Re: [PATCH 01/12] KVM: SVM: Add KVM_SEV SEND_START command

[Steve Rutherford <srutherford@google.com>]

On Mon, Mar 9, 2020 at 2:28 PM Steve Rutherford <srutherford@google.com> wrote:
>
> On Wed, Feb 12, 2020 at 5:15 PM Ashish Kalra <Ashish.Kalra@amd.com> wrote:
> >
> > From: Brijesh Singh <brijesh.singh@amd.com>
> >
> > The command is used to create an outgoing SEV guest encryption context.
> >
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > Cc: "Radim Krčmář" <rkrcmar@redhat.com>
> > Cc: Joerg Roedel <joro@8bytes.org>
> > Cc: Borislav Petkov <bp@suse.de>
> > Cc: Tom Lendacky <thomas.lendacky@amd.com>
> > Cc: x86@kernel.org
> > Cc: kvm@vger.kernel.org
> > Cc: linux-kernel@vger.kernel.org
> > Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> > Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
> > ---
> >  .../virt/kvm/amd-memory-encryption.rst        |  27 ++++
> >  arch/x86/kvm/svm.c                            | 125 ++++++++++++++++++
> >  include/linux/psp-sev.h                       |   8 +-
> >  include/uapi/linux/kvm.h                      |  12 ++
> >  4 files changed, 168 insertions(+), 4 deletions(-)
> >
> > diff --git a/Documentation/virt/kvm/amd-memory-encryption.rst b/Documentation/virt/kvm/amd-memory-encryption.rst
> > index d18c97b4e140..826911f41f3b 100644
> > --- a/Documentation/virt/kvm/amd-memory-encryption.rst
> > +++ b/Documentation/virt/kvm/amd-memory-encryption.rst
> > @@ -238,6 +238,33 @@ Returns: 0 on success, -negative on error
> >                  __u32 trans_len;
> >          };
> >
> > +10. KVM_SEV_SEND_START
> > +----------------------
> > +
> > +The KVM_SEV_SEND_START command can be used by the hypervisor to create an
> > +outgoing guest encryption context.
> > +
> > +Parameters (in): struct kvm_sev_send_start
> > +
> > +Returns: 0 on success, -negative on error
> > +
> > +::
> > +        struct kvm_sev_send_start {
> > +                __u32 policy;                 /* guest policy */
> > +
> > +                __u64 pdh_cert_uaddr;         /* platform Diffie-Hellman certificate */
> > +                __u32 pdh_cert_len;
> > +
> > +                __u64 plat_certs_uadr;        /* platform certificate chain */
> > +                __u32 plat_certs_len;
> > +
> > +                __u64 amd_certs_uaddr;        /* AMD certificate */
> > +                __u32 amd_cert_len;
> > +
> > +                __u64 session_uaddr;          /* Guest session information */
> > +                __u32 session_len;
> > +        };
> > +
> >  References
> >  ==========
> >
> > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > index a3e32d61d60c..3a7e2cac51de 100644
> > --- a/arch/x86/kvm/svm.c
> > +++ b/arch/x86/kvm/svm.c
> > @@ -7140,6 +7140,128 @@ static int sev_launch_secret(struct kvm *kvm, struct kvm_sev_cmd *argp)
> >         return ret;
> >  }
> >
> > +/* Userspace wants to query session length. */
> > +static int
> > +__sev_send_start_query_session_length(struct kvm *kvm, struct kvm_sev_cmd *argp,
> > +                                     struct kvm_sev_send_start *params)
> > +{
> > +       struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> > +       struct sev_data_send_start *data;
> > +       int ret;
> > +
> > +       data = kzalloc(sizeof(*data), GFP_KERNEL_ACCOUNT);
> > +       if (data == NULL)
> > +               return -ENOMEM;
> > +
> > +       data->handle = sev->handle;
> > +       ret = sev_issue_cmd(kvm, SEV_CMD_SEND_START, data, &argp->error);
> > +
> > +       params->session_len = data->session_len;
> > +       if (copy_to_user((void __user *)(uintptr_t)argp->data, params,
> > +                               sizeof(struct kvm_sev_send_start)))
> > +               ret = -EFAULT;
> > +
> > +       kfree(data);
> > +       return ret;
> > +}
> > +
> > +static int sev_send_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
