linux-kernel - Re: [PATCH v17 08/10] PM: hibernate: disable when there are active secretmem users

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Date:   Mon, 22 Feb 2021 11:21:50 -0800
From:   James Bottomley <jejb@...ux.ibm.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        Mike Rapoport <rppt@...nel.org>
Cc:     Matthew Garrett <mjg59@...f.ucam.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christopher Lameter <cl@...ux.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Michal Hocko <mhocko@...e.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>, Will Deacon <will@...nel.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-kselftest@...r.kernel.org,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        linux-riscv@...ts.infradead.org, X86 ML <x86@...nel.org>,
        Hagen Paul Pfeifer <hagen@...u.net>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>
Subject: Re: [PATCH v17 08/10] PM: hibernate: disable when there are active
 secretmem users

On Mon, 2021-02-22 at 11:17 -0800, Dan Williams wrote:
> On Mon, Feb 22, 2021 at 2:24 AM Mike Rapoport <rppt@...nel.org>
> wrote:
> > On Mon, Feb 22, 2021 at 07:34:52AM +0000, Matthew Garrett wrote:
> > > On Mon, Feb 08, 2021 at 10:49:18AM +0200, Mike Rapoport wrote:
> > > 
> > > > It is unsafe to allow saving of secretmem areas to the
> > > > hibernation snapshot as they would be visible after the resume
> > > > and this essentially will defeat the purpose of secret memory
> > > > mappings.
> > > 
> > > Sorry for being a bit late to this - from the point of view of
> > > running processes (and even the kernel once resume is complete),
> > > hibernation is effectively equivalent to suspend to RAM. Why do
> > > they need to be handled differently here?
> > 
> > Hibernation leaves a copy of the data on the disk which we want to
> > prevent.
> 
> Why not document that users should use data at rest protection
> mechanisms for their hibernation device? Just because secretmem can't
> assert its disclosure guarantee does not mean the hibernation device
> is untrustworthy.

It's not just data at rest.  Part of the running security guarantees
are that the kernel never gets access to the data.  To support
hibernate and swap we have to break that, so it reduces the runtime
security posture as well as the data at rest one.

This argues we could do it with a per region flags (something like less
secure or more secure mappings), but when you give users that choice
most of them rarely choose less secure.

James