linux-kernel - Re: [PATCH v14 09/19] x86/mm: x86/sgx: Signal SEGV_SGXERR for #PFs w/ PF

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Date:   Thu, 27 Sep 2018 17:21:58 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Andrew Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        Platform Driver <platform-driver-x86@...r.kernel.org>,
        nhorman@...hat.com, npmccallum@...hat.com,
        "Ayoun, Serge" <serge.ayoun@...el.com>, shay.katz-zamir@...el.com,
        linux-sgx@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v14 09/19] x86/mm: x86/sgx: Signal SEGV_SGXERR for #PFs
 w/ PF_SGX

On Wed, Sep 26, 2018 at 03:37:45PM -0700, Andy Lutomirski wrote:
> Yeah.  Maybe like this: > > xorl %eax,%eax > eenter_insn:
> ENCLU[whatever]
> eenter_landing_pad:
> ret
> 
> And the kernel would use the existing vdso2c vdso-symbol-finding
> mechanism to do the fixup.
> 
> >
> > How would a syscall work, though?  I assume we can't just enter the
> > enclave from ring0.
> 
> My understanding of how AEX works is a bit vague, but maybe a syscall
> could reuse the mechanism?  The vDSO approach seems considerably
> simpler.
> 
> We do need to make sure that a fault that happens on or after return
> from an AEX event does the right thing.  But I'm still vague on how
> that works, sigh.
> 
> --Andy

Returning from AEX does not differ from any other memory access event so
AFAIK it should be handled right with the proposed solution already.
For convenience I think we could have a fixed trampoline for AEX e.g.
this how it is implemented in the open source LE that I did:

sgx_get_token:
	push	%rbx
	mov	$0x02, %rax
	mov	%rsi, %rbx
	mov	%rdx, %rsi
	mov	$sgx_async_exit, %rcx
sgx_async_exit:
	ENCLU
	pop	%rbx
	ret

BTW, if I converted the in-kernel LE as a standalone test program, would
that be useful for basic testing of the series?

/Jarkko