linux-kernel - Re: [PATCH v21 16/28] x86/sgx: Add the Linux SGX Enclave Driver

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Date:   Mon, 5 Aug 2019 09:16:44 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-sgx@...r.kernel.org, akpm@...ux-foundation.org,
        dave.hansen@...el.com, nhorman@...hat.com, npmccallum@...hat.com,
        serge.ayoun@...el.com, shay.katz-zamir@...el.com,
        haitao.huang@...el.com, andriy.shevchenko@...ux.intel.com,
        tglx@...utronix.de, kai.svahn@...el.com, bp@...en8.de,
        josh@...htriplett.org, luto@...nel.org, kai.huang@...el.com,
        rientjes@...gle.com, cedric.xing@...el.com,
        Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: [PATCH v21 16/28] x86/sgx: Add the Linux SGX Enclave Driver

On Sat, Jul 13, 2019 at 08:07:52PM +0300, Jarkko Sakkinen wrote:
> +static unsigned long sgx_get_unmapped_area(struct file *file,
> +					   unsigned long addr,
> +					   unsigned long len,
> +					   unsigned long pgoff,
> +					   unsigned long flags)
> +{
> +	if (flags & MAP_PRIVATE)
> +		return -EINVAL;
> +
> +	if (flags & MAP_FIXED)
> +		return addr;
> +
> +	if (len < 2 * PAGE_SIZE || len & (len - 1))
> +		return -EINVAL;
> +
> +	addr = current->mm->get_unmapped_area(file, addr, 2 * len, pgoff,
> +					      flags);
> +	if (IS_ERR_VALUE(addr))
> +		return addr;
> +
> +	addr = (addr + (len - 1)) & ~(len - 1);
> +
> +	return addr;
> +}

Thinking about this more, I don't think the driver should verify or adjust
@addr and @len during non-fixed mmap().  There is no requirement that
userspace must map the full ELRANGE, or that an enclave's ELRANGE can
*never* be mapped to non-EPC.  The architectural requirement is that an
access that hits ELRANGE must map to the EPC *if* the CPU is executing the
associated enclave.

This means that a process can have random mappings that overlap an
enclave's ELRANGE while the enclave is active, it just can't access that
memory from within the enclave.

A good example is a large enclave, e.g. 3gb of actual code/data, that
userspace wants to locate below 4gb for some reason, e.g. running an
unmodified non-enclave binary under graphene.  Requiring @addr+@len to be
naturally sized and aligned will force @addr to 0x0, which is often
disallowed by the kernel.