linux-kernel - Re: [PATCH v40 11/24] x86/sgx: Add SGX misc driver interface

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Date:   Thu, 5 Nov 2020 03:10:43 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     x86@...nel.org, linux-sgx@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.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>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Suresh Siddha <suresh.b.siddha@...el.com>,
        andriy.shevchenko@...ux.intel.com, asapek@...gle.com, bp@...en8.de,
        cedric.xing@...el.com, chenalexchen@...gle.com,
        conradparker@...gle.com, cyhanish@...gle.com,
        dave.hansen@...el.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 v40 11/24] x86/sgx: Add SGX misc driver interface

Noticed couple of minor glitches.

On Wed, Nov 04, 2020 at 04:54:17PM +0200, Jarkko Sakkinen wrote:
> +int sgx_encl_may_map(struct sgx_encl *encl, unsigned long start,
> +		     unsigned long end, unsigned long vm_flags)
> +{
> +	unsigned long vm_prot_bits = vm_flags & (VM_READ | VM_WRITE | VM_EXEC);
> +	struct sgx_encl_page *page;
> +	unsigned long count = 0;
> +	int ret = 0;
> +
> +	XA_STATE(xas, &encl->page_array, PFN_DOWN(start));
> +
> +	/*
> +	 * Disallow READ_IMPLIES_EXEC tasks as their VMA permissions might
> +	 * conflict with the enclave page permissions.
> +	 */
> +	if (current->personality & READ_IMPLIES_EXEC)
> +		return -EACCES;
> +
> +	mutex_lock(&encl->lock);
> +	xas_lock(&xas);
> +	xas_for_each(&xas, page, PFN_DOWN(end - 1)) {
> +		if (!page)
> +			break;

A redundant check, can be removed.

> +
> +		if (~page->vm_max_prot_bits & vm_prot_bits) {
> +			ret = -EACCES;
> +			break;
> +		}
> +
> +		/* Reschedule on every XA_CHECK_SCHED iteration. */
> +		if (!(++count % XA_CHECK_SCHED)) {
> +			xas_pause(&xas);
> +			xas_unlock(&xas);
> +			mutex_unlock(&encl->lock);
> +
> +			cond_resched();
> +
> +			mutex_lock(&encl->lock);
> +			xas_lock(&xas);
> +		}
> +	}
> +	xas_unlock(&xas);
> +	mutex_unlock(&encl->lock);
> +
> +	return ret;
> +}
> +
> +static int sgx_vma_mprotect(struct vm_area_struct *vma,
> +			    struct vm_area_struct **pprev, unsigned long start,
> +			    unsigned long end, unsigned long newflags)
> +{
> +	int ret;
> +
> +	ret = sgx_encl_may_map(vma->vm_private_data, start, end, newflags);
> +	if (ret)
> +		return ret;
> +
> +	return mprotect_fixup(vma, pprev, start, end, newflags);
> +}
> +
> +const struct vm_operations_struct sgx_vm_ops = {
> +	.fault = sgx_vma_fault,
> +	.mprotect = sgx_vma_mprotect,
> +};
> +
> +/**
> + * sgx_encl_find - find an enclave
> + * @mm:		mm struct of the current process
> + * @addr:	address in the ELRANGE
> + * @vma:	the resulting VMA
> + *
> + * Find an enclave identified by the given address. Give back a VMA that is
> + * part of the enclave and located in that address. The VMA is given back if it
> + * is a proper enclave VMA even if an &sgx_encl instance does not exist yet
> + * (enclave creation has not been performed).
> + *
> + * Return:
> + *   0 on success,
> + *   -EINVAL if an enclave was not found,
> + *   -ENOENT if the enclave has not been created yet
> + */
> +int sgx_encl_find(struct mm_struct *mm, unsigned long addr,
> +		  struct vm_area_struct **vma)
> +{
> +	struct vm_area_struct *result;
> +	struct sgx_encl *encl;
> +
> +	result = find_vma(mm, addr);
> +	if (!result || result->vm_ops != &sgx_vm_ops || addr < result->vm_start)
> +		return -EINVAL;
> +
> +	encl = result->vm_private_data;
> +	*vma = result;
> +
> +	return encl ? 0 : -ENOENT;
> +}

Since v20 there has been 1:1 assocition between enclaves and files.
In other words, this can never return -ENOENT.

With this reduction the function turns into:

int sgx_encl_find(struct mm_struct *mm, unsigned long addr,
		  struct vm_area_struct **vma)
{
	struct vm_area_struct *result;

	result = find_vma(mm, addr);
	if (!result || result->vm_ops != &sgx_vm_ops || addr < result->vm_start)
		return -EINVAL;

	*vma = result;

	return 0;
}

There are only two call sites:

1. sgx_encl_test_and_clear_young()
2. sgx_reclaimer_block()

I.e. would not be a big trouble to tune the signature a bit:

struct vm_area_struct *sgx_encl_find_vma(struct mm_struct *mm, unsigned long addr)
{
	struct vm_area_struct *result;

	result = find_vma(mm, addr);
	if (!result || result->vm_ops != &sgx_vm_ops || addr < result->vm_start)
		return NULL;

	return result;
}

There is a function called sgx_encl_find_mm(), which is *unrelated* to
this function and has only one call sites. Its flow is very linear. In
order to avoid confusion, I'd open code that into sgx_encl_mm_add().

/Jarkko