Re: [PATCH v5 01/14] virt: sev-guest: Use AES GCM crypto library

Tom Lendacky
2023-10-30T17:51:50Z

On 10/30/23 01:36, Nikunj A Dadhania wrote:
> The sev-guest driver encryption code uses Crypto API for SNP guest
> messaging to interact with AMD Security processor. For enabling SecureTSC,
> SEV-SNP guests need to send a TSC_INFO request guest message before the
> smpboot phase starts. Details from the TSC_INFO response will be used to
> program the VMSA before the secondary CPUs are brought up. The Crypto API
> is not available this early in the boot phase.
>
> In preparation of moving the encryption code out of sev-guest driver to
> support SecureTSC and make reviewing the diff easier, start using AES GCM
> library implementation instead of Crypto API.
>
> CC: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
> Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

I just a few nit comments that might be nice to cover if you have to do a
v6...

> ---
> drivers/virt/coco/sev-guest/Kconfig | 4 +-
> drivers/virt/coco/sev-guest/sev-guest.c | 163 ++++++------------------
> drivers/virt/coco/sev-guest/sev-guest.h | 3 +
> 3 files changed, 44 insertions(+), 126 deletions(-)
>
> diff --git a/drivers/virt/coco/sev-guest/Kconfig b/drivers/virt/coco/sev-guest/Kconfig
> index da2d7ca531f0..bcc760bfb468 100644
> --- a/drivers/virt/coco/sev-guest/Kconfig
> +++ b/drivers/virt/coco/sev-guest/Kconfig
> @@ -2,9 +2,7 @@ config SEV_GUEST
> tristate "AMD SEV Guest driver"
> default m
> depends on AMD_MEM_ENCRYPT
> - select CRYPTO
> - select CRYPTO_AEAD2
> - select CRYPTO_GCM
> + select CRYPTO_LIB_AESGCM
> help
> SEV-SNP firmware provides the guest a mechanism to communicate with
> the PSP without risk from a malicious hypervisor who wishes to read,
> diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
> index 97dbe715e96a..68044c436866 100644
> --- a/drivers/virt/coco/sev-guest/sev-guest.c
> +++ b/drivers/virt/coco/sev-guest/sev-guest.c
> @@ -16,8 +16,7 @@
> #include <linux/miscdevice.h>
> #include <linux/set_memory.h>
> #include <linux/fs.h>
> -#include <crypto/aead.h>
> -#include <linux/scatterlist.h>
> +#include <crypto/gcm.h>
> #include <linux/psp-sev.h>
> #include <uapi/linux/sev-guest.h>
> #include <uapi/linux/psp-sev.h>
> @@ -28,24 +27,16 @@
> #include "sev-guest.h"
>
> #define DEVICE_NAME "sev-guest"
> -#define AAD_LEN 48
> -#define MSG_HDR_VER 1
>
> #define SNP_REQ_MAX_RETRY_DURATION (60*HZ)
> #define SNP_REQ_RETRY_DELAY (2*HZ)
>
> -struct snp_guest_crypto {
> - struct crypto_aead *tfm;
> - u8 *iv, *authtag;
> - int iv_len, a_len;
> -};
> -
> struct snp_guest_dev {
> struct device *dev;
> struct miscdevice misc;
>
> void *certs_data;
> - struct snp_guest_crypto *crypto;
> + struct aesgcm_ctx *ctx;
> /* request and response are in unencrypted memory */
> struct snp_guest_msg *request, *response;
>
> @@ -152,132 +143,59 @@ static inline struct snp_guest_dev *to_snp_dev(struct file *file)
> return container_of(dev, struct snp_guest_dev, misc);
> }
>
> -static struct snp_guest_crypto *init_crypto(struct snp_guest_dev *snp_dev, u8 *key, size_t keylen)
> +static struct aesgcm_ctx *snp_init_crypto(u8 *key, size_t keylen)
> {
> - struct snp_guest_crypto *crypto;
> + struct aesgcm_ctx *ctx;
>
> - crypto = kzalloc(sizeof(*crypto), GFP_KERNEL_ACCOUNT);
> - if (!crypto)
> + ctx = kzalloc(sizeof(*ctx), GFP_KERNEL_ACCOUNT);
> + if (!ctx)
> return NULL;
>
