linux-kernel - Re: [PATCH v19,RESEND 24/27] x86/vdso: Add __vdso_sgx_enter_enclave() to wrap SGX enclave transitions

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Message-ID: <c7da5f00-f266-48a7-2149-49d7c8ba44a5@fortanix.com>
Date:   Wed, 20 Mar 2019 19:02:06 +0000
From:   Jethro Beekman <jethro@...tanix.com>
To:     "Xing, Cedric" <cedric.xing@...el.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>
CC:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        "nhorman@...hat.com" <nhorman@...hat.com>,
        "npmccallum@...hat.com" <npmccallum@...hat.com>,
        "Ayoun, Serge" <serge.ayoun@...el.com>,
        "Katz-zamir, Shay" <shay.katz-zamir@...el.com>,
        "Huang, Haitao" <haitao.huang@...el.com>,
        "andriy.shevchenko@...ux.intel.com" 
        <andriy.shevchenko@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Svahn, Kai" <kai.svahn@...el.com>, "bp@...en8.de" <bp@...en8.de>,
        "josh@...htriplett.org" <josh@...htriplett.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "Huang, Kai" <kai.huang@...el.com>,
        "rientjes@...gle.com" <rientjes@...gle.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Haitao Huang <haitao.huang@...ux.intel.com>,
        "Dr . Greg Wettstein" <greg@...ellic.com>
Subject: Re: [PATCH v19,RESEND 24/27] x86/vdso: Add __vdso_sgx_enter_enclave()
 to wrap SGX enclave transitions

On 2019-03-20 11:30, Xing, Cedric wrote:
>> +/**
>> + * __vdso_sgx_enter_enclave() - Enter an SGX enclave
>> + *
>> + * %eax:        ENCLU leaf, must be EENTER or ERESUME
>> + * %rbx:        TCS, must be non-NULL
>> + * %rcx:        Optional pointer to 'struct sgx_enclave_exception'
>> + *
>> + * Return:
>> + *  0 on a clean entry/exit to/from the enclave
>> + *  -EINVAL if ENCLU leaf is not allowed or if TCS is NULL
>> + *  -EFAULT if ENCLU or the enclave faults
>> + *
>> + * Note that __vdso_sgx_enter_enclave() is not compliant with the x86-
>> 64 ABI.
>> + * All registers except RSP must be treated as volatile from the
>> +caller's
>> + * perspective, including but not limited to GPRs, EFLAGS.DF, MXCSR,
>> FCW, etc...
>> + * Conversely, the enclave being run must preserve the untrusted RSP
>> and stack.
> 
> By requiring preservation of RSP at both AEX and EEXIT, this precludes the possibility of using the untrusted stack as temporary storage by enclaves. While that looks reasonable at first glance, I'm afraid it isn't the case in reality. The untrusted stack is inarguably the most convenient way for data exchange between an enclave and its enclosing process, and is in fact being used for that purpose by almost all existing enclaves to date. Given the expectation that this API will be used by all future SGX application, it looks unwise to ban the most convenient and commonly used approach for data exchange.

For reference, here's the code in the Intel toolchain responsible for 
this: 
https://github.com/intel/linux-sgx/blob/6a0b5ac71f8d16f04e0376f3b2168e80c773dd23/sdk/trts/trts.cpp#L125-L140

Regarding "almost all existing enclaves to date", enclaves built with 
the Fortanix toolchain don't touch the untrusted stack.

--
Jethro Beekman | Fortanix


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