Re: [PATCH v38 16/24] x86/sgx: Add a page reclaimer

From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Borislav Petkov <bp@alien8.de>
Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
	x86@kernel.org, linux-sgx@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Jethro Beekman <jethro@fortanix.com>,
	Jordan Hand <jorhand@linux.microsoft.com>,
	Nathaniel McCallum <npmccallum@redhat.com>,
	Chunyang Hui <sanqian.hcy@antfin.com>,
	Seth Moore <sethmo@google.com>,
	akpm@linux-foundation.org, andriy.shevchenko@linux.intel.com,
	asapek@google.com, cedric.xing@intel.com,
	chenalexchen@google.com, conradparker@google.com,
	cyhanish@google.com, dave.hansen@intel.com,
	haitao.huang@intel.com, josh@joshtriplett.org,
	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
Subject: Re: [PATCH v38 16/24] x86/sgx: Add a page reclaimer
Date: Tue, 22 Sep 2020 09:24:38 -0700	[thread overview]
Message-ID: <20200922162437.GA30827@linux.intel.com> (raw)
In-Reply-To: <20200922104538.GE22660@zn.tnic>

On Tue, Sep 22, 2020 at 12:45:38PM +0200, Borislav Petkov wrote:
> On Tue, Sep 15, 2020 at 02:28:34PM +0300, Jarkko Sakkinen wrote:
> > + * %SGX_ENCL_PAGE_VA_OFFSET_MASK:	Holds the offset in the Version Array
> > + *					(VA) page for a swapped page.
> >   * %SGX_ENCL_PAGE_ADDR_MASK:		Holds the virtual address of the page.
> >   *
> >   * The page address for SECS is zero and is used by the subsystem to recognize
> 
> ...
> 
> > @@ -86,24 +123,34 @@ static int sgx_encl_create(struct sgx_encl *encl, struct sgx_secs *secs)
> >  {
> >  	unsigned long encl_size = secs->size + PAGE_SIZE;
> >  	struct sgx_epc_page *secs_epc;
> > +	struct sgx_va_page *va_page;
> >  	struct sgx_pageinfo pginfo;
> >  	struct sgx_secinfo secinfo;
> >  	struct file *backing;
> >  	long ret;
> >  
> > +	va_page = sgx_encl_grow(encl);
> > +	if (IS_ERR(va_page))
> > +		return PTR_ERR(va_page);
> > +	else if (va_page)
> 
> Not "else" simply?
> 
> AFAICT, sgx_encl_grow() would either return an ERR_PTR or the actual
> page...
> 

The "else if" is correct.  Version Array (VA) pages have 512 slots that hold
metadata for evicted EPC pages, i.e. swapping a page out of the EPC requires
a VA slot.  For simplicity (LOL), the approach we are taking for initial
support is to reserve a VA slot when adding a page to the enclave[*].  In most
cases, reserving a slot does not require allocating a new VA page, e.g. to
reserve slots 1-511 of the "current" VA page.   The if-elif is handling the
case where the current VA page is fully reserved and a new one needs to be
allocated. The if handles the error, the elif handles success, i.e.

	if (IS_ERR(va_page)) <- needed a new VA page, allocation failed
		return PTR_ERR(va_page);
	else if (va_page)    <- needed a new VA page, allocation succeeded
		list_add(&va_page->list, &encl->va_pages);
	else
		             <- reused the current VA page

When reusing a VA page, we obviously don't want to readd the page to the list
of va_pages, and the error handling path also shouldn't free the VA page.

> Also, should the growing happen *after* the SECS validation?
> 
> > +		list_add(&va_page->list, &encl->va_pages);
> > +
> >  	if (sgx_validate_secs(secs)) {
> >  		pr_debug("invalid SECS\n");
> > -		return -EINVAL;
> > +		ret = -EINVAL;
> > +		goto err_out_shrink;
> >  	}