linux-kernel - Re: [PATCH V4 16/31] x86/sgx: Support adding of pages to an initialized enclave

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Date:   Thu, 14 Apr 2022 09:31:46 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>, <dave.hansen@...ux.intel.com>,
        <tglx@...utronix.de>, <bp@...en8.de>, <luto@...nel.org>,
        <mingo@...hat.com>, <linux-sgx@...r.kernel.org>, <x86@...nel.org>,
        <shuah@...nel.org>, <linux-kselftest@...r.kernel.org>
CC:     <seanjc@...gle.com>, <kai.huang@...el.com>,
        <cathy.zhang@...el.com>, <cedric.xing@...el.com>,
        <haitao.huang@...el.com>, <mark.shanahan@...el.com>,
        <vijay.dhanraj@...el.com>, <hpa@...or.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V4 16/31] x86/sgx: Support adding of pages to an
 initialized enclave

Hi Jarkko,

On 4/14/2022 4:20 AM, Jarkko Sakkinen wrote:
> On Wed, 2022-04-13 at 14:10 -0700, Reinette Chatre wrote:
>> With SGX1 an enclave needs to be created with its maximum memory demands
>> allocated. Pages cannot be added to an enclave after it is initialized.
>> SGX2 introduces a new function, ENCLS[EAUG], that can be used to add
>> pages to an initialized enclave. With SGX2 the enclave still needs to
>> set aside address space for its maximum memory demands during enclave
>> creation, but all pages need not be added before enclave initialization.
>> Pages can be added during enclave runtime.
>>
>> Add support for dynamically adding pages to an initialized enclave,
>> architecturally limited to RW permission at creation but allowed to
>> obtain RWX permissions after trusted enclave runs EMODPE. Add pages
>> via the page fault handler at the time an enclave address without a
>> backing enclave page is accessed, potentially directly reclaiming
>> pages if no free pages are available.
>>
>> The enclave is still required to run ENCLU[EACCEPT] on the page before
>> it can be used. A useful flow is for the enclave to run ENCLU[EACCEPT]
>> on an uninitialized address. This will trigger the page fault handler
>> that will add the enclave page and return execution to the enclave to
>> repeat the ENCLU[EACCEPT] instruction, this time successful.
>>
>> If the enclave accesses an uninitialized address in another way, for
>> example by expanding the enclave stack to a page that has not yet been
>> added, then the page fault handler would add the page on the first
>> write but upon returning to the enclave the instruction that triggered
>> the page fault would be repeated and since ENCLU[EACCEPT] was not run
>> yet it would trigger a second page fault, this time with the SGX flag
>> set in the page fault error code. This can only be recovered by entering
>> the enclave again and directly running the ENCLU[EACCEPT] instruction on
>> the now initialized address.
>>
>> Accessing an uninitialized address from outside the enclave also
>> triggers this flow but the page will remain inaccessible (access will
>> result in #PF) until accepted from within the enclave via
>> ENCLU[EACCEPT].
>>
>> Reviewed-by: Jarkko Sakkinen <jarkko@...nel.org>
>> Tested-by: Jarkko Sakkinen <jarkko@...nel.org>
>> Signed-off-by: Reinette Chatre <reinette.chatre@...el.com>
> 
> I'm presuming that Haitao tested with this applied, right?

Yes, I will move his Tested-by to this patch.

Reinette