> - crypto->tfm = crypto_alloc_aead("gcm(aes)", 0, 0);
> - if (IS_ERR(crypto->tfm))
> - goto e_free;
> -
> - if (crypto_aead_setkey(crypto->tfm, key, keylen))
> - goto e_free_crypto;
> -
> - crypto->iv_len = crypto_aead_ivsize(crypto->tfm);
> - crypto->iv = kmalloc(crypto->iv_len, GFP_KERNEL_ACCOUNT);
> - if (!crypto->iv)
> - goto e_free_crypto;
> -
> - if (crypto_aead_authsize(crypto->tfm) > MAX_AUTHTAG_LEN) {
> - if (crypto_aead_setauthsize(crypto->tfm, MAX_AUTHTAG_LEN)) {
> - dev_err(snp_dev->dev, "failed to set authsize to %d\n", MAX_AUTHTAG_LEN);
> - goto e_free_iv;
> - }
> + if (aesgcm_expandkey(ctx, key, keylen, AUTHTAG_LEN)) {
> + pr_err("SNP: crypto init failed\n");
> + kfree(ctx);
> + return NULL;
> }
>
> - crypto->a_len = crypto_aead_authsize(crypto->tfm);
> - crypto->authtag = kmalloc(crypto->a_len, GFP_KERNEL_ACCOUNT);
> - if (!crypto->authtag)
> - goto e_free_iv;
> -
> - return crypto;
> -
> -e_free_iv:
> - kfree(crypto->iv);
> -e_free_crypto:
> - crypto_free_aead(crypto->tfm);
> -e_free:
> - kfree(crypto);
> -
> - return NULL;
> -}
> -
> -static void deinit_crypto(struct snp_guest_crypto *crypto)
> -{
> - crypto_free_aead(crypto->tfm);
> - kfree(crypto->iv);
> - kfree(crypto->authtag);
> - kfree(crypto);
> -}
> -
> -static int enc_dec_message(struct snp_guest_crypto *crypto, struct snp_guest_msg *msg,
> - u8 *src_buf, u8 *dst_buf, size_t len, bool enc)
> -{
> - struct snp_guest_msg_hdr *hdr = &msg->hdr;
> - struct scatterlist src[3], dst[3];
> - DECLARE_CRYPTO_WAIT(wait);
> - struct aead_request *req;
> - int ret;
> -
> - req = aead_request_alloc(crypto->tfm, GFP_KERNEL);
> - if (!req)
> - return -ENOMEM;
> -
> - /*
> - * AEAD memory operations:
> - * +------ AAD -------+------- DATA -----+---- AUTHTAG----+
> - * | msg header | plaintext | hdr->authtag |
> - * | bytes 30h - 5Fh | or | |
> - * | | cipher | |
> - * +------------------+------------------+----------------+
> - */
> - sg_init_table(src, 3);
> - sg_set_buf(&src[0], &hdr->algo, AAD_LEN);
> - sg_set_buf(&src[1], src_buf, hdr->msg_sz);
> - sg_set_buf(&src[2], hdr->authtag, crypto->a_len);
> -
> - sg_init_table(dst, 3);
> - sg_set_buf(&dst[0], &hdr->algo, AAD_LEN);
> - sg_set_buf(&dst[1], dst_buf, hdr->msg_sz);
> - sg_set_buf(&dst[2], hdr->authtag, crypto->a_len);
> -
> - aead_request_set_ad(req, AAD_LEN);
> - aead_request_set_tfm(req, crypto->tfm);
> - aead_request_set_callback(req, 0, crypto_req_done, &wait);
> -
> - aead_request_set_crypt(req, src, dst, len, crypto->iv);
> - ret = crypto_wait_req(enc ? crypto_aead_encrypt(req) : crypto_aead_decrypt(req), &wait);
> -
> - aead_request_free(req);
> - return ret;
> + return ctx;
> }
>
> -static int __enc_payload(struct snp_guest_dev *snp_dev, struct snp_guest_msg *msg,
> +static int __enc_payload(struct aesgcm_ctx *ctx, struct snp_guest_msg *msg,
> void *plaintext, size_t len)
> {
> - struct snp_guest_crypto *crypto = snp_dev->crypto;
> struct snp_guest_msg_hdr *hdr = &msg->hdr;
> + u8 iv[GCM_AES_IV_SIZE] = {};
>
> - memset(crypto->iv, 0, crypto->iv_len);
> - memcpy(crypto->iv, &hdr->msg_seqno, sizeof(hdr->msg_seqno));
> + if (WARN_ON((hdr->msg_sz + ctx->authsize) > sizeof(msg->payload)))
> + return -EBADMSG;
>
> - return enc_dec_message(crypto, msg, plaintext, msg->payload, len, true);
> + memcpy(iv, &hdr->msg_seqno, sizeof(hdr->msg_seqno));
> + aesgcm_encrypt(ctx, msg->payload, plaintext, len, &hdr->algo, AAD_LEN,
> + iv, hdr->authtag);
> + return 0;
> }

