Re: [PATCH v17 07/10] mm: introduce memfd_secret system call to create "secret" memory areas

From: James Bottomley <jejb@linux.ibm.com>
To: David Hildenbrand <david@redhat.com>, Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andy Lutomirski <luto@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Borislav Petkov <bp@alien8.de>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Christopher Lameter <cl@linux.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Elena Reshetova <elena.reshetova@intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Matthew Wilcox <willy@infradead.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
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	Shuah Khan <shuah@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Tycho Andersen <tycho@tycho.ws>, Will Deacon <will@kernel.org>,
	linux-api@vger.kernel.org, linux-arch@vger.kernel.org,
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	x86@kernel.org, Hagen Paul Pfeifer <hagen@jauu.net>,
	Palmer Dabbelt <palmerdabbelt@google.com>
Subject: Re: [PATCH v17 07/10] mm: introduce memfd_secret system call to create "secret" memory areas
Date: Wed, 17 Feb 2021 08:19:26 -0800	[thread overview]
Message-ID: <b58debfe598331791ecc238a6bf8d2cf1762203a.camel@linux.ibm.com> (raw)
In-Reply-To: <5a8567a9-6940-c23f-0927-e4b5c5db0d5e@redhat.com>

On Tue, 2021-02-16 at 18:16 +0100, David Hildenbrand wrote:
[...]
> > >   The discussion regarding migratability only really popped up
> > > because this is a user-visible thing and not being able to
> > > migrate can be a real problem (fragmentation, ZONE_MOVABLE, ...).
> > 
> > I think the biggest use will potentially come from hardware
> > acceleration.  If it becomes simple to add say encryption to a
> > secret page with no cost, then no flag needed.  However, if we only
> > have a limited number of keys so once we run out no more encrypted
> > memory then it becomes a costly resource and users might want a
> > choice of being backed by encryption or not.
> 
> Right. But wouldn't HW support with configurable keys etc. need more 
> syscall parameters (meaning, even memefd_secret() as it is would not
> be sufficient?). I suspect the simplistic flag approach might not
> be sufficient. I might be wrong because I have no clue about MKTME
> and friends.

The theory I was operating under is key management is automatic and
hidden, but key scarcity can't be, so if you flag requesting hardware
backing then you either get success (the kernel found a key) or failure
(the kernel is out of keys).  If we actually want to specify the key
then we need an extra argument and we *must* have a new system call.

> Anyhow, I still think extending memfd_create() might just be good
> enough - at least for now.

I really think this is the wrong approach for a user space ABI.  If we
think we'll ever need to move to a separate syscall, we should begin
with one.  The pain of trying to shift userspace from memfd_create to a
new syscall would be enormous.  It's not impossible (see clone3) but
it's a pain we should avoid if we know it's coming.

>  Things like HW support might have requirements we don't even know
> yet and that we cannot even model in memfd_secret() right now.

This is the annoying problem with our Linux unbreakable ABI policy: we
get to plan when the ABI is introduced for stuff we don't yet even know
about.

James