linux-kernel - Re: [PATCH v7 01/16] virt: sev-guest: Use AES GCM crypto library

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Date: Thu, 25 Jan 2024 11:36:43 +0100
From: Borislav Petkov <bp@...en8.de>
To: Nikunj A Dadhania <nikunj@....com>
Cc: linux-kernel@...r.kernel.org, thomas.lendacky@....com, x86@...nel.org,
	kvm@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de,
	dave.hansen@...ux.intel.com, dionnaglaze@...gle.com,
	pgonda@...gle.com, seanjc@...gle.com, pbonzini@...hat.com
Subject: Re: [PATCH v7 01/16] virt: sev-guest: Use AES GCM crypto library

On Wed, Dec 20, 2023 at 08:43:43PM +0530, Nikunj A Dadhania wrote:
> @@ -307,11 +197,16 @@ 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);
> +	memcpy(iv, &resp_hdr->msg_seqno, sizeof(resp_hdr->msg_seqno));

sizeof(iv) != sizeof(resp_hdr->msg_seqno) and it fits now.

However, for protection against future bugs, this should be:

	memcpy(iv, &resp_hdr->msg_seqno, min(sizeof(iv), sizeof(resp_hdr->msg_seqno)));

> +	if (!aesgcm_decrypt(ctx, payload, resp->payload, resp_hdr->msg_sz,
> +			    &resp_hdr->algo, AAD_LEN, iv, resp_hdr->authtag))
> +		return -EBADMSG;
> +
> +	return 0;
>  }
>  
>  static int enc_payload(struct snp_guest_dev *snp_dev, u64 seqno, int version, u8 type,
> @@ -319,6 +214,8 @@ static int enc_payload(struct snp_guest_dev *snp_dev, u64 seqno, int version, u8
>  {
>  	struct snp_guest_msg *req = &snp_dev->secret_request;
>  	struct snp_guest_msg_hdr *hdr = &req->hdr;
> +	struct aesgcm_ctx *ctx = snp_dev->ctx;
> +	u8 iv[GCM_AES_IV_SIZE] = {};
>  
>  	memset(req, 0, sizeof(*req));
>  
> @@ -338,7 +235,14 @@ 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);
> +	if (WARN_ON((sz + ctx->authsize) > sizeof(req->payload)))
> +		return -EBADMSG;
> +
> +	memcpy(iv, &hdr->msg_seqno, sizeof(hdr->msg_seqno));

Ditto.

> +	aesgcm_encrypt(ctx, req->payload, payload, sz, &hdr->algo, AAD_LEN,
> +		       iv, hdr->authtag);
> +
> +	return 0;

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette