linux-kernel - Re: [PATCH v39 21/24] x86/vdso: Implement a vDSO for Intel SGX enclave call

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Date:   Tue, 6 Oct 2020 18:36:39 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     x86@...nel.org, linux-sgx@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        Jethro Beekman <jethro@...tanix.com>,
        Cedric Xing <cedric.xing@...el.com>, akpm@...ux-foundation.org,
        andriy.shevchenko@...ux.intel.com, asapek@...gle.com, bp@...en8.de,
        chenalexchen@...gle.com, conradparker@...gle.com,
        cyhanish@...gle.com, dave.hansen@...el.com, haitao.huang@...el.com,
        kai.huang@...el.com, kai.svahn@...el.com, kmoy@...gle.com,
        ludloff@...gle.com, luto@...nel.org, nhorman@...hat.com,
        npmccallum@...hat.com, puiterwijk@...hat.com, rientjes@...gle.com,
        tglx@...utronix.de, yaozhangx@...gle.com, mikko.ylinen@...el.com
Subject: Re: [PATCH v39 21/24] x86/vdso: Implement a vDSO for Intel SGX
 enclave call

On Mon, Oct 05, 2020 at 07:57:05PM -0700, Sean Christopherson wrote:
> On Sat, Oct 03, 2020 at 07:50:56AM +0300, Jarkko Sakkinen wrote:
> > From: Sean Christopherson <sean.j.christopherson@...el.com>
> > +	/* Validate that the reserved area contains only zeros. */
> > +	push	%rax
> > +	push	%rbx
> > +	mov	$SGX_ENCLAVE_RUN_RESERVED_START, %rbx
> > +1:
> > +	mov	(%rcx, %rbx), %rax
> > +	cmpq	$0, %rax
> > +	jne	.Linvalid_input
> > +
> > +	add	$8, %rbx
> > +	cmpq	$SGX_ENCLAVE_RUN_RESERVED_END, %rbx
> > +	jne	1b
> > +	pop	%rbx
> > +	pop	%rax
> 
> This can more succinctly be (untested):
> 
> 	movq	SGX_ENCLAVE_RUN_RESERVED_1(%rbp), %rbx	
> 	orq	SGX_ENCLAVE_RUN_RESERVED_2(%rbp), %rbx	
> 	orq	SGX_ENCLAVE_RUN_RESERVED_3(%rbp), %rbx	
> 	jnz	.Linvalid_input
> 
> Note, %rbx is getting clobbered anyways, no need to save/restore it.

Right of course, because TCS comes through the run-struct. I've created
a backlog entry for this. Thank you.

> > diff --git a/arch/x86/include/uapi/asm/sgx.h b/arch/x86/include/uapi/asm/sgx.h
> > index b6ba036a9b82..3dd2df44d569 100644
> > --- a/arch/x86/include/uapi/asm/sgx.h
> > +++ b/arch/x86/include/uapi/asm/sgx.h
> > @@ -74,4 +74,102 @@ struct sgx_enclave_provision {
> >  	__u64 attribute_fd;
> >  };
> >  
> > +struct sgx_enclave_run;
> > +
> > +/**
> > + * typedef sgx_enclave_user_handler_t - Exit handler function accepted by
> > + *					__vdso_sgx_enter_enclave()
> > + * @run:	Pointer to the caller provided struct sgx_enclave_run
> > + *
> > + * The register parameters contain the snapshot of their values at enclave
> > + * exit
> > + *
> > + * Return:
> > + *  0 or negative to exit vDSO
> > + *  positive to re-enter enclave (must be EENTER or ERESUME leaf)
> > + */
> > +typedef int (*sgx_enclave_user_handler_t)(long rdi, long rsi, long rdx,
> > +					  long rsp, long r8, long r9,
> > +					  struct sgx_enclave_run *run);
> > +
> > +/**
> > + * struct sgx_enclave_run - the execution context of __vdso_sgx_enter_enclave()
> > + * @tcs:			TCS used to enter the enclave
> > + * @user_handler:		User provided callback run on exception
> > + * @user_data:			Data passed to the user handler
> > + * @leaf:			The ENCLU leaf we were at (EENTER/ERESUME/EEXIT)
> > + * @exception_vector:		The interrupt vector of the exception
> > + * @exception_error_code:	The exception error code pulled out of the stack
> > + * @exception_addr:		The address that triggered the exception
> > + * @reserved			Reserved for possible future use
> > + */
> > +struct sgx_enclave_run {
> > +	__u64 tcs;
> > +	__u64 user_handler;
> > +	__u64 user_data;
> > +	__u32 leaf;
> 
> I am still very strongly opposed to omitting exit_reason.  It is not at all
> difficult to imagine scenarios where 'leaf' alone is insufficient for the
> caller or its handler to deduce why the CPU exited the enclave.  E.g. see
> Jethro's request for intercepting interrupts.
> 
> I don't buy the argument that the N bytes needed for the exit_reason are at
> all expensive.

It's not used for anything.

> > +	__u16 exception_vector;
> > +	__u16 exception_error_code;
> > +	__u64 exception_addr;
> > +	__u8  reserved[24];
> 
> I also think it's a waste of space to bother with multiple reserved fields.
> 24 bytes isn't so much that it guarantees we'll never run into problems in
> the future.  But I care far less about this than I do about exit_reason.

/Jarkko