linux-kernel - Re: [PATCH v39 13/24] x86/sgx: Add SGX_IOC_ENCLAVE_ADD

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Date:   Mon, 19 Oct 2020 14:44:19 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>, x86@...nel.org,
        linux-sgx@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jethro Beekman <jethro@...tanix.com>,
        Haitao Huang <haitao.huang@...ux.intel.com>,
        Chunyang Hui <sanqian.hcy@...fin.com>,
        Jordan Hand <jorhand@...ux.microsoft.com>,
        Nathaniel McCallum <npmccallum@...hat.com>,
        Seth Moore <sethmo@...gle.com>,
        Darren Kenny <darren.kenny@...cle.com>,
        Suresh Siddha <suresh.b.siddha@...el.com>,
        akpm@...ux-foundation.org, andriy.shevchenko@...ux.intel.com,
        asapek@...gle.com, bp@...en8.de, cedric.xing@...el.com,
        chenalexchen@...gle.com, conradparker@...gle.com,
        cyhanish@...gle.com, haitao.huang@...el.com, kai.huang@...el.com,
        kai.svahn@...el.com, kmoy@...gle.com, ludloff@...gle.com,
        luto@...nel.org, nhorman@...hat.com, puiterwijk@...hat.com,
        rientjes@...gle.com, tglx@...utronix.de, yaozhangx@...gle.com,
        mikko.ylinen@...el.com
Subject: Re: [PATCH v39 13/24] x86/sgx: Add SGX_IOC_ENCLAVE_ADD_PAGES

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.