linux-kernel - Re: [PATCH 04/12] KVM: SVM: Add support for KVM_SEV_RECEIVE

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Message-ID: <20200312003855.GA26448@ashkalra_ubuntu_server>
Date:   Thu, 12 Mar 2020 00:38:55 +0000
From:   Ashish Kalra <ashish.kalra@....com>
To:     Steve Rutherford <srutherford@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        David Rientjes <rientjes@...gle.com>, X86 ML <x86@...nel.org>,
        KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, brijesh.singh@....com
Subject: Re: [PATCH 04/12] KVM: SVM: Add support for KVM_SEV_RECEIVE_START
 command

On Mon, Mar 09, 2020 at 06:41:02PM -0700, Steve Rutherford wrote:
> > +static int sev_receive_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
> > +{
> > +       struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> > +       struct sev_data_receive_start *start;
> > +       struct kvm_sev_receive_start params;
> > +       int *error = &argp->error;
> > +       void *session_data;
> > +       void *pdh_data;
> > +       int ret;
> > +
> > +       if (!sev_guest(kvm))
> > +               return -ENOTTY;
> > +
> > +       /* Get parameter from the userspace */
> > +       if (copy_from_user(&params, (void __user *)(uintptr_t)argp->data,
> > +                       sizeof(struct kvm_sev_receive_start)))
> > +               return -EFAULT;
> > +
> > +       /* some sanity checks */
> > +       if (!params.pdh_uaddr || !params.pdh_len ||
> > +           !params.session_uaddr || !params.session_len)
> > +               return -EINVAL;
> > +
> > +       pdh_data = psp_copy_user_blob(params.pdh_uaddr, params.pdh_len);
> > +       if (IS_ERR(pdh_data))
> > +               return PTR_ERR(pdh_data);
> > +
> > +       session_data = psp_copy_user_blob(params.session_uaddr,
> > +                       params.session_len);
> > +       if (IS_ERR(session_data)) {
> > +               ret = PTR_ERR(session_data);
> > +               goto e_free_pdh;
> > +       }
> > +
> > +       ret = -ENOMEM;
> > +       start = kzalloc(sizeof(*start), GFP_KERNEL);
> > +       if (!start)
> > +               goto e_free_session;
> > +
> > +       start->handle = params.handle;
> > +       start->policy = params.policy;
> > +       start->pdh_cert_address = __psp_pa(pdh_data);
> > +       start->pdh_cert_len = params.pdh_len;
> > +       start->session_address = __psp_pa(session_data);
> > +       start->session_len = params.session_len;
> > +
> > +       /* create memory encryption context */
> 
> Set ret to a different value here, since otherwise this will look like -ENOMEM.

But, ret will be the value returned by __sev_issue_cmd(), so why will it
look like -ENOMEM ?

> 
> > +       ret = __sev_issue_cmd(argp->sev_fd, SEV_CMD_RECEIVE_START, start,
> > +                               error);
> > +       if (ret)
> > +               goto e_free;
> > +
> > +       /* Bind ASID to this guest */
> 
> Ideally, set ret to another distinct value, since the error spaces for
> these commands overlap, so you won't be sure which had the problem.
> You also wouldn't be sure if one succeeded and the other failed vs
> both failing.

Both commands "may" return the same error code as set by sev_do_cmd(), but
then we need that very specific error code, sev_do_cmd() can't return
different error codes for each command it is issuing ?

> 
> > +       ret = sev_bind_asid(kvm, start->handle, error);
> > +       if (ret)
> > +               goto e_free;
> > +

Thanks,
Ashish