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Date:   Sat, 15 Jun 2019 01:41:31 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>, x86@kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@amacapital.net>,
        David Howells <dhowells@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Kai Huang <kai.huang@linux.intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        Alison Schofield <alison.schofield@intel.com>,
        linux-mm@kvack.org, kvm@vger.kernel.org, keyrings@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH, RFC 13/62] x86/mm: Add hooks to allocate and free
 encrypted pages
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On Fri, Jun 14, 2019 at 03:43:35PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 14, 2019 at 04:28:36PM +0300, Kirill A. Shutemov wrote:
> > On Fri, Jun 14, 2019 at 01:04:58PM +0200, Peter Zijlstra wrote:
> > > On Fri, Jun 14, 2019 at 11:34:09AM +0200, Peter Zijlstra wrote:
> > > > On Wed, May 08, 2019 at 05:43:33PM +0300, Kirill A. Shutemov wrote:
> > > > 
> > > > > +		lookup_page_ext(page)->keyid = keyid;
> > > 
> > > > > +		lookup_page_ext(page)->keyid = 0;
> > > 
> > > Also, perhaps paranoid; but do we want something like:
> > > 
> > > static inline void page_set_keyid(struct page *page, int keyid)
> > > {
> > > 	/* ensure nothing creeps after changing the keyid */
> > > 	barrier();
> > > 	WRITE_ONCE(lookup_page_ext(page)->keyid, keyid);
> > > 	barrier();
> > > 	/* ensure nothing creeps before changing the keyid */
> > > }
> > > 
> > > And this is very much assuming there is no concurrency through the
> > > allocator locks.
> > 
> > There's no concurrency for this page: it has been off the free list, but
> > have not yet passed on to user. Nobody else sees the page before
> > allocation is finished.
> > 
> > And barriers/WRITE_ONCE() looks excessive to me. It's just yet another bit
> > of page's metadata and I don't see why it's has to be handled in a special
> > way.
> > 
> > Does it relax your paranoia? :P
> 
> Not really, it all 'works' because clflush_cache_range() includes mb()
> and page_address() has an address dependency on the store, and there are
> no other sites that will ever change 'keyid', which is all kind of
> fragile.

Hm. I don't follow how the mb() in clflush_cache_range() relevant...

Any following access of page's memory by kernel will go through
page_keyid() and therefore I believe there's always address dependency on
the store.

Am I missing something?

> At the very least that should be explicitly called out in a comment.
> 

-- 
 Kirill A. Shutemov