From nobody Thu Apr 9 07:43:15 2026 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id C3EE2C43217 for ; Wed, 16 Nov 2022 17:56:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232974AbiKPR4W (ORCPT ); Wed, 16 Nov 2022 12:56:22 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38410 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239012AbiKPR4K (ORCPT ); Wed, 16 Nov 2022 12:56:10 -0500 Received: from mail-pj1-x1049.google.com (mail-pj1-x1049.google.com [IPv6:2607:f8b0:4864:20::1049]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1BA5262383 for ; Wed, 16 Nov 2022 09:56:07 -0800 (PST) Received: by mail-pj1-x1049.google.com with SMTP id x14-20020a17090a2b0e00b002134b1401ddso1686865pjc.8 for ; Wed, 16 Nov 2022 09:56:07 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=cc:to:from:subject:mime-version:message-id:date:from:to:cc:subject :date:message-id:reply-to; bh=KlfWE2cqIDu+7jwt9BbCyjCmf2/9BDPosXGyqPGOT/M=; b=YqHanX6E4mgbT+antD03PMvEdQAruUauwGv55O3VagwqEnQWmlXKqePkHqf+JD8EFO Zm3/kbyQd5sHKHSwikWlTewuUH5ZcduUQspOux4DbRqVApmJwCSDChNeqEjCf2kXZpA3 qIREzJmILSklQWgbeteJSpj6AtqOMLnHdX5NKDUv5YNi6gBWNXyx6hfwrQRKopgJWqTu iNeARYeKKHi87HMZNYpECxwJvpSY1sJV5GNNwKlwwxJ1F1ujji9+0EQY8fddZZLzgIZ4 vD8uvlM39B11eol6MO87tSS4ZFEaxziITSPdX8nWko60CqELWyTQTSIyD7cqpTIWjOLa 0kEw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:from:subject:mime-version:message-id:date:x-gm-message-state :from:to:cc:subject:date:message-id:reply-to; bh=KlfWE2cqIDu+7jwt9BbCyjCmf2/9BDPosXGyqPGOT/M=; b=syH1x5IeXgtvA6YYU7K+HD7eSVn8eZ4V12aeawgD0dymZ9E2bfO/lrBySMBTGcsT79 JNG9vDWH9s+Kdumfc/goN3R5zZiZosboEnySRB6697DiIn3gAq3iTeUYSwmw079xY/uv 0Rdq1WNNqR4B84z2VDm+t8FNYPXLA0yxbDsm46yRBhbJVHe07yfC+yx5uLUKFov4KrLd uQlV8azuA256z+Q0O5vtZN0XrLRxWoNNgNaSrVsPfG6tpZ9lzuJsLV5GntVfy0e88XJu dMPhMZJl8ew7ybxsgGRk47RTkW7TAv4sY2JWrrNUoktYl9Kyy2b6VdliEUsZ9Jj+J4Fx S3BA== X-Gm-Message-State: ANoB5pk20Akl92SlHnexo+MS1RJEU2AdhLxL7zpohmLZppQD+YhD8W2t eOuZtGdzZiNoSVVGEGPYBmvFQ7C0iic= X-Google-Smtp-Source: AA0mqf5dF4oJxqtS3V6FHFqeA6L+G7+fa9gyTELuMQYaN++HsvSIMV5L1/3KQoZyTXyg8m9g/KDEdLC31Wo= X-Received: from pgonda1.kir.corp.google.com ([2620:0:1008:11:e28:e984:f232:1178]) (user=pgonda job=sendgmr) by 2002:a05:6a00:15d2:b0:563:1231:1da with SMTP id o18-20020a056a0015d200b00563123101damr24728098pfu.5.1668621366488; Wed, 16 Nov 2022 09:56:06 -0800 (PST) Date: Wed, 16 Nov 2022 09:55:58 -0800 Message-Id: <20221116175558.2373112-1-pgonda@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.38.1.493.g58b659f92b-goog Subject: [PATCH V5] virt: sev: Prevent IV reuse in SNP guest driver From: Peter Gonda To: thomas.lendacky@amd.com Cc: Peter Gonda , Borislav Petkov , Michael Roth , Haowen Bai , Yang Yingliang , Marc Orr , David Rientjes , Dionna Glaze , Ashish Kalra , stable@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" The AMD Secure Processor (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: "Authentication Failures in NIST version of GCM" - Antoine Joux et al. In order to address this make handle_guest_request() delete the VMPCK on any non successful return. To allow userspace querying the cert_data length make handle_guest_request() safe the number of pages required by the host, then handle_guest_request() retry the request without requesting the extended data, then return the number of pages required back to userspace. Fixes: fce96cf044308 ("virt: Add SEV-SNP guest driver") Signed-off-by: Peter Gonda Reported-by: Peter Gonda Cc: Borislav Petkov Cc: Tom Lendacky Cc: Michael Roth Cc: Haowen Bai Cc: