Re: [PATCH Part1 RFC v3 21/22] x86/sev: Register SNP guest request platform device

[Brijesh Singh <brijesh.singh@amd.com>]

On 6/14/21 12:15 PM, Dr. David Alan Gilbert wrote:
> * Tom Lendacky (thomas.lendacky@amd.com) wrote:
>> On 6/9/21 2:24 PM, Dr. David Alan Gilbert wrote:
>>> * Brijesh Singh (brijesh.singh@amd.com) wrote:
>>>> Version 2 of GHCB specification provides NAEs that can be used by the SNP
>>>> guest to communicate with the PSP without risk from a malicious hypervisor
>>>> who wishes to read, alter, drop or replay the messages sent.
>>>>
>>>> The hypervisor uses the SNP_GUEST_REQUEST command interface provided by
>>>> the SEV-SNP firmware to forward the guest messages to the PSP.
>>>>
>>>> In order to communicate with the PSP, the guest need to locate the secrets
>>>> page inserted by the hypervisor during the SEV-SNP guest launch. The
>>>> secrets page contains the communication keys used to send and receive the
>>>> encrypted messages between the guest and the PSP.
>>>>
>>>> The secrets page is located either through the setup_data cc_blob_address
>>>> or EFI configuration table.
>>>>
>>>> Create a platform device that the SNP guest driver can bind to get the
>>>> platform resources. The SNP guest driver can provide userspace interface
>>>> to get the attestation report, key derivation etc.
>>>>
>>>> The helper snp_issue_guest_request() will be used by the drivers to
>>>> send the guest message request to the hypervisor. The guest message header
>>>> contains a message count. The message count is used in the IV. The
>>>> firmware increments the message count by 1, and expects that next message
>>>> will be using the incremented count.
>>>>
>>>> The helper snp_msg_seqno() will be used by driver to get and message
>>>> sequence counter, and it will be automatically incremented by the
>>>> snp_issue_guest_request(). The incremented value is be saved in the
>>>> secrets page so that the kexec'ed kernel knows from where to begin.
>>>>
>>>> See SEV-SNP and GHCB spec for more details.
>>>>
>>>> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
>>>> ---
>>>>  arch/x86/include/asm/sev.h      |  12 +++
>>>>  arch/x86/include/uapi/asm/svm.h |   2 +
>>>>  arch/x86/kernel/sev.c           | 176 ++++++++++++++++++++++++++++++++
>>>>  arch/x86/platform/efi/efi.c     |   2 +
>>>>  include/linux/efi.h             |   1 +
>>>>  include/linux/sev-guest.h       |  76 ++++++++++++++
>>>>  6 files changed, 269 insertions(+)
>>>>  create mode 100644 include/linux/sev-guest.h
>>>>
>>>> +u64 snp_msg_seqno(void)
>>>> +{
>>>> +	struct snp_secrets_page_layout *layout;
>>>> +	u64 count;
>>>> +
>>>> +	layout = snp_map_secrets_page();
>>>> +	if (layout == NULL)
>>>> +		return 0;
>>>> +
>>>> +	/* Read the current message sequence counter from secrets pages */
>>>> +	count = readl(&layout->os_area.msg_seqno_0);
>>> Why is this seqno_0 - is that because it's the count of talking to the
>>> PSP?
>> Yes, the sequence number is an ever increasing value that is used in
>> communicating with the PSP. The PSP maintains the next expected sequence
>> number and will reject messages which have a sequence number that is not
>> in sync with the PSP. The 0 refers to the VMPL level. Each VMPL level has
>> its own sequence number.
> Can you just clarify; is that the VMPL of the caller or the destination?
> What I'm partially asking here is whether it matters which VMPL the
> kernel is running at (which I'm assuming could well be non-0)


The caller's VMPL number. Each VMPL have different communicate keys,
please see the secrets page layout as described in the SEV-SNP firmware
spec 8.14.2.5[1].

As indicated in the cover letter, the guest and hypervisor patches are
targeted to for VMPL0 so we are using sequence number and key from the
vmpl0 only.

[1] https://www.amd.com/system/files/TechDocs/56860.pdf

>
>>>> +	iounmap(layout);
>>>> +
>>>> +	/*
>>>> +	 * The message sequence counter for the SNP guest request is a 64-bit value
>>>> +	 * but the version 2 of GHCB specification defines the 32-bit storage for the
>>>> +	 * it.
>>>> +	 */
>>>> +	if ((count + 1) >= INT_MAX)
>>>> +		return 0;
>>> Is that UINT_MAX?
>>>
>>>> +
>>>> +	return count + 1;
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(snp_msg_seqno);
>>>> +
>>>> +static void snp_gen_msg_seqno(void)
>>>> +{
>>>> +	struct snp_secrets_page_layout *layout;
>>>> +	u64 count;
>>>> +
>>>> +	layout = snp_map_secrets_page();
>>>> +	if (layout == NULL)
>>>> +		return;
>>>> +
>>>> +	/* Increment the sequence counter by 2 and save in secrets page. */
>>>> +	count = readl(&layout->os_area.msg_seqno_0);
>>>> +	count += 2;
>>> Why 2 not 1 ?
>> The return message by the PSP also increments the sequence number, hence
>> the increment by 2 instead of 1 for the next message to be submitted.
> OK
>
> Dave
>
>> I'll let Brijesh address the other questions.
>>
>> Thanks,
>> Tom
>>