Re: [PATCH 2/4] x86/sgx: Put enclaves into anonymous files

From: Jethro Beekman <jethro@fortanix.com>
To: Andy Lutomirski <luto@kernel.org>,
	Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc: Topi Miettinen <toiwoton@gmail.com>,
	Casey Schaufler <casey@schaufler-ca.com>,
	"Schaufler, Casey" <casey.schaufler@intel.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	linux-sgx@vger.kernel.org, "Svahn, Kai" <kai.svahn@intel.com>,
	"Schlobohm, Bruce" <bruce.schlobohm@intel.com>,
	Stephen Smalley <sds@tycho.nsa.gov>,
	Haitao Huang <haitao.huang@linux.intel.com>,
	Ben Hutchings <ben@decadent.org.uk>
Subject: Re: [PATCH 2/4] x86/sgx: Put enclaves into anonymous files
Date: Tue, 7 Apr 2020 09:15:06 +0200	[thread overview]
Message-ID: <3912545a-1c57-4f95-21a4-566c2bcea194@fortanix.com> (raw)
In-Reply-To: <CALCETrWkaUS2RU61_4KoNhn3RkW-J+SWiCQTZ623wu4b7snJ0w@mail.gmail.com>

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On 2020-04-07 01:18, Andy Lutomirski wrote:
> On Mon, Apr 6, 2020 at 2:24 PM Jarkko Sakkinen
> <jarkko.sakkinen@linux.intel.com> wrote:
>>
>> On Mon, Apr 06, 2020 at 12:53:41PM -0700, Andy Lutomirski wrote:
>>>> sgxfs is somewhat trivial to implement and has one stakeholder less to
>>>> worry about. It is not really a huge stretch.
>>>>
>>>> Overally, I think it is something that we could live with. At least it
>>>> is something that does not step on others toes.
>>>>
>>>> Haitao: If we go with sgxfs route, then you can for the moment do what
>>>> Andy suggested: bind mount it to /dev/sgx.
>>>
>>> That also needs userspace support.
>>>
>>> I’ll start a thread on the udev list.
>>
>> Haven't written any code yet but just by drafting the idea I already
>> pinpointed something.
>>
>> You would add roughly this to the existing codebase:
>>
>> * struct file_system_type sgxfs
>> * struct fs_context_operations sgxfs_ctx_ops
>> * int sgxfs_get_tree()
>> * int sgxfs_fill_super()
>> * const struct tree_descr sgxfs_files[]
>>
>> The files would be defined as like this:
>>
>> static const struct tree_descr sgxfs_files[] {
>>         [SGXFS_ENCLAVE] = { "enclave", &sgxfs_encl_ops, /* ? */ },
>>         [SGXFS_PROVISION] = { "provision", &sgxfs_encl_ops, /* ? */ },
>> };
>>
>> The permissions would need to be completely static, which feels nasty
>> from maintainability viewpoint :-( And I see no gain anywhere.
>>
>> In my opinion udev defining the whole /dev as noexec has zero technical
>> merits. It is same as they would say that "we don't trust our own
>> database". There are no real security benefits as long as dev nodes are
>> configured correctly.
> 
> I tend to agree, but it could be seen as a sort-of-valid hardening measure.

Yeah it seems to me `noexec` is overly broad in not allowing mmap(PROT_EXEC). I'd expect it to disallow only execve(), which is fine for /dev.

--
Jethro Beekman | Fortanix

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