[PATCH v9 3/4] virt: sev-guest: Remove err in handle_guest_request

Dionna Glaze
2022-12-07T01:02:50Z

The err variable may not be set in the call to snp_issue_guest_request,
yet it is unconditionally written back to fw_err if fw_err is non-null.
This is undefined behavior, and currently returns uninitialized kernel
stack memory to user space.

The fw_err argument is better to just pass through to
snp_issue_guest_request, so that's done by passing along the ioctl
argument. This removes the need for an argument to handle_guest_request.

Cc: Tom Lendacky <Thomas.Lendacky@amd.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Joerg Roedel <jroedel@suse.de>
Cc: Peter Gonda <pgonda@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Haowen Bai <baihaowen@meizu.com>
Cc: Liam Merwick <liam.merwick@oracle.com>
Cc: Yang Yingliang <yangyingliang@huawei.com>

Fixes: fce96cf04430 ("virt: Add SEV-SNP guest driver")
Reviewed-by: Tom Lendacky <Thomas.Lendacky@amd.com>
Signed-off-by: Dionna Glaze <dionnaglaze@google.com>
---
drivers/virt/coco/sev-guest/sev-guest.c | 35 ++++++++++++-------------
1 file changed, 17 insertions(+), 18 deletions(-)

diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
index 1ea6d2e5b218..4a2a0a02985f 100644
--- a/drivers/virt/coco/sev-guest/sev-guest.c
+++ b/drivers/virt/coco/sev-guest/sev-guest.c
@@ -320,11 +320,11 @@ static int enc_payload(struct snp_guest_dev *snp_dev, u64 seqno, int version, u8
return __enc_payload(snp_dev, req, payload, sz);
}

-static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, int msg_ver,
+static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code,
+ struct snp_guest_request_ioctl *arg,
u8 type, void *req_buf, size_t req_sz, void *resp_buf,
- u32 resp_sz, __u64 *fw_err)
+ u32 resp_sz)
{
- unsigned long err;
u64 seqno;
int rc;

@@ -336,7 +336,8 @@ static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, in
memset(snp_dev->response, 0, sizeof(struct snp_guest_msg));

/* Encrypt the userspace provided payload */
- rc = enc_payload(snp_dev, seqno, msg_ver, type, req_buf, req_sz);
+ rc = enc_payload(snp_dev, seqno, arg->msg_version, type, req_buf,
+ req_sz);
if (rc)
return rc;

@@ -346,7 +347,8 @@ static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, in
* sequence number must be incremented or the VMPCK must be deleted to
* prevent reuse of the IV.
*/
- rc = snp_issue_guest_request(exit_code, &snp_dev->input, &err);
+ rc = snp_issue_guest_request(exit_code, &snp_dev->input,
+ &arg->fw_err);

/*
* If the extended guest request fails due to having too small of a
@@ -368,24 +370,22 @@ 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);
+ rc = snp_issue_guest_request(exit_code, &snp_dev->input,
+ &arg->fw_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;
+ arg->fw_err = SNP_GUEST_REQ_INVALID_LEN;
snp_dev->input.data_npages = certs_npages;
}

- if (fw_err)
- *fw_err = err;
-
if (rc) {
dev_alert(snp_dev->dev,
"Detected error from ASP request. rc: %d, fw_err: %llu\n",
- rc, *fw_err);
+ rc, arg->fw_err);
goto disable_vmpck;
}

@@ -432,9 +432,9 @@ static int get_report(struct snp_guest_dev *snp_dev, struct snp_guest_request_io
if (!resp)
return -ENOMEM;

- rc = handle_guest_request(snp_dev, SVM_VMGEXIT_GUEST_REQUEST, arg->msg_version,
+ rc = handle_guest_request(snp_dev, SVM_VMGEXIT_GUEST_REQUEST, arg,
SNP_MSG_REPORT_REQ, &req, sizeof(req), resp->data,
- resp_len, &arg->fw_err);
+ resp_len);
if (rc)
goto e_free;

@@ -472,9 +472,8 @@ static int get_derived_key(struct snp_guest_dev *snp_dev, struct snp_guest_reque
if (copy_from_user(&req, (void __user *)arg->req_data, sizeof(req)))
return -EFAULT;

- rc = handle_guest_request(snp_dev, SVM_VMGEXIT_GUEST_REQUEST, arg->msg_version,
- SNP_MSG_KEY_REQ, &req, sizeof(req), buf, resp_len,
- &arg->fw_err);
+ rc = handle_guest_request(snp_dev, SVM_VMGEXIT_GUEST_REQUEST, arg,
+ SNP_MSG_KEY_REQ, &req, sizeof(req), buf, resp_len);
if (rc)
return rc;

@@ -534,9 +533,9 @@ static int get_ext_report(struct snp_guest_dev *snp_dev, struct snp_guest_reques
return -ENOMEM;

snp_dev->input.data_npages = npages;
- ret = handle_guest_request(snp_dev, SVM_VMGEXIT_EXT_GUEST_REQUEST, arg->msg_version,
+ ret = handle_guest_request(snp_dev, SVM_VMGEXIT_EXT_GUEST_REQUEST, arg,
SNP_MSG_REPORT_REQ, &req.data,
- sizeof(req.data), resp->data, resp_len, &arg->fw_err);
+ sizeof(req.data), resp->data, resp_len);

/* If certs length is invalid then copy the returned length */
if (arg->fw_err == SNP_GUEST_REQ_INVALID_LEN) {