Yang Yingliang Cc: Marc Orr Cc: David Rientjes Cc: Dionna Glaze Cc: Ashish Kalra Cc: stable@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: kvm@vger.kernel.org Reviewed-by: Tom Lendacky --- drivers/virt/coco/sev-guest/sev-guest.c | 83 ++++++++++++++++++++----- 1 file changed, 69 insertions(+), 14 deletions(-) diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/se= v-guest/sev-guest.c index f422f9c58ba79..64b4234c14da8 100644 --- a/drivers/virt/coco/sev-guest/sev-guest.c +++ b/drivers/virt/coco/sev-guest/sev-guest.c @@ -67,8 +67,27 @@ static bool is_vmpck_empty(struct snp_guest_dev *snp_dev) return true; } =20 +/* + * If an error is received from the host or AMD Secure Processor (ASP) the= re + * are two options. Either retry the exact same encrypted request or disco= ntinue + * 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 seque= nce + * number. + * + * So if the sequence number were to be reused the encryption scheme is + * vulnerable. If the sequence number were incremented for a fresh IV the = ASP + * will reject the 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 =3D NULL; } @@ -321,34 +340,70 @@ static int handle_guest_request(struct snp_guest_dev = *snp_dev, u64 exit_code, in if (rc) return rc; =20 - /* Call firmware to process the request */ + /* + * Call firmware to process the request. In this function the encrypted + * message enters shared memory with the host. So after this call the + * sequence number must be incremented or the VMPCK must be deleted to + * prevent reuse of the IV. + */ rc =3D snp_issue_guest_request(exit_code, &snp_dev->input, &err); + + /* + * If the extended guest request fails due to having too small of a + * certificate data buffer retry the same guest request without the + * extended data request in order to not have to reuse the IV. + */ + if (exit_code =3D=3D SVM_VMGEXIT_EXT_GUEST_REQUEST && + err =3D=3D SNP_GUEST_REQ_INVALID_LEN) { + const unsigned int certs_npages =3D snp_dev->input.data_npages; + + exit_code =3D SVM_VMGEXIT_GUEST_REQUEST; + + /* + * If this call to the firmware succeeds the sequence number can + * be incremented allowing for continued use of the VMPCK. If + * there is an error reflected in the return value, this value + * is checked further down and the result will be the deletion + * of the VMPCK and the error code being propagated back to the + * user as an IOCLT return code. + */ + rc =3D snp_issue_guest_request(exit_code, &snp_dev->input, &err); + + /* + * Override the error to inform callers the given extended + * request buffer size was too small and give the caller the + * required buffer size. + */ + err =3D SNP_GUEST_REQ_INVALID_LEN; + snp_dev->input.data_npages =3D certs_npages; + } + if (fw_err) *fw_err =3D err; =20 - 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; + } =20 - /* - * The verify_and_dec_payload() will fail only if the hypervisor is - * actively modifying the message header or corrupting the encrypted payl= oad. - * 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 e= nsure - * that AES-GCM does not use the same IV while encrypting the request pay= load. - */ rc =3D 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; } =20 /* Increment to new message sequence after payload decryption was success= ful. */ snp_inc_msg_seqno(snp_dev); =20 return 0; + +disable_vmpck: + snp_disable_vmpck(snp_dev); + return rc; } =20 static int get_report(struct snp_guest_dev *snp_dev, struct snp_guest_requ= est_ioctl *arg) --=20 2.38.1.493.g58b659f92b-goog