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

enh enh at google.com
Thu May 23 15:44:12 UTC 2019

On Thu, May 23, 2019 at 7:45 AM Catalin Marinas <catalin.marinas at arm.com> wrote:
> On Wed, May 22, 2019 at 01:47:36PM -0700, Kees Cook 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?
> Yes. I mentioned this just to make sure people don't expect different
> kernel builds for different hardware features.
> There is also the obvious requirement which I didn't mention: new user
> space continues to run on new/subsequent kernel versions. That's one of
> the points of contention for this series (ignoring MTE) with the
> maintainers having to guarantee this without much effort. IOW, do the
> 500K+ new lines in a subsequent kernel version break any user space out
> there? I'm only talking about the relaxed TBI ABