On Fri, Oct 21, 2022 at 1:02 PM Tom Lendacky <thomas.lendacky@xxxxxxx> wrote: > > On 10/21/22 12:33, Peter Gonda wrote: > > 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. Instead of requesting > > the cert length from userspace use the size of the drivers allocated > > shared buffer. Then copy that buffer to userspace, or give userspace an > > error depending on the size of the buffer given by userspace. > > > > Fixes: fce96cf044308 ("virt: Add SEV-SNP guest driver") > > Signed-off-by: Peter Gonda <pgonda@xxxxxxxxxx> > > Reported-by: Peter Gonda <pgonda@xxxxxxxxxx> > > Reviewed-by: 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 > > --- > > drivers/virt/coco/sev-guest/sev-guest.c | 93 ++++++++++++++++--------- > > 1 file changed, 62 insertions(+), 31 deletions(-) > > > > diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c > > index f422f9c58ba7..8c54ea84bc57 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; > > } > > @@ -326,29 +345,29 @@ static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, in > > 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) > > @@ -437,7 +456,7 @@ static int get_ext_report(struct snp_guest_dev *snp_dev, struct snp_guest_reques > > struct snp_guest_crypto *crypto = snp_dev->crypto; > > struct snp_ext_report_req req; > > struct snp_report_resp *resp; > > - int ret, npages = 0, resp_len; > > + int ret, resp_len, req_cert_len, resp_cert_len; > > > > lockdep_assert_held(&snp_cmd_mutex); > > > > @@ -448,14 +467,15 @@ static int get_ext_report(struct snp_guest_dev *snp_dev, struct snp_guest_reques > > return -EFAULT; > > > > /* userspace does not want certificate data */ > > - if (!req.certs_len || !req.certs_address) > > + req_cert_len = req.certs_len; > > + if (!req_cert_len || !req.certs_address) > > goto cmd; > > > > - if (req.certs_len > SEV_FW_BLOB_MAX_SIZE || > > - !IS_ALIGNED(req.certs_len, PAGE_SIZE)) > > + if (req_cert_len > sizeof(*snp_dev->certs_data) || > > + !IS_ALIGNED(req_cert_len, PAGE_SIZE)) > > return -EINVAL; > > > > - if (!access_ok((const void __user *)req.certs_address, req.certs_len)) > > + if (!access_ok((const void __user *)req.certs_address, req_cert_len)) > > return -EFAULT; > > > > /* > > @@ -464,8 +484,7 @@ static int get_ext_report(struct snp_guest_dev *snp_dev, struct snp_guest_reques > > * the host. If host does not supply any certs in it, then copy > > * zeros to indicate that certificate data was not provided. > > */ > > - memset(snp_dev->certs_data, 0, req.certs_len); > > - npages = req.certs_len >> PAGE_SHIFT; > > + memset(snp_dev->certs_data, 0, sizeof(*snp_dev->certs_data)); > > cmd: > > /* > > * The intermediate response buffer is used while decrypting the > > @@ -477,25 +496,37 @@ static int get_ext_report(struct snp_guest_dev *snp_dev, struct snp_guest_reques > > if (!resp) > > return -ENOMEM; > > > > - snp_dev->input.data_npages = npages; > > + snp_dev->input.data_npages = sizeof(*snp_dev->certs_data) >> PAGE_SHIFT; > > ret = handle_guest_request(snp_dev, SVM_VMGEXIT_EXT_GUEST_REQUEST, arg->msg_version, > > SNP_MSG_REPORT_REQ, &req.data, > > sizeof(req.data), resp->data, resp_len, &arg->fw_err); > > > > + resp_cert_len = snp_dev->input.data_npages << PAGE_SHIFT; > > The hypervisor is not required to update the number of pages that the > certificates actually used/required if enough pages were supplied. So this > value could always remain as 4 (based on SEV_FW_BLOB_MAX_SIZE) on > successful return. > > And if that's the case, we could always just return 4 for the number of > pages no matter what. Otherwise you'll have to update the logic here if > you want to obtain the actual number. Are you asking for this to just hard code the userspace requirement to 4 pages? We could leave this as written here, that would leave the guest open to a new GHCB spec where "State from Hypervisor: on error will contain the number of guest contiguous pages required to hold the data to be returned" Is instead: "State from Hypervisor: contain the number of guest contiguous pages required to hold the data to be returned" I think this would be a non-breaking change since the spec says nothing of the non-error case currently. Fine with your preference here. Either Dionna or I can follow up with a series to allow more than 4pages if needed. The logic required would be parsing the GUID table? I think we'd rather keep that out of the kernel driver, right? > > Thanks, > Tom > > > + > > /* If certs length is invalid then copy the returned length */ > > if (arg->fw_err == SNP_GUEST_REQ_INVALID_LEN) { > > - req.certs_len = snp_dev->input.data_npages << PAGE_SHIFT; > > + dev_alert(snp_dev->dev, > > + "Certificate data from host: %d, Max size allocated by driver: %lu.\n", > > + resp_cert_len, sizeof(*snp_dev->certs_data)); > > + ret = -EFAULT; > > + } > > + > > + if (ret) > > + goto e_free; > > + > > + /* Pass the actual certificate data size back to userspace */ > > + req.certs_len = resp_cert_len; > > + if (resp_cert_len > req_cert_len) { > > + arg->fw_err = SNP_GUEST_REQ_INVALID_LEN; > > > > if (copy_to_user((void __user *)arg->req_data, &req, sizeof(req))) > > ret = -EFAULT; > > - } > > > > - if (ret) > > goto e_free; > > + } > > > > - if (npages && > > - copy_to_user((void __user *)req.certs_address, snp_dev->certs_data, > > - req.certs_len)) { > > + if (copy_to_user((void __user *)req.certs_address, snp_dev->certs_data, > > + resp_cert_len)) { > > ret = -EFAULT; > > goto e_free; > > } > > @@ -676,7 +707,7 @@ 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 +734,7 @@ 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 +748,7 @@ 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);
