Re: [PATCH V4 16/31] x86/sgx: Support adding of pages to an initialized enclave

From: Jarkko Sakkinen <jarkko@kernel.org>
To: Reinette Chatre <reinette.chatre@intel.com>,
	dave.hansen@linux.intel.com, tglx@linutronix.de, bp@alien8.de,
	luto@kernel.org, mingo@redhat.com, linux-sgx@vger.kernel.org,
	x86@kernel.org, shuah@kernel.org,
	linux-kselftest@vger.kernel.org
Cc: seanjc@google.com, kai.huang@intel.com, cathy.zhang@intel.com,
	cedric.xing@intel.com, haitao.huang@intel.com,
	mark.shanahan@intel.com, vijay.dhanraj@intel.com, hpa@zytor.com,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH V4 16/31] x86/sgx: Support adding of pages to an initialized enclave
Date: Thu, 14 Apr 2022 14:20:36 +0300	[thread overview]
Message-ID: <8b36c54def8948747a704676491cc18dd08b48e9.camel@kernel.org> (raw)
In-Reply-To: <2293bdca025ce54bd2996783e187e209f9c0f5f2.1649878359.git.reinette.chatre@intel.com>

On Wed, 2022-04-13 at 14:10 -0700, Reinette Chatre wrote:
> With SGX1 an enclave needs to be created with its maximum memory demands
> allocated. Pages cannot be added to an enclave after it is initialized.
> SGX2 introduces a new function, ENCLS[EAUG], that can be used to add
> pages to an initialized enclave. With SGX2 the enclave still needs to
> set aside address space for its maximum memory demands during enclave
> creation, but all pages need not be added before enclave initialization.
> Pages can be added during enclave runtime.
> 
> Add support for dynamically adding pages to an initialized enclave,
> architecturally limited to RW permission at creation but allowed to
> obtain RWX permissions after trusted enclave runs EMODPE. Add pages
> via the page fault handler at the time an enclave address without a
> backing enclave page is accessed, potentially directly reclaiming
> pages if no free pages are available.
> 
> The enclave is still required to run ENCLU[EACCEPT] on the page before
> it can be used. A useful flow is for the enclave to run ENCLU[EACCEPT]
> on an uninitialized address. This will trigger the page fault handler
> that will add the enclave page and return execution to the enclave to
> repeat the ENCLU[EACCEPT] instruction, this time successful.
> 
> If the enclave accesses an uninitialized address in another way, for
> example by expanding the enclave stack to a page that has not yet been
> added, then the page fault handler would add the page on the first
> write but upon returning to the enclave the instruction that triggered
> the page fault would be repeated and since ENCLU[EACCEPT] was not run
> yet it would trigger a second page fault, this time with the SGX flag
> set in the page fault error code. This can only be recovered by entering
> the enclave again and directly running the ENCLU[EACCEPT] instruction on
> the now initialized address.
> 
> Accessing an uninitialized address from outside the enclave also
> triggers this flow but the page will remain inaccessible (access will
> result in #PF) until accepted from within the enclave via
> ENCLU[EACCEPT].
> 
> Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
> Tested-by: Jarkko Sakkinen <jarkko@kernel.org>
> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>

I'm presuming that Haitao tested with this applied, right?

BR, Jarkko