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

Evgenii Stepanov eugenis at google.com
Wed May 22 23:03:36 UTC 2019

On Wed, May 22, 2019 at 1:47 PM Kees Cook <keescook at chromium.org> wrote:
> 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.

MTE instructions are not NOP. Most of them have side effects (changing
register values, zeroing memory).
This only matters for stack tagging, though. Heap tagging is a runtime
decision in the allocator.

If an image needs to run on old hardware, it will have to do heap tagging only.

> 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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