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Message-ID: <f3ce0718-064d-48e4-a681-7058157127b0@arm.com>
Date: Mon, 14 Oct 2024 15:41:07 +0100
From: Steven Price <steven.price@....com>
To: Suzuki K Poulose <suzuki.poulose@....com>, Gavin Shan <gshan@...hat.com>,
kvm@...r.kernel.org, kvmarm@...ts.linux.dev
Cc: Sami Mujawar <sami.mujawar@....com>,
Catalin Marinas <catalin.marinas@....com>, Marc Zyngier <maz@...nel.org>,
Will Deacon <will@...nel.org>, James Morse <james.morse@....com>,
Oliver Upton <oliver.upton@...ux.dev>, Zenghui Yu <yuzenghui@...wei.com>,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
Joey Gouly <joey.gouly@....com>, Alexandru Elisei
<alexandru.elisei@....com>, Christoffer Dall <christoffer.dall@....com>,
Fuad Tabba <tabba@...gle.com>, linux-coco@...ts.linux.dev,
Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>,
Shanker Donthineni <sdonthineni@...dia.com>, Alper Gun
<alpergun@...gle.com>, Dan Williams <dan.j.williams@...el.com>,
"Aneesh Kumar K . V" <aneesh.kumar@...nel.org>
Subject: Re: [PATCH v6 10/11] virt: arm-cca-guest: TSM_REPORT support for
realms
On 14/10/2024 09:56, Suzuki K Poulose wrote:
> On 12/10/2024 07:06, Gavin Shan wrote:
>> On 10/12/24 2:22 AM, Suzuki K Poulose wrote:
>>> On 11/10/2024 15:14, Steven Price wrote:
>>>> On 08/10/2024 05:12, Gavin Shan wrote:
>>>>> On 10/5/24 12:43 AM, Steven Price wrote:
>>>>>> From: Sami Mujawar <sami.mujawar@....com>
>>>>>>
>>>>>> Introduce an arm-cca-guest driver that registers with
>>>>>> the configfs-tsm module to provide user interfaces for
>>>>>> retrieving an attestation token.
>>>>>>
>>>>>> When a new report is requested the arm-cca-guest driver
>>>>>> invokes the appropriate RSI interfaces to query an
>>>>>> attestation token.
>>>>>>
>>>>>> The steps to retrieve an attestation token are as follows:
>>>>>> 1. Mount the configfs filesystem if not already mounted
>>>>>> mount -t configfs none /sys/kernel/config
>>>>>> 2. Generate an attestation token
>>>>>> report=/sys/kernel/config/tsm/report/report0
>>>>>> mkdir $report
>>>>>> dd if=/dev/urandom bs=64 count=1 > $report/inblob
>>>>>> hexdump -C $report/outblob
>>>>>> rmdir $report
>>>>>>
>>>>>> Signed-off-by: Sami Mujawar <sami.mujawar@....com>
>>>>>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>>>>>> Signed-off-by: Steven Price <steven.price@....com>
>>>>>> ---
>>>>>> v3: Minor improvements to comments and adapt to the renaming of
>>>>>> GRANULE_SIZE to RSI_GRANULE_SIZE.
>>>>>> ---
>>>>>> drivers/virt/coco/Kconfig | 2 +
>>>>>> drivers/virt/coco/Makefile | 1 +
>>>>>> drivers/virt/coco/arm-cca-guest/Kconfig | 11 +
>>>>>> drivers/virt/coco/arm-cca-guest/Makefile | 2 +
>>>>>> .../virt/coco/arm-cca-guest/arm-cca-guest.c | 211
>>>>>> ++++++++++++ ++++++
>>>>>> 5 files changed, 227 insertions(+)
>>>>>> create mode 100644 drivers/virt/coco/arm-cca-guest/Kconfig
>>>>>> create mode 100644 drivers/virt/coco/arm-cca-guest/Makefile
>>>>>> create mode 100644 drivers/virt/coco/arm-cca-guest/arm-cca-guest.c
>>
>> [...]
>>
>>>>>> +/**
>>>>>> + * arm_cca_report_new - Generate a new attestation token.
>>>>>> + *
>>>>>> + * @report: pointer to the TSM report context information.
>>>>>> + * @data: pointer to the context specific data for this module.
>>>>>> + *
>>>>>> + * Initialise the attestation token generation using the
>>>>>> challenge data
>>>>>> + * passed in the TSM descriptor. Allocate memory for the attestation
>>>>>> token
>>>>>> + * and schedule calls to retrieve the attestation token on the
>>>>>> same CPU
>>>>>> + * on which the attestation token generation was initialised.
