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

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Date:   Wed, 8 Sep 2021 16:44:37 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part1 v5 37/38] virt: sevguest: Add support to derive key



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