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From:   Jethro Beekman <jethro@fortanix.com>
To:     "Xing, Cedric" <cedric.xing@intel.com>,
        Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
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        "x86@kernel.org" <x86@kernel.org>,
        "linux-sgx@vger.kernel.org" <linux-sgx@vger.kernel.org>
CC:     "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "Hansen, Dave" <dave.hansen@intel.com>,
        "Christopherson, Sean J" <sean.j.christopherson@intel.com>,
        "nhorman@redhat.com" <nhorman@redhat.com>,
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        "Ayoun, Serge" <serge.ayoun@intel.com>,
        "Katz-zamir, Shay" <shay.katz-zamir@intel.com>,
        "Huang, Haitao" <haitao.huang@intel.com>,
        "andriy.shevchenko@linux.intel.com" 
        <andriy.shevchenko@linux.intel.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "Svahn, Kai" <kai.svahn@intel.com>, "bp@alien8.de" <bp@alien8.de>,
        "josh@joshtriplett.org" <josh@joshtriplett.org>,
        "luto@kernel.org" <luto@kernel.org>,
        "Huang, Kai" <kai.huang@intel.com>,
        "rientjes@google.com" <rientjes@google.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Haitao Huang <haitao.huang@linux.intel.com>,
        "Dr . Greg Wettstein" <greg@enjellic.com>
Subject: Re: [PATCH v19,RESEND 24/27] x86/vdso: Add __vdso_sgx_enter_enclave()
 to wrap SGX enclave transitions
Thread-Topic: [PATCH v19,RESEND 24/27] x86/vdso: Add
 __vdso_sgx_enter_enclave() to wrap SGX enclave transitions
Thread-Index: AQHU3zmldgnOY/iBI0GzP+2+92cI76YU14YAgAAI1YA=
Date:   Wed, 20 Mar 2019 19:02:06 +0000
Message-ID: <c7da5f00-f266-48a7-2149-49d7c8ba44a5@fortanix.com>
References: <20190320162119.4469-1-jarkko.sakkinen@linux.intel.com>
 <20190320162119.4469-25-jarkko.sakkinen@linux.intel.com>
 <960B34DE67B9E140824F1DCDEC400C0F4E85C484@ORSMSX116.amr.corp.intel.com>
In-Reply-To: <960B34DE67B9E140824F1DCDEC400C0F4E85C484@ORSMSX116.amr.corp.intel.com>
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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.
>=20
> By requiring preservation of RSP at both AEX and EEXIT, this precludes =
the possibility of using the untrusted stack as temporary storage by encl=
aves. While that looks reasonable at first glance, I'm afraid it isn't th=
e case in reality. The untrusted stack is inarguably the most convenient =
way for data exchange between an enclave and its enclosing process, and i=
s 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=20
this:=20
https://github.com/intel/linux-sgx/blob/6a0b5ac71f8d16f04e0376f3b2168e80c=
773dd23/sdk/trts/trts.cpp#L125-L140

Regarding "almost all existing enclaves to date", enclaves built with=20
the Fortanix toolchain don't touch the untrusted stack.

--
Jethro Beekman | Fortanix


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