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Date: Fri, 21 Oct 2022 14:01:59 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Peter Gonda <pgonda@...gle.com>
Cc: Dionna Glaze <dionnaglaze@...gle.com>,
Borislav Petkov <bp@...e.de>,
Michael Roth <michael.roth@....com>,
Haowen Bai <baihaowen@...zu.com>,
Yang Yingliang <yangyingliang@...wei.com>,
Marc Orr <marcorr@...gle.com>,
David Rientjes <rientjes@...gle.com>,
Ashish Kalra <Ashish.Kalra@....com>,
linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH V2] virt: Prevent IV reuse in SNP guest driver
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@...gle.com>
> Reported-by: Peter Gonda <pgonda@...gle.com>
> Reviewed-by: Dionna Glaze <dionnaglaze@...gle.com>
> Cc: Borislav Petkov <bp@...e.de>
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Cc: Michael Roth <michael.roth@....com>
> Cc: Haowen Bai <baihaowen@...zu.com>
> Cc: Yang Yingliang <yangyingliang@...wei.com>
> Cc: Marc Orr <marcorr@...gle.com>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Ashish Kalra <Ashish.Kalra@....com>
> Cc: linux-kernel@...r.kernel.org
> Cc: kvm@...r.kernel.org
> ---
> 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.
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);
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