Re: [PATCH v2 0/3] x86/sgx: eextend ioctl

[Jethro Beekman <jethro@fortanix.com>]

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On 2021-04-12 18:40, Dave Hansen wrote:
> On 4/12/21 8:58 AM, Jethro Beekman wrote:
>> On 2021-04-12 17:36, Dave Hansen wrote:
>>> On 4/12/21 1:59 AM, Raoul Strackx wrote:
>>>> This patch set adds a new ioctl to enable userspace to execute EEXTEND
>>>> leaf functions per 256 bytes of enclave memory. With this patch in place,
>>>> Linux will be able to build all valid SGXv1 enclaves.
>>> This didn't cover why we need a *NEW* ABI for this instead of relaxing
>>> the page alignment rules in the existing one.
>>>
>> In executing the ECREATE, EADD, EEXTEND, EINIT sequence, you currently have 2 options for EADD/EEXTEND using the SGX_IOC_ENCLAVE_ADD_PAGES ioctl:
>> - execute EADD on any address
>> - execute EADD on any address followed by 16× EEXTEND for that address span
> 
> I think you forgot a key piece of the explanation here.  The choice as
> to whether you just EADD or EADD+16xEEXTEND is governed by the addition
> of the: SGX_PAGE_MEASURE flag.
> 
>> Could you be more specific on how you're suggesting that the current ioctl is modified to in addition support the following?
>> - execute EEXTEND on any address
> 
> I'm still not convinced you *NEED* EEXTEND on arbitrary addresses.
> 
> Right now, we have (roughly):
> 
> 	 ioctl(ADD_PAGES, ptr, PAGE_SIZE, MEASURE)
> 
> which translates in the kernel to:
> 
> 	__eadd(ptr, epc)
> 	if (flags & MEASURE) {
> 		for (i = 0; i < PAGE_SIZE/256; i++)
> 			__eextend(epc + i*256);
> 	}
> 
> Instead, we could allow add_arg.src and add_arg.offset to be
> non-page-aligned.  Then, we still do the same __eadd(), but modify the
> __eextend() loop to only cover the actual range referred to by 'add_arg'.
> 
> The downside is that you only get a single range of measured data per
> page.  Let's say a 'X' means measured (EEXTEND'ed) and '_' means not.
> You could have patterns like:
> 
> 	XXXXXXXXXXXXXXXX
> or
> 	XXXXXXXXXXXXXXX_
> or
> 	____XXXXXXXXXXXX
> 
> but not:
> 
> 	_X_X_X_X_X_X_X_X
> or
> 	_XXXXXXXXXXXXXX_
> 
> 
> Is that a problem?
> 

Yes this still doesn't let one execute all possible ECREATE, EADD, EEXTEND, EINIT sequences.

--
Jethro Beekman | Fortanix



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