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 14:20:36 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Reinette Chatre <reinette.chatre@...el.com>,
        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

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?

BR, Jarkko