__enc_payload() is pretty small now and can probably just be part of the
only function that calls it, enc_payload().

>
> -static int dec_payload(struct snp_guest_dev *snp_dev, struct snp_guest_msg *msg,
> +static int dec_payload(struct aesgcm_ctx *ctx, struct snp_guest_msg *msg,
> void *plaintext, size_t len)
> {
> - struct snp_guest_crypto *crypto = snp_dev->crypto;
> struct snp_guest_msg_hdr *hdr = &msg->hdr;
> + u8 iv[GCM_AES_IV_SIZE] = {};
>
> - /* Build IV with response buffer sequence number */
> - memset(crypto->iv, 0, crypto->iv_len);
> - memcpy(crypto->iv, &hdr->msg_seqno, sizeof(hdr->msg_seqno));
> -
> - return enc_dec_message(crypto, msg, msg->payload, plaintext, len, false);
> + memcpy(iv, &hdr->msg_seqno, sizeof(hdr->msg_seqno));
> + if (aesgcm_decrypt(ctx, plaintext, msg->payload, len, &hdr->algo,
> + AAD_LEN, iv, hdr->authtag))
> + return 0;
> + else
> + return -EBADMSG;

This would look cleaner / read easier to me to have as:

if (!aesgcm_decrypt(...))
return -EBADMSG;

return 0;

But just my opinion.

And ditto here on the size now, can probably just be part of
verify_and_dec_payload() now.