> > +{
> > +       struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> > +       struct sev_data_send_start *data;
> > +       struct kvm_sev_send_start params;
> > +       void *amd_certs, *session_data;
> > +       void *pdh_cert, *plat_certs;
> > +       int ret;
> > +
> > +       if (!sev_guest(kvm))
> > +               return -ENOTTY;
> > +
> > +       if (copy_from_user(&params, (void __user *)(uintptr_t)argp->data,
> > +                               sizeof(struct kvm_sev_send_start)))
> > +               return -EFAULT;
> > +
> > +       /* if session_len is zero, userspace wants t query the session length */
>
> /t/to/
> >
> > +       if (!params.session_len)
> > +               return __sev_send_start_query_session_length(kvm, argp,
> > +                               &params);
> Document this behavior with the command.
>
> > +
> > +       /* some sanity checks */
> > +       if (!params.pdh_cert_uaddr || !params.pdh_cert_len ||
> > +           !params.session_uaddr || params.session_len > SEV_FW_BLOB_MAX_SIZE)
> > +               return -EINVAL;
> > +
> > +       /* allocate the memory to hold the session data blob */
> > +       session_data = kmalloc(params.session_len, GFP_KERNEL_ACCOUNT);
> > +       if (!session_data)
> > +               return -ENOMEM;
> > +
> > +       /* copy the certificate blobs from userspace */
> > +       pdh_cert = psp_copy_user_blob(params.pdh_cert_uaddr,
> > +                               params.pdh_cert_len);
> > +       if (IS_ERR(pdh_cert)) {
> > +               ret = PTR_ERR(pdh_cert);
> > +               goto e_free_session;
> > +       }
> > +
> > +       plat_certs = psp_copy_user_blob(params.plat_certs_uaddr,
> > +                               params.plat_certs_len);
> > +       if (IS_ERR(plat_certs)) {
> > +               ret = PTR_ERR(plat_certs);
> > +               goto e_free_pdh;
> > +       }
> > +
> > +       amd_certs = psp_copy_user_blob(params.amd_certs_uaddr,
> > +                               params.amd_certs_len);
> > +       if (IS_ERR(amd_certs)) {
> > +               ret = PTR_ERR(amd_certs);
> > +               goto e_free_plat_cert;
> > +       }
> > +
> > +       data = kzalloc(sizeof(*data), GFP_KERNEL_ACCOUNT);
> > +       if (data == NULL) {
> > +               ret = -ENOMEM;
> > +               goto e_free_amd_cert;
> > +       }
> > +
> > +       /* populate the FW SEND_START field with system physical address */
> > +       data->pdh_cert_address = __psp_pa(pdh_cert);
> > +       data->pdh_cert_len = params.pdh_cert_len;
> > +       data->plat_certs_address = __psp_pa(plat_certs);
> > +       data->plat_certs_len = params.plat_certs_len;
> > +       data->amd_certs_address = __psp_pa(amd_certs);
> > +       data->amd_certs_len = params.amd_certs_len;
> > +       data->session_address = __psp_pa(session_data);
> > +       data->session_len = params.session_len;
> > +       data->handle = sev->handle;
> > +
> > +       ret = sev_issue_cmd(kvm, SEV_CMD_SEND_START, data, &argp->error);
> sev_issue_cmd can fail. I think you want to handle those errors here
> (e.g. it can return -ebadf or a number of others). Right now they
> could get clobbered by a later copy_to_user error.
>
> It's also worth documenting what the error argp->error is filled in
> with. I didn't see anything in the docs mentioning the status codes
> (may have missed it).
>
> > +
> > +       if (copy_to_user((void __user *)(uintptr_t) params.session_uaddr,
> > +                       session_data, params.session_len)) {
> > +               ret = -EFAULT;
> > +               goto e_free;
> > +       }

One additional aspect, which also comes up for other commands, is that
it's not clear if the command succeeded if you get back -EFAULT. Any
of the copy_to_users could have failed, on either side of the
sev_issue_cmd call.

If userspace filled in the error with a reserved value (e.g.
0xFFFF0000, which is larger than the largest possible error code), it
could observe that that value was clobbered and infer that
sev_issue_cmd succeeded/failed/etc. This is particularly error prone
since the success code is zero, which is almost certainly what people
will initialize the error field as, unless they go out of their way.

I think the cleanest answer would be to write in a reserved value to
the error at the start of sev_send_* and have sev_issue_command
clobber that value with the expected value. This way, userspace can
know which GSTATE the guest transitioned to, even if it sees -EFAULT.
> > +
> > +       params.policy = data->policy;
> > +       params.session_len = data->session_len;
> > +       if (copy_to_user((void __user *)(uintptr_t)argp->data, &params,
> > +                               sizeof(struct kvm_sev_send_start)))
> > +               ret = -EFAULT;
> > +
> > +e_free:
> > +       kfree(data);
> > +e_free_amd_cert:
> > +       kfree(amd_certs);
> > +e_free_plat_cert:
> > +       kfree(plat_certs);
> > +e_free_pdh:
> > +       kfree(pdh_cert);
> > +e_free_session:
> > +       kfree(session_data);
> > +       return ret;
> > +}
> > +
> >  static int svm_mem_enc_op(struct kvm *kvm, void __user *argp)
> >  {
> >         struct kvm_sev_cmd sev_cmd;
> > @@ -7181,6 +7303,9 @@ static int svm_mem_enc_op(struct kvm *kvm, void __user *argp)
> >         case KVM_SEV_LAUNCH_SECRET:
> >                 r = sev_launch_secret(kvm, &sev_cmd);
> >                 break;
> > +       case KVM_SEV_SEND_START:
> > +               r = sev_send_start(kvm, &sev_cmd);
> > +               break;
> >         default:
> >                 r = -EINVAL;
> >                 goto out;
> > diff --git a/include/linux/psp-sev.h b/include/linux/psp-sev.h
> > index 5167bf2bfc75..9f63b9d48b63 100644
> > --- a/include/linux/psp-sev.h
> > +++ b/include/linux/psp-sev.h
> > @@ -323,11 +323,11 @@ struct sev_data_send_start {
> >         u64 pdh_cert_address;                   /* In */
> >         u32 pdh_cert_len;                       /* In */
> >         u32 reserved1;
> > -       u64 plat_cert_address;                  /* In */
> > -       u32 plat_cert_len;                      /* In */
> > +       u64 plat_certs_address;                 /* In */
> > +       u32 plat_certs_len;                     /* In */
> >         u32 reserved2;
> > -       u64 amd_cert_address;                   /* In */
> > -       u32 amd_cert_len;                       /* In */
> > +       u64 amd_certs_address;                  /* In */
> > +       u32 amd_certs_len;                      /* In */
> >         u32 reserved3;
> >         u64 session_address;                    /* In */
> >         u32 session_len;                        /* In/Out */
> > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > index 4b95f9a31a2f..17bef4c245e1 100644
> > --- a/include/uapi/linux/kvm.h
> > +++ b/include/uapi/linux/kvm.h
> > @@ -1558,6 +1558,18 @@ struct kvm_sev_dbg {
> >         __u32 len;
> >  };
> >
> > +struct kvm_sev_send_start {
> > +       __u32 policy;
> > +       __u64 pdh_cert_uaddr;
> > +       __u32 pdh_cert_len;
> > +       __u64 plat_certs_uaddr;
> > +       __u32 plat_certs_len;
> > +       __u64 amd_certs_uaddr;
> > +       __u32 amd_certs_len;
> > +       __u64 session_uaddr;
> > +       __u32 session_len;
> > +};
> > +
> >  #define KVM_DEV_ASSIGN_ENABLE_IOMMU    (1 << 0)
> >  #define KVM_DEV_ASSIGN_PCI_2_3         (1 << 1)
> >  #define KVM_DEV_ASSIGN_MASK_INTX       (1 << 2)
> > --
> > 2.17.1
> >
>
> Looks pretty reasonable overall.