Re: [PATCH v39 13/24] x86/sgx: Add SGX_IOC_ENCLAVE_ADD_PAGES

From: Dave Hansen <dave.hansen@intel.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
	x86@kernel.org, linux-sgx@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Jethro Beekman <jethro@fortanix.com>,
	Haitao Huang <haitao.huang@linux.intel.com>,
	Chunyang Hui <sanqian.hcy@antfin.com>,
	Jordan Hand <jorhand@linux.microsoft.com>,
	Nathaniel McCallum <npmccallum@redhat.com>,
	Seth Moore <sethmo@google.com>,
	Darren Kenny <darren.kenny@oracle.com>,
	Suresh Siddha <suresh.b.siddha@intel.com>,
	akpm@linux-foundation.org, andriy.shevchenko@linux.intel.com,
	asapek@google.com, bp@alien8.de, cedric.xing@intel.com,
	chenalexchen@google.com, conradparker@google.com,
	cyhanish@google.com, haitao.huang@intel.com, kai.huang@intel.com,
	kai.svahn@intel.com, kmoy@google.com, ludloff@google.com,
	luto@kernel.org, nhorman@redhat.com, puiterwijk@redhat.com,
	rientjes@google.com, tglx@linutronix.de, yaozhangx@google.com,
	mikko.ylinen@intel.com
Subject: Re: [PATCH v39 13/24] x86/sgx: Add SGX_IOC_ENCLAVE_ADD_PAGES
Date: Mon, 19 Oct 2020 14:44:19 -0700	[thread overview]
Message-ID: <564152e4-2334-2c82-1a12-4c980b08a944@intel.com> (raw)
In-Reply-To: <20201019211525.GC23072@linux.intel.com>

On 10/19/20 2:15 PM, Sean Christopherson wrote:
>>>> Yeah...  Don't we need to do another access_ok() check here, if we
>>>> needed one above since we are moving away from addrp.src?
>>> I don't think so because the page is pinned with get_user_pages().
>> No, get_user_pages() is orthogonal.
>>
>> Looking at this again, you _might_ be OK since you validated addp.length
>> against encl->size.  But, it's all very convoluted and doesn't look very
>> organized or obviously right.
> The easiest fix would be to have the existing access_ok() check the entire
> range, no?  Or am I missing something obvious?

In general, I want the actual userspace access to be as close as
possible and 1:1 with the access_ok() checks.  That way, it's blatantly
obvious that the pointers have been checked.

*But* get_user_pages() has access_ok() checks inside of its
implementation, which makes sense.  *But*, that begs the question of
what the top-level one was doing in the first place.  Maybe it was just
superfluous.

Either way, it still doesn't explain what this is doing:

> +       ret = get_user_pages(src, 1, 0, &src_page, NULL);
> +       if (ret < 1)
> +               return -EFAULT;
> +
> +       pginfo.secs = (unsigned long)sgx_get_epc_addr(encl->secs.epc_page);
> +       pginfo.addr = SGX_ENCL_PAGE_ADDR(encl_page);
> +       pginfo.metadata = (unsigned long)secinfo;
> +       pginfo.contents = (unsigned long)kmap_atomic(src_page);
> +
> +       ret = __eadd(&pginfo, sgx_get_epc_addr(epc_page));
> +
> +       kunmap_atomic((void *)pginfo.contents);

I think the point is to create a stable kernel alias address for
'src_page' so that any mucking with the userspace mapping doesn't screw
up the __eadd() and any failures aren't due to reclaim or MADV_DONTNEED.

If this isn't even touching the userspace mapping, it didn't need
access_ok() in the first place.