From mboxrd@z Thu Jan 1 00:00:00 1970 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 lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 8D221C433EF for ; Fri, 4 Mar 2022 19:10:55 +0000 (UTC) Received: from localhost ([::1]:51236 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1nQDKI-0000jL-Bg for qemu-devel@archiver.kernel.org; Fri, 04 Mar 2022 14:10:54 -0500 Received: from eggs.gnu.org ([209.51.188.92]:53556) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nQDFD-0001Dq-Td for qemu-devel@nongnu.org; Fri, 04 Mar 2022 14:05:42 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:23845) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nQDFB-0001mx-78 for qemu-devel@nongnu.org; Fri, 04 Mar 2022 14:05:38 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1646420736; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:in-reply-to:in-reply-to: references:references; bh=QsEZdECL9CO/8Rbo/YAaICdmr+wHlhPaRNfeTYH5570=; b=hXCGYQEHI4hNqL3EAv7Tbv9MuscGx2+xFoli64l8UNoAt18QZy80S4hkiu2lxgIZAwzD0o MBuQ1eIozdFx4J9T5niVMWb0yIqSU3pB+DRUKLIDdTj5sEiUJpAiOBoGYW/VkEuNcP0kYl rg29uUj9WFlFNGHOcBRlJRw6L7YeBwY= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-648-OIWayi5wNH2UNMu7atr7PA-1; Fri, 04 Mar 2022 14:05:29 -0500 X-MC-Unique: OIWayi5wNH2UNMu7atr7PA-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 1138A1006AA7; Fri, 4 Mar 2022 19:05:29 +0000 (UTC) Received: from redhat.com (unknown [10.39.194.222]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 44F1110016F2; Fri, 4 Mar 2022 19:05:27 +0000 (UTC) Date: Fri, 4 Mar 2022 19:05:24 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Tyler Fanelli Subject: Re: [PATCH] i386/sev: Ensure attestation report length is valid before retrieving Message-ID: References: <20220304183930.502777-1-tfanelli@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20220304183930.502777-1-tfanelli@redhat.com> User-Agent: Mutt/2.1.5 (2021-12-30) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: pbonzini@redhat.com, mtosatti@redhat.com, qemu-devel@nongnu.org, kvm@vger.kernel.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Mar 04, 2022 at 01:39:32PM -0500, Tyler Fanelli wrote: > The length of the attestation report buffer is never checked to be > valid before allocation is made. If the length of the report is returned > to be 0, the buffer to retrieve the attestation report is allocated with > length 0 and passed to the kernel to fill with contents of the attestation > report. Leaving this unchecked is dangerous and could lead to undefined > behavior. I don't see the undefined behaviour risk here. The KVM_SEV_ATTESTATION_REPORT semantics indicate that the kernel will fill in a valid length if the buffer we provide is too small and we can re-call with that buffer. If the kernel tells us the buffer is 0 bytes, then it should be fine having a second call with length 0. If not, then the kernel is broken and we're doomed. The QEMU code looks like it would cope with a zero length, unless I'm mistaken, it'll just return a zero length attestation report. > Signed-off-by: Tyler Fanelli > --- > target/i386/sev.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/target/i386/sev.c b/target/i386/sev.c > index 025ff7a6f8..215acd7c6b 100644 > --- a/target/i386/sev.c > +++ b/target/i386/sev.c > @@ -616,6 +616,8 @@ static SevAttestationReport *sev_get_attestation_report(const char *mnonce, > return NULL; > } > > + input.len = 0; The declaration of 'input' already zero initializes. > /* Query the report length */ > ret = sev_ioctl(sev->sev_fd, KVM_SEV_GET_ATTESTATION_REPORT, > &input, &err); > @@ -626,6 +628,11 @@ static SevAttestationReport *sev_get_attestation_report(const char *mnonce, > ret, err, fw_error_to_str(err)); > return NULL; > } > + } else if (input.len <= 0) { It can't be less than 0 because 'len' is an unsigned integer. > + error_setg(errp, "SEV: Failed to query attestation report:" > + " length returned=%d", > + input.len); > + return NULL; > } Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|