sw_exit_info_2) {
 		/* Number of expected pages are returned in RBX */
 		if (exit_code == SVM_VMGEXIT_EXT_GUEST_REQUEST &&
-		    ghcb->save.sw_exit_info_2 == SNP_GUEST_REQ_INVALID_LEN)
+		    ghcb->save.sw_exit_info_2 == SNP_GUEST_REQ_INVALID_LEN) {
 			input->data_npages = ghcb_get_rbx(ghcb);
-
-		*fw_err = ghcb->save.sw_exit_info_2;
-
-		ret = -EIO;
+			ret = -ENOSPC;
+		} else {
+			ret = -EIO;
+		}
 	}
 
 e_put:
diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
index edaf6031c6d9..5b4cddf44a3a 100644
--- a/drivers/virt/coco/sev-guest/sev-guest.c
+++ b/drivers/virt/coco/sev-guest/sev-guest.c
@@ -322,7 +322,8 @@ static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, in
 				u8 type, void *req_buf, size_t req_sz, void *resp_buf,
 				u32 resp_sz, __u64 *fw_err)
 {
-	unsigned long err;
+	unsigned long err, override_err = 0;
+	unsigned int override_npages = 0;
 	u64 seqno;
 	int rc;
 
@@ -338,6 +339,7 @@ static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, in
 	if (rc)
 		return rc;
 
+retry_request:
 	/*
 	 * Call firmware to process the request. In this function the encrypted
 	 * message enters shared memory with the host. So after this call the
@@ -346,17 +348,24 @@ static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, in
 	 */
 	rc = snp_issue_guest_request(exit_code, &snp_dev->input, &err);
 
-	/*
-	 * If the extended guest request fails due to having too small of a
-	 * certificate data buffer, retry the same guest request without the
-	 * extended data request in order to increment the sequence number
-	 * and thus avoid IV reuse.
-	 */
-	if (exit_code == SVM_VMGEXIT_EXT_GUEST_REQUEST &&
-	    err == SNP_GUEST_REQ_INVALID_LEN) {
-		const unsigned int certs_npages = snp_dev->input.data_npages;
+	switch (rc) {
+	case -ENOSPC:
+		/*
+		 * If the extended guest request fails due to having too
+		 * small of a certificate data buffer, retry the same
+		 * guest request without the extended data request in
+		 * order to increment the sequence number and thus avoid
+		 * IV reuse.
+		 */
+		override_npages = snp_dev->input.data_npages;
+		exit_code	= SVM_VMGEXIT_GUEST_REQUEST;
 
-		exit_code = SVM_VMGEXIT_GUEST_REQUEST;
+		/*
+		 * Override the error to inform callers the given extended
+		 * request buffer size was too small and give the caller the
+		 * required buffer size.
+		 */
+		override_err	= SNP_GUEST_REQ_INVALID_LEN;
 
 		/*
 		 * If this call to the firmware succeeds, the sequence number can
@@ -366,19 +375,14 @@ static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, in
 		 * of the VMPCK and the error code being propagated back to the
 		 * user as an ioctl() return code.
 		 */
-		rc = snp_issue_guest_request(exit_code, &snp_dev->input, &err);
-
-		/*
-		 * Override the error to inform callers the given extended
-		 * request buffer size was too small and give the caller the
-		 * required buffer size.
-		 */
-		err = SNP_GUEST_REQ_INVALID_LEN;
-		snp_dev->input.data_npages = certs_npages;
+		goto retry_request;
 	}
 
 	if (fw_err)
-		*fw_err = err;
+		*fw_err = override_err ?: err;
+
+	if (override_npages)
+		snp_dev->input.data_npages = override_npages;
 
 	if (rc) {
 		dev_alert(snp_dev->dev,
-- 
2.35.1