The ASP and an SNP guest use a series of AES-GCM keys called VMPCKs to communicate securely with each other. The IV to this scheme is a sequence number that both the ASP and the guest track. Currently this sequence number in a guest request must exactly match the sequence number tracked by the ASP. This means that if the guest sees an error from the host during a request it can only retry that exact request or disable the VMPCK to prevent an IV reuse. AES-GCM cannot tolerate IV reuse see: https://csrc.nist.gov/csrc/media/projects/block-cipher-techniques/documents/bcm/comments/800-38-series-drafts/gcm/joux_comments.pdf To handle userspace querying the cert_data length handle_guest_request() now: saves the number of pages required by the host, retries the request without requesting the extended data, then returns the number of pages required. Fixes: fce96cf044308 ("virt: Add SEV-SNP guest driver") Signed-off-by: Peter Gonda <pgonda@xxxxxxxxxx> Reported-by: Peter Gonda <pgonda@xxxxxxxxxx> Cc: Dionna Glaze <dionnaglaze@xxxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxx> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx> Cc: Michael Roth <michael.roth@xxxxxxx> Cc: Haowen Bai <baihaowen@xxxxxxxxx> Cc: Yang Yingliang <yangyingliang@xxxxxxxxxx> Cc: Marc Orr <marcorr@xxxxxxxxxx> Cc: David Rientjes <rientjes@xxxxxxxxxx> Cc: Ashish Kalra <Ashish.Kalra@xxxxxxx> Cc: linux-kernel@xxxxxxxxxxxxxxx Cc: kvm@xxxxxxxxxxxxxxx --- Tested by placing each of the guest requests: attestation quote, extended attestation quote, and get key. Then tested the extended attestation quote certificate length querying. V4 * As suggested by Dionna moved the extended request retry logic into the driver. * Due to big change in patch dropped any reviewed-by tags. --- drivers/virt/coco/sev-guest/sev-guest.c | 70 +++++++++++++++++++------ 1 file changed, 53 insertions(+), 17 deletions(-) diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c index f422f9c58ba79..7dd6337ebdd5b 100644 --- a/drivers/virt/coco/sev-guest/sev-guest.c +++ b/drivers/virt/coco/sev-guest/sev-guest.c @@ -41,7 +41,7 @@ struct snp_guest_dev { struct device *dev; struct miscdevice misc; - void *certs_data; + u8 (*certs_data)[SEV_FW_BLOB_MAX_SIZE]; struct snp_guest_crypto *crypto; struct snp_guest_msg *request, *response; struct snp_secrets_page_layout *layout; @@ -67,8 +67,27 @@ static bool is_vmpck_empty(struct snp_guest_dev *snp_dev) return true; } +/* + * If we receive an error from the host or ASP we have two options. We can + * either retry the exact same encrypted request or we can discontinue using the + * VMPCK. + * + * This is because in the current encryption scheme GHCB v2 uses AES-GCM to + * encrypt the requests. The IV for this scheme is the sequence number. GCM + * cannot tolerate IV reuse. + * + * The ASP FW v1.51 only increments the sequence numbers on a successful + * guest<->ASP back and forth and only accepts messages at its exact sequence + * number. + * + * So if we were to reuse the sequence number the encryption scheme is + * vulnerable. If we encrypt the sequence number for a fresh IV the ASP will + * reject our request. + */ static void snp_disable_vmpck(struct snp_guest_dev *snp_dev) { + dev_alert(snp_dev->dev, "Disabling vmpck_id: %d to prevent IV reuse.\n", + vmpck_id); memzero_explicit(snp_dev->vmpck, VMPCK_KEY_LEN); snp_dev->vmpck = NULL; } @@ -323,32 +342,49 @@ static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, in /* Call firmware to process the request */ rc = snp_issue_guest_request(exit_code, &snp_dev->input, &err); + + /* + * If the extended guest request fails due to having to small of a + * certificate data buffer retry the same guest request without the + * extended data request. + */ + if (exit_code == SVM_VMGEXIT_EXT_GUEST_REQUEST && + err == SNP_GUEST_REQ_INVALID_LEN) { + const unsigned int certs_npages = snp_dev->input.data_npages; + + exit_code = SVM_VMGEXIT_GUEST_REQUEST; + rc = snp_issue_guest_request(exit_code, &snp_dev->input, &err); + + err = SNP_GUEST_REQ_INVALID_LEN; + snp_dev->input.data_npages = certs_npages; + } + if (fw_err) *fw_err = err; - if (rc) - return rc; + if (rc) { + dev_alert(snp_dev->dev, + "Detected error from ASP request. rc: %d, fw_err: %llu\n", + rc, *fw_err); + goto disable_vmpck; + } - /* - * The verify_and_dec_payload() will fail only if the hypervisor is - * actively modifying the message header or corrupting the encrypted payload. - * This hints that hypervisor is acting in a bad faith. Disable the VMPCK so that - * the key cannot be used for any communication. The key is disabled to ensure - * that AES-GCM does not use the same IV while encrypting the request payload. - */ rc = verify_and_dec_payload(snp_dev, resp_buf, resp_sz); if (rc) { dev_alert(snp_dev->dev, - "Detected unexpected decode failure, disabling the vmpck_id %d\n", - vmpck_id); - snp_disable_vmpck(snp_dev); - return rc; + "Detected unexpected decode failure from ASP. rc: %d\n", + rc); + goto disable_vmpck; } /* Increment to new message sequence after payload decryption was successful. */ snp_inc_msg_seqno(snp_dev); return 0; + +disable_vmpck: + snp_disable_vmpck(snp_dev); + return rc; } static int get_report(struct snp_guest_dev *snp_dev, struct snp_guest_request_ioctl *arg) @@ -676,7 +712,7 @@ static int __init sev_guest_probe(struct platform_device *pdev) if (!snp_dev->response) goto e_free_request; - snp_dev->certs_data = alloc_shared_pages(dev, SEV_FW_BLOB_MAX_SIZE); + snp_dev->certs_data = alloc_shared_pages(dev, sizeof(*snp_dev->certs_data)); if (!snp_dev->certs_data) goto e_free_response; @@ -703,7 +739,7 @@ static int __init sev_guest_probe(struct platform_device *pdev) return 0; e_free_cert_data: - free_shared_pages(snp_dev->certs_data, SEV_FW_BLOB_MAX_SIZE); + free_shared_pages(snp_dev->certs_data, sizeof(*snp_dev->certs_data)); e_free_response: free_shared_pages(snp_dev->response, sizeof(struct snp_guest_msg)); e_free_request: @@ -717,7 +753,7 @@ static int __exit sev_guest_remove(struct platform_device *pdev) { struct snp_guest_dev *snp_dev = platform_get_drvdata(pdev); - free_shared_pages(snp_dev->certs_data, SEV_FW_BLOB_MAX_SIZE); + free_shared_pages(snp_dev->certs_data, sizeof(*snp_dev->certs_data)); free_shared_pages(snp_dev->response, sizeof(struct snp_guest_msg)); free_shared_pages(snp_dev->request, sizeof(struct snp_guest_msg)); deinit_crypto(snp_dev->crypto); -- 2.38.1.273.g43a17bfeac-goog
