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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 13 Jun 2019 09:16:12 -0700
Message-ID: <CALCETrUH4xDeyJh9N19Pf4k5ibRG7phCJy8PEfiJbvr6WZL0MA@mail.gmail.com>
Subject: Re: [RFC 00/10] Process-local memory allocations for hiding KVM secrets
To:     Nadav Amit <namit@vmware.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Marius Hillenbrand <mhillenb@amazon.de>,
        kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Linux-MM <linux-mm@kvack.org>, Alexander Graf <graf@amazon.de>,
        David Woodhouse <dwmw@amazon.co.uk>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
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On Wed, Jun 12, 2019 at 6:50 PM Nadav Amit <namit@vmware.com> wrote:
>
> > On Jun 12, 2019, at 6:30 PM, Andy Lutomirski <luto@kernel.org> wrote:
> >
> > On Wed, Jun 12, 2019 at 1:27 PM Andy Lutomirski <luto@amacapital.net> w=
rote:
> >>> On Jun 12, 2019, at 12:55 PM, Dave Hansen <dave.hansen@intel.com> wro=
te:
> >>>
> >>>> On 6/12/19 10:08 AM, Marius Hillenbrand wrote:
> >>>> This patch series proposes to introduce a region for what we call
> >>>> process-local memory into the kernel's virtual address space.
> >>>
> >>> It might be fun to cc some x86 folks on this series.  They might have
> >>> some relevant opinions. ;)
> >>>
> >>> A few high-level questions:
> >>>
> >>> Why go to all this trouble to hide guest state like registers if all =
the
> >>> guest data itself is still mapped?
> >>>
> >>> Where's the context-switching code?  Did I just miss it?
> >>>
> >>> We've discussed having per-cpu page tables where a given PGD is only =
in
> >>> use from one CPU at a time.  I *think* this scheme still works in suc=
h a
> >>> case, it just adds one more PGD entry that would have to context-swit=
ched.
> >>
> >> Fair warning: Linus is on record as absolutely hating this idea. He mi=
ght change his mind, but it=E2=80=99s an uphill battle.
> >
> > I looked at the patch, and it (sensibly) has nothing to do with
> > per-cpu PGDs.  So it's in great shape!
> >
> > Seriously, though, here are some very high-level review comments:
> >
> > Please don't call it "process local", since "process" is meaningless.
> > Call it "mm local" or something like that.
> >
> > We already have a per-mm kernel mapping: the LDT.  So please nix all
> > the code that adds a new VA region, etc, except to the extent that
> > some of it consists of valid cleanups in and of itself.  Instead,
> > please refactor the LDT code (arch/x86/kernel/ldt.c, mainly) to make
> > it use a more general "mm local" address range, and then reuse the
> > same infrastructure for other fancy things.  The code that makes it
> > KASLR-able should be in its very own patch that applies *after* the
> > code that makes it all work so that, when the KASLR part causes a
> > crash, we can bisect it.
> >
> > + /*
> > + * Faults in process-local memory may be caused by process-local
> > + * addresses leaking into other contexts.
> > + * tbd: warn and handle gracefully.
> > + */
> > + if (unlikely(fault_in_process_local(address))) {
> > + pr_err("page fault in PROCLOCAL at %lx", address);
> > + force_sig_fault(SIGSEGV, SEGV_MAPERR, (void __user *)address, current=
);
> > + }
> > +
> >
> > Huh?  Either it's an OOPS or you shouldn't print any special
> > debugging.  As it is, you're just blatantly leaking the address of the
> > mm-local range to malicious user programs.
> >
> > Also, you should IMO consider using this mechanism for kmap_atomic().
> > Hi, Nadav!
>
> Well, some context for the =E2=80=9Chi=E2=80=9D would have been helpful. =
(Do I have a bug
> and I still don=E2=80=99t understand it?)

Fair enough :)

>
> Perhaps you regard some use-case for a similar mechanism that I mentioned
> before. I did implement something similar (but not the way that you wante=
d)
> to improve the performance of seccomp and system-calls when retpolines ar=
e
> used. I set per-mm code area that held code that used direct calls to inv=
oke
> seccomp filters and frequently used system-calls.
>
> My mechanism, I think, is more not suitable for this use-case. I needed m=
y
> code-page to be at the same 2GB range as the kernel text/modules, which d=
oes
> complicate things. Due to the same reason, it is also limited in the size=
 of
> the data/code that it can hold.
>

I actually meant the opposite.  If we had a general-purpose per-mm
kernel address range, could it be used to optimize kmap_atomic() by
limiting the scope of any shootdowns?  As a rough sketch, we'd have
some kmap_atomic slots for each cpu *in the mm-local region*.  I'm not
entirely sure this is a win.

--Andy