linux-kernel - Re: [PATCH v39 11/24] x86/sgx: Add SGX enclave driver

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Date:   Mon, 5 Oct 2020 00:50:49 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Matthew Wilcox <willy@...radead.org>
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>,
        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 v39 11/24] x86/sgx: Add SGX enclave driver

On Sat, Oct 03, 2020 at 08:54:40PM +0100, Matthew Wilcox wrote:
> On Sat, Oct 03, 2020 at 07:50:46AM +0300, Jarkko Sakkinen wrote:
> > +	XA_STATE(xas, &encl->page_array, idx_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;
> > +
> > +	xas_for_each(&xas, page, idx_end)
> > +		if (!page || (~page->vm_max_prot_bits & vm_prot_bits))
> > +			return -EACCES;
> 
> You're iterating the array without holding any lock that the XArray knows
> about.  If you're OK with another thread adding/removing pages behind your
> back, or there's a higher level lock (the mmap_sem?) protecting the XArray
> from being modified while you walk it, then hold the rcu_read_lock()
> while walking the array.  Otherwise you can prevent modification by
> calling xas_lock(&xas) and xas_unlock()..

I backtracked this. The locks have been there from v21-v35. This is a
refactoring mistake in radix_tree to xarray migration happened in v36.
It's by no means intentional.

What is shoukd take is encl->lock.

The loop was pre-v36 like:

	idx_start = PFN_DOWN(start);
	idx_end = PFN_DOWN(end - 1);

	for (idx = idx_start; idx <= idx_end; ++idx) {
		mutex_lock(&encl->lock);
		page = radix_tree_lookup(&encl->page_tree, idx);
		mutex_unlock(&encl->lock);

		if (!page || (~page->vm_max_prot_bits & vm_prot_bits))
			return -EACCES;
	}

Looking at xarray.h and filemap.c, I'm thinking something along the
lines of:

	for (idx = idx_start; idx <= idx_end; ++idx) {
		mutex_lock(&encl->lock);
		page = xas_find(&xas, idx + 1);
		mutex_unlock(&encl->lock);

		if (!page || (~page->vm_max_prot_bits & vm_prot_bits))
			return -EACCES;
	}

Does this look about right?

/Jarkko