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

From: Jethro Beekman <jethro@fortanix.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"x86@kernel.org" <x86@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Haitao Huang <haitao.huang@linux.intel.com>,
	"linux-sgx@vger.kernel.org" <linux-sgx@vger.kernel.org>,
	"Dr. Greg" <greg@enjellic.com>
Subject: Re: [RFC PATCH v2 4/4] x86/vdso: Add __vdso_sgx_enter_enclave() to wrap SGX enclave transitions
Date: Fri, 7 Dec 2018 18:15:04 +0000	[thread overview]
Message-ID: <f2b7df05-20ef-ac16-fc99-258c39d84666@fortanix.com> (raw)
In-Reply-To: <20181206221922.31012-5-sean.j.christopherson@intel.com>

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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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