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Message-ID: <56d9edcb-2574-43fe-8ebb-65cc4fdbc3d0@arm.com>
Date: Mon, 14 Oct 2024 15:46:47 +0100
From: Suzuki K Poulose <suzuki.poulose@....com>
To: Steven Price <steven.price@....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 15:41, Steven Price wrote:
> 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()).
That sounds good to me.
Suzuki
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