linux-kernel - Re: [RFC PATCH v2 4/4] x86/vdso: Add __vdso_sgx_enter_enclave() to wrap SGX enclave transitions

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Date:   Fri, 7 Dec 2018 18:15:04 +0000
From:   Jethro Beekman <jethro@...tanix.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "x86@...nel.org" <x86@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
CC:     "H. Peter Anvin" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Josh Triplett <josh@...htriplett.org>,
        Haitao Huang <haitao.huang@...ux.intel.com>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        "Dr. Greg" <greg@...ellic.com>
Subject: Re: [RFC PATCH v2 4/4] x86/vdso: Add __vdso_sgx_enter_enclave() to
 wrap SGX enclave transitions

On 2018-12-07 03:49, Sean Christopherson wrote:

...

> diff --git a/arch/x86/entry/vdso/vsgx_enter_enclave.c b/arch/x86/entry/vdso/vsgx_enter_enclave.c
> new file mode 100644
> index 000000000000..896c2eb079bb
> --- /dev/null
> +++ b/arch/x86/entry/vdso/vsgx_enter_enclave.c

...

> +enter_enclave:
> +	asm volatile(
> +		/*
> +		 * When an event occurs in an enclave, hardware first exits the
> +		 * enclave to the AEP, switching CPU context along the way, and
> +		 * *then* delivers the event as usual.  As part of the context
> +		 * switching, registers are loaded with synthetic state (except
> +		 * BP and SP, which are saved/restored).  The defined synthetic
> +		 * state loads registers so that simply executing ENCLU will do
> +		 * ERESUME, e.g. RAX=4, RBX=TCS and RCX=AEP after an AEE.  So,
> +		 * we only need to load RAX, RBX and RCX for the initial entry.
> +		 * The AEP can point at that same ENCLU, fixup will jump us out
> +		 * if an exception was unhandled.
> +		 */
> +		"	lea	1f(%%rip), %%rcx\n"
> +		"1:	enclu\n"
> +		"2:\n"
> +
> +		".pushsection .fixup, \"ax\" \n"
> +		"3:	jmp 2b\n"
> +		".popsection\n"
> +		_ASM_VDSO_EXTABLE_HANDLE(1b, 3b)
> +
> +		: "=a"(leaf), "=D" (rdi), "=S" (rsi), "=d" (rdx)
> +		: "a" (leaf), "b" (tcs), "D" (priv)
> +		: "cc", "memory",
> +		  "rcx", "r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15"
> +	);

This is not sufficient to support the Fortanix SGX ABI calling 
convention, which was designed to be mostly compatible with the SysV 
64-bit calling convention. The following registers need to be passed in 
to an enclave from userspace: RDI, RSI, RDX, R8, R9, R10. The following 
registers need to be passed out from an enclave to userspace: RDI, RSI, 
RDX, R8, R9.

You can find the ABI specification at 
https://github.com/fortanix/rust-sgx/blob/master/doc/FORTANIX-SGX-ABI.md#enclave-calling-convention

--
Jethro Beekman | Fortanix


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