>>>>>> + *
>>>>>> + * The challenge data must be at least 32 bytes and no more than 64
>>>>>> bytes. If
>>>>>> + * less than 64 bytes are provided it will be zero padded to 64
>>>>>> bytes.
>>>>>> + *
>>>>>> + * Return:
>>>>>> + * * %0 - Attestation token generated successfully.
>>>>>> + * * %-EINVAL - A parameter was not valid.
>>>>>> + * * %-ENOMEM - Out of memory.
>>>>>> + * * %-EFAULT - Failed to get IPA for memory page(s).
>>>>>> + * * A negative status code as returned by
>>>>>> smp_call_function_single().
>>>>>> + */
>>>>>> +static int arm_cca_report_new(struct tsm_report *report, void *data)
>>>>>> +{
>>>>>> + int ret;
>>>>>> + int cpu;
>>>>>> + long max_size;
>>>>>> + unsigned long token_size;
>>>>>> + struct arm_cca_token_info info;
>>>>>> + void *buf;
>>>>>> + u8 *token __free(kvfree) = NULL;
>>>>>> + struct tsm_desc *desc = &report->desc;
>>>>>> +
>>>>>> + if (!report)
>>>>>> + return -EINVAL;
>>>>>> +
>>>>>
>>>>> This check seems unnecessary and can be dropped.
>>>>
>>>> Ack
>>>>
>>>>>> + if (desc->inblob_len < 32 || desc->inblob_len > 64)
>>>>>> + return -EINVAL;
>>>>>> +
>>>>>> + /*
>>>>>> + * Get a CPU on which the attestation token generation will be
>>>>>> + * scheduled and initialise the attestation token generation.
>>>>>> + */
>>>>>> + cpu = get_cpu();
>>>>>> + max_size = rsi_attestation_token_init(desc->inblob,
>>>>>> desc->inblob_len);
>>>>>> + put_cpu();
>>>>>> +
>>>>>
>>>>> It seems that put_cpu() is called early, meaning the CPU can go
>>>>> away before
>>>>> the subsequent call to arm_cca_attestation_continue() ?
>>>>
>>>> Indeed, good spot. I'll move it to the end of the function and update
>>>> the error paths below.
>>>
>>> Actually this was on purpose, not to block the CPU hotplug. The
>>> attestation must be completed on the same CPU.
>>>
>>> We can detect the failure from "smp_call" further down and make sure
>>> we can safely complete the operation or restart it.
>>>
>>
>> Yes, It's fine to call put_cpu() early since we're tolerant to error
>> introduced
>> by CPU unplug. It's a bit confused that rsi_attestation_token_init()
>> is called
>> on the local CPU while arm_cca_attestation_continue() is called on
>> same CPU
>> with help of smp_call_function_single(). Does it make sense to unify
>> so that
>> both will be invoked with the help of smp_call_function_single() ?
>>
>> int cpu = smp_processor_id();
>>
>> /*
>> * The calling and target CPU can be different after the calling
>> process
>> * is migrated to another different CPU. It's guaranteed the
>> attestatation
>> * always happen on the target CPU with smp_call_function_single().
>> */
>> ret = smp_call_function_single(cpu,
>> rsi_attestation_token_init_wrapper,
>> (void *)&info, true);
>
> Well, we want to allocate sufficient size buffer (size returned from
> token_init()) outside an atomic context (thus not in smp_call_function()).
>
> May be we could make this "allocation" restriction in a comment to
> make it clear, why we do it this way.
So if I've followed this correctly the get_cpu() route doesn't work
because of the need to allocate outblob. So using
smp_call_function_single() for all calls seems to be the best approach,
along with a comment explaining what's going on. So how about:
/*
* The attestation token 'init' and 'continue' calls must be
* performed on the same CPU. smp_call_function_single() is used
* instead of simply calling get_cpu() because of the need to
* allocate outblob based on the returned value from the 'init'
* call and that cannot be done in an atomic context.
*/
cpu = smp_processor_id();
info.challenge = desc->inblob;
info.challenge_size = desc->inblob_len;
ret = smp_call_function_single(cpu, arm_cca_attestation_init,
&info, true);
if (ret)
return ret;
max_size = info.result;
(with appropriate updates to the 'info' struct and a new
arm_cca_attestation_init() wrapper for rsi_attestation_token_init()).
Steve
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