Re: [RFC PATCH v2.1 14/30] x86/sgx: Support restricting of enclave page permissions

From: Jarkko Sakkinen <jarkko@kernel.org>
To: linux-sgx@vger.kernel.org
Cc: Nathaniel McCallum <nathaniel@profian.com>,
	Reinette Chatre <reinette.chatre@intel.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
	<linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH v2.1 14/30] x86/sgx: Support restricting of enclave page permissions
Date: Wed, 9 Mar 2022 11:35:14 +0200	[thread overview]
Message-ID: <Yih00gFMaJhLIgxa@iki.fi> (raw)
In-Reply-To: <YihqwiU3Dr5mvMQx@iki.fi>

On Wed, Mar 09, 2022 at 10:52:22AM +0200, Jarkko Sakkinen wrote:
> On Fri, Mar 04, 2022 at 11:35:08AM +0200, Jarkko Sakkinen wrote:
> > +#define SGX_IOC_ENCLAVE_RESTRICT_PERMISSIONS \
> > +	_IOWR(SGX_MAGIC, 0x05, struct sgx_enclave_restrict_perm)
> 
> What if this was replaced with just SGX_IOC_ENCLAVE_RESET_PAGES, which
> would simply do EMODPR with PROT_NONE? The main ingredient of EMODPR is to
> flush out the TLB's, and move a page to pending state, which cannot be done
> from inside the enclave.
> 
> It's there because of microarchitecture constraints, and less so to work as
> a reasonable permission control mechanism (actually it does terrible job on
> that side and only confuses).
> 
> Once you have this magic TLB reset button in place you can just do one
> EACCEPT and EMODPE inside the enclave and you're done.
> 
> This is also kind of atomic in the sense that EACCEPT free's a page with no
> rights so no misuse can happend before EMODPE has tuned EPCM.

I wonder if this type of pattern could be made work out for Graphene:

1. SGX_IOC_ENCLAVE_RESET_PAGES
2. EACCEPT + EMODPE

This kind of delivers EMODP that everyone has been looking for.

BR, Jarkko