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Subject: Re: [PATCH v26 10/22] x86/sgx: Linux Enclave Driver
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        linux-sgx@vger.kernel.org, akpm@linux-foundation.org,
        dave.hansen@intel.com, nhorman@redhat.com, npmccallum@redhat.com,
        haitao.huang@intel.com, andriy.shevchenko@linux.intel.com,
        tglx@linutronix.de, kai.svahn@intel.com, bp@alien8.de,
        josh@joshtriplett.org, luto@kernel.org, kai.huang@intel.com,
        rientjes@google.com, cedric.xing@intel.com, puiterwijk@redhat.com,
        linux-security-module@vger.kernel.org,
        Haitao Huang <haitao.huang@linux.intel.com>
References: <20200209212609.7928-1-jarkko.sakkinen@linux.intel.com>
 <20200209212609.7928-11-jarkko.sakkinen@linux.intel.com>
 <d17c50a7-6900-731b-43a2-d6e49b8eb44d@fortanix.com>
 <20200213180737.GC18610@linux.intel.com>
From:   Jethro Beekman <jethro@fortanix.com>
Message-ID: <a4d9a58d-6984-5894-f6c8-73f2b2b466aa@fortanix.com>
Date:   Fri, 14 Feb 2020 10:24:10 +0100
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On 2020-02-13 19:07, Sean Christopherson wrote:
> On Thu, Feb 13, 2020 at 02:59:52PM +0100, Jethro Beekman wrote:
>> On 2020-02-09 22:25, Jarkko Sakkinen wrote:
>>> +/**
>>> + * struct sgx_enclave_add_pages - parameter structure for the
>>> + *                                %SGX_IOC_ENCLAVE_ADD_PAGE ioctl
>>> + * @src:	start address for the page data
>>> + * @offset:	starting page offset
>>> + * @length:	length of the data (multiple of the page size)
>>> + * @secinfo:	address for the SECINFO data
>>> + * @flags:	page control flags
>>> + * @count:	number of bytes added (multiple of the page size)
>>> + */
>>> +struct sgx_enclave_add_pages {
>>> +	__u64	src;
>>> +	__u64	offset;
>>> +	__u64	length;
>>> +	__u64	secinfo;
>>> +	__u64	flags;
>>> +	__u64	count;
>>> +};
>>
>> Compared to the last time I looked at the patch set, this API removes =
the
>> ability to measure individual pages chunks. That is not acceptable.
>=20
> Why is it not acceptable?  E.g. what specific use case do you have that=

> _requires_ on measuring partial 4k pages of an enclave?

The use case is someone gives me an enclave and I want to load it. If I d=
on't load it exactly as the enclave author specified, the enclave hash wi=
ll be different, and it won't work.

>> On 2019-10-11 16:37, Sean Christopherson wrote:
>>> Hiding the 256-byte granualarity from userspace is a good idea as it'=
s not
>>> intrinsically tied to the SGX architecture and exists only because of=

>>> latency requirements.
>>
>> What do you mean by "it's not intrinsically tied to the SGX architectu=
re"?
>> This is a fundamental part of the SGX instruction set. This is the
>> instruction definition from the SDM: "EEXTEND=E2=80=94Extend Uninitial=
ized Enclave
>> Measurement by 256 Bytes".
>=20
> SGX fundamentally works at a 4k granularity.  EEXTEND is special cased
> because extending the measurement is a slow operation, i.e. EEXTEND on =
more
> than 256 byte chunks, *with the current implementation*, would exceeded=

> latency requirements, e.g. block interrupts for too long and hose the
> kernel.
>=20
> A future implementation of SGX could change the latency of extending th=
e
> measurement, e.g. a different algorithm that is slower/faster, and so c=
ould
> introduce EEXTEND2 which would work at a different granularity than EEX=
TEND.
>=20
> EEXTEND could have avoided the latency problems via other methods, e.g.=
 by
> being interruptible a la EINIT and/or by being restartable.  But that s=
hip
> has sailed, so to avoid future complication in the kernel's ABI we're
> proposing/advocating supporting only measuring at a 4k granularity.

It doesn't really matter what the reason for the current EEXTEND implemen=
tation is. It's there now in the ISA, it needs to be supported. If EEXTEN=
D2 (or whatever) is added to the ISA, it will likely influence the enclav=
e hash, so userspace would need to specify what instruction is used for m=
easuring anyway.

--
Jethro Beekman | Fortanix


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