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Subject: Re: [PATCH Part1 v5 37/38] virt: sevguest: Add support to derive key
To:     Dov Murik <dovmurik@linux.ibm.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        linux-efi@vger.kernel.org, platform-driver-x86@vger.kernel.org,
        linux-coco@lists.linux.dev, linux-mm@kvack.org
References: <20210820151933.22401-1-brijesh.singh@amd.com>
 <20210820151933.22401-38-brijesh.singh@amd.com>
 <a6841be9-a2ca-8d92-3346-af8513b528fc@linux.ibm.com>
From:   Brijesh Singh <brijesh.singh@amd.com>
Message-ID: <fd9fadae-a493-1d8d-6777-e1c789a5113f@amd.com>
Date:   Tue, 31 Aug 2021 16:04:12 -0500
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Hi Dov,


On 8/31/21 1:59 PM, Dov Murik wrote:
>> +
>> +	/*
>> +	 * The intermediate response buffer is used while decrypting the
>> +	 * response payload. Make sure that it has enough space to cover the
>> +	 * authtag.
>> +	 */
>> +	resp_len = sizeof(resp->data) + crypto->a_len;
>> +	resp = kzalloc(resp_len, GFP_KERNEL_ACCOUNT);
> 
> The length of resp->data is 64 bytes; I assume crypto->a_len is not a
> lot more (and probably known in advance for AES GCM).  Maybe use a
> buffer on the stack instead of allocating and freeing?
> 

The authtag size can be up to 16 bytes, so I guess I can allocate 80 
bytes on stack and avoid the kzalloc().

> 
>> +	if (!resp)
>> +		return -ENOMEM;
>> +
>> +	/* Issue the command to get the attestation report */
>> +	rc = handle_guest_request(snp_dev, req.msg_version, SNP_MSG_KEY_REQ,
>> +				  &req.data, sizeof(req.data), resp->data, resp_len,
>> +				  &arg->fw_err);
>> +	if (rc)
>> +		goto e_free;
>> +
>> +	/* Copy the response payload to userspace */
>> +	if (copy_to_user((void __user *)arg->resp_data, resp, sizeof(*resp)))
>> +		rc = -EFAULT;
>> +
>> +e_free:
>> +	kfree(resp);
> 
> Since resp contains key material, I think you should explicit_memzero()
> it before freeing, so the key bytes don't linger around in unused
> memory.  I'm not sure if any copies are made inside the
> handle_guest_request call above; maybe zero these as well.
> 

I can do that, but I guess I am trying to find a reason for it. The resp 
buffer is encrypted page, so, the key is protected from the hypervisor 
access. Are you thinking about an attack within the VM guest OS ?

-Brijesh