Thanks,
Tom

> }
>
> static int verify_and_dec_payload(struct snp_guest_dev *snp_dev, void *payload, u32 sz)
> {
> - struct snp_guest_crypto *crypto = snp_dev->crypto;
> struct snp_guest_msg *resp = &snp_dev->secret_response;
> struct snp_guest_msg *req = &snp_dev->secret_request;
> struct snp_guest_msg_hdr *req_hdr = &req->hdr;
> struct snp_guest_msg_hdr *resp_hdr = &resp->hdr;
> + struct aesgcm_ctx *ctx = snp_dev->ctx;
>
> dev_dbg(snp_dev->dev, "response [seqno %lld type %d version %d sz %d]\n",
> resp_hdr->msg_seqno, resp_hdr->msg_type, resp_hdr->msg_version, resp_hdr->msg_sz);
> @@ -298,11 +216,11 @@ static int verify_and_dec_payload(struct snp_guest_dev *snp_dev, void *payload,
> * If the message size is greater than our buffer length then return
> * an error.
> */
> - if (unlikely((resp_hdr->msg_sz + crypto->a_len) > sz))
> + if (unlikely((resp_hdr->msg_sz + ctx->authsize) > sz))
> return -EBADMSG;
>
> /* Decrypt the payload */
> - return dec_payload(snp_dev, resp, payload, resp_hdr->msg_sz + crypto->a_len);
> + return dec_payload(ctx, resp, payload, resp_hdr->msg_sz);
> }
>
> static int enc_payload(struct snp_guest_dev *snp_dev, u64 seqno, int version, u8 type,
> @@ -329,7 +247,7 @@ static int enc_payload(struct snp_guest_dev *snp_dev, u64 seqno, int version, u8
> dev_dbg(snp_dev->dev, "request [seqno %lld type %d version %d sz %d]\n",
> hdr->msg_seqno, hdr->msg_type, hdr->msg_version, hdr->msg_sz);
>
> - return __enc_payload(snp_dev, req, payload, sz);
> + return __enc_payload(snp_dev->ctx, req, payload, sz);
> }
>
> static int __handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code,
> @@ -472,7 +390,6 @@ static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code,
>
> static int get_report(struct snp_guest_dev *snp_dev, struct snp_guest_request_ioctl *arg)
> {
> - struct snp_guest_crypto *crypto = snp_dev->crypto;
> struct snp_report_resp *resp;
> struct snp_report_req req;
> int rc, resp_len;
> @@ -490,7 +407,7 @@ static int get_report(struct snp_guest_dev *snp_dev, struct snp_guest_request_io
> * response payload. Make sure that it has enough space to cover the
> * authtag.
> */
> - resp_len = sizeof(resp->data) + crypto->a_len;
> + resp_len = sizeof(resp->data) + snp_dev->ctx->authsize;
> resp = kzalloc(resp_len, GFP_KERNEL_ACCOUNT);
> if (!resp)
> return -ENOMEM;
> @@ -511,7 +428,6 @@ static int get_report(struct snp_guest_dev *snp_dev, struct snp_guest_request_io
>
> static int get_derived_key(struct snp_guest_dev *snp_dev, struct snp_guest_request_ioctl *arg)
> {
> - struct snp_guest_crypto *crypto = snp_dev->crypto;
> struct snp_derived_key_resp resp = {0};
> struct snp_derived_key_req req;
> int rc, resp_len;
> @@ -528,7 +444,7 @@ static int get_derived_key(struct snp_guest_dev *snp_dev, struct snp_guest_reque
> * response payload. Make sure that it has enough space to cover the
> * authtag.
> */
> - resp_len = sizeof(resp.data) + crypto->a_len;
> + resp_len = sizeof(resp.data) + snp_dev->ctx->authsize;
> if (sizeof(buf) < resp_len)
> return -ENOMEM;
>
> @@ -552,7 +468,6 @@ static int get_derived_key(struct snp_guest_dev *snp_dev, struct snp_guest_reque
>
> static int get_ext_report(struct snp_guest_dev *snp_dev, struct snp_guest_request_ioctl *arg)
> {
> - 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;
> @@ -590,7 +505,7 @@ static int get_ext_report(struct snp_guest_dev *snp_dev, struct snp_guest_reques
> * response payload. Make sure that it has enough space to cover the
> * authtag.
> */
> - resp_len = sizeof(resp->data) + crypto->a_len;
> + resp_len = sizeof(resp->data) + snp_dev->ctx->authsize;
> resp = kzalloc(resp_len, GFP_KERNEL_ACCOUNT);
> if (!resp)
> return -ENOMEM;
> @@ -802,8 +717,8 @@ static int __init sev_guest_probe(struct platform_device *pdev)
> goto e_free_response;
>
> ret = -EIO;
> - snp_dev->crypto = init_crypto(snp_dev, snp_dev->vmpck, VMPCK_KEY_LEN);
> - if (!snp_dev->crypto)
> + snp_dev->ctx = snp_init_crypto(snp_dev->vmpck, VMPCK_KEY_LEN);
> + if (!snp_dev->ctx)
> goto e_free_cert_data;
>
> misc = &snp_dev->misc;
> @@ -818,11 +733,13 @@ static int __init sev_guest_probe(struct platform_device *pdev)
>
> ret = misc_register(misc);
> if (ret)
> - goto e_free_cert_data;
> + goto e_free_ctx;
>
> dev_info(dev, "Initialized SEV guest driver (using vmpck_id %d)\n", vmpck_id);
> return 0;
>
> +e_free_ctx:
> + kfree(snp_dev->ctx);
> e_free_cert_data:
> free_shared_pages(snp_dev->certs_data, SEV_FW_BLOB_MAX_SIZE);
> e_free_response:
> @@ -841,7 +758,7 @@ static int __exit sev_guest_remove(struct platform_device *pdev)
> free_shared_pages(snp_dev->certs_data, SEV_FW_BLOB_MAX_SIZE);
> 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);
> + kfree(snp_dev->ctx);
> misc_deregister(&snp_dev->misc);
>
> return 0;
> diff --git a/drivers/virt/coco/sev-guest/sev-guest.h b/drivers/virt/coco/sev-guest/sev-guest.h
> index 21bda26fdb95..ceb798a404d6 100644
> --- a/drivers/virt/coco/sev-guest/sev-guest.h
> +++ b/drivers/virt/coco/sev-guest/sev-guest.h
> @@ -13,6 +13,9 @@
> #include <linux/types.h>
>
> #define MAX_AUTHTAG_LEN 32
> +#define AUTHTAG_LEN 16
> +#define AAD_LEN 48
> +#define MSG_HDR_VER 1
>
> /* See SNP spec SNP_GUEST_REQUEST section for the structure */
> enum msg_type {