Subject: Re: [PATCH v15 00/17] arm64: untag user pointers passed to the kernel

From: Kees Cook <[email-masked]>
Date: Wed, 22 May 2019 13:47:36 -0700

On Wed, May 22, 2019 at 05:35:27PM +0100, Catalin Marinas wrote:
> The two hard requirements I have for supporting any new hardware feature
> in Linux are (1) a single kernel image binary continues to run on old
> hardware while making use of the new feature if available and (2) old
> user space continues to run on new hardware while new user space can
> take advantage of the new feature.

Agreed! And I think the series meets these requirements, yes?

> For MTE, we just can't enable it by default since there are applications
> who use the top byte of a pointer and expect it to be ignored rather
> than failing with a mismatched tag. Just think of a hwasan compiled
> binary where TBI is expected to work and you try to run it with MTE
> turned on.

Ah! Okay, here's the use-case I wasn't thinking of: the concern is TBI
conflicting with MTE. And anything that starts using TBI suddenly can't
run in the future because it's being interpreted as MTE bits? (Is that
the ABI concern? I feel like we got into the weeds about ioctl()s and
one-off bugs...)

So there needs to be some way to let the kernel know which of three
things it should be doing:
1- leaving userspace addresses as-is (present)
2- wiping the top bits before using (this series)
3- wiping the top bits for most things, but retaining them for MTE as
   needed (the future)

I expect MTE to be the "default" in the future. Once a system's libc has
grown support for it, everything will be trying to use MTE. TBI will be
the special case (but TBI is effectively a prerequisite).

AFAICT, the only difference I see between 2 and 3 will be the tag handling
in usercopy (all other places will continue to ignore the top bits). Is
that accurate?

Is "1" a per-process state we want to keep? (I assume not, but rather it
is available via no TBI/MTE CONFIG or a boot-time option, if at all?)

To choose between "2" and "3", it seems we need a per-process flag to
opt into TBI (and out of MTE). For userspace, how would a future binary
choose TBI over MTE? If it's a library issue, we can't use an ELF bit,
since the choice may be "late" after ELF load (this implies the need
for a prctl().) If it's binary-only ("built with HWKASan") then an ELF
bit seems sufficient. And without the marking, I'd expect the kernel to
enforce MTE when there are high bits.

> I would also expect the C library or dynamic loader to check for the
> presence of a HWCAP_MTE bit before starting to tag memory allocations,
> otherwise it would get SIGILL on the first MTE instruction it tries to
> execute.

I've got the same question as Elliot: aren't MTE instructions just NOP
to older CPUs? I.e. if the CPU (or kernel) don't support it, it just
gets entirely ignored: checking is only needed to satisfy curiosity
or behavioral expectations.

To me, the conflict seems to be using TBI in the face of expecting MTE to
be the default state of the future. (But the internal changes needed
for TBI -- this series -- is a prereq for MTE.)

-- 
Kees Cook

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