Re: [PATCH Part1 v5 37/38] virt: sevguest: Add support to derive key

[Brijesh Singh]

On 9/8/21 9:00 AM, Borislav Petkov wrote:
> On Fri, Aug 20, 2021 at 10:19:32AM -0500, Brijesh Singh wrote:
> > +2.2 SNP_GET_DERIVED_KEY
> > +-----------------------
> > +:Technology: sev-snp
> > +:Type: guest ioctl
> > +:Parameters (in): struct snp_derived_key_req
> > +:Returns (out): struct snp_derived_key_req on success, -negative on error
> > +
> > +The SNP_GET_DERIVED_KEY ioctl can be used to get a key derive from a root key.
> > +The derived key can be used by the guest for any purpose, such as sealing keys
> > +or communicating with external entities.
> > +
> > +The ioctl uses the SNP_GUEST_REQUEST (MSG_KEY_REQ) command provided by the
> > +SEV-SNP firmware to derive the key. See SEV-SNP specification for further details
> > +on the various fileds passed in the key derivation request.
> > +
> > +On success, the snp_derived_key_resp.data will contains the derived key
>
> "will contain"

Noted.

> > +
> > +	/* Copy the request payload from the userspace */
>
> "from userspace"

Noted.


> > +
> >   static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long arg)
> >   {
> >   	struct snp_guest_dev *snp_dev = to_snp_dev(file);
> > @@ -320,6 +364,10 @@ static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long
> >   		ret = get_report(snp_dev, &input);
> >   		break;
> >   	}
> > +	case SNP_GET_DERIVED_KEY: {
> > +		ret = get_derived_key(snp_dev, &input);
> > +		break;
> > +	}
>
> {} brackets are not needed.
>
> What, however, is bothering me more in this function is that you call
> the respective ioctl function which might fail, you do not look at the
> return value and copy_to_user() unconditionally.
>
> Looking at get_derived_key(), for example, if it returns after:
>
>          if (!arg->req_data || !arg->resp_data)
>                  return -EINVAL;
>
> you will be copying the same thing back to the user, you copied in
> earlier. That doesn't make any sense to me.

I will look into improving it to copy back to userspace only if there is 
firmware error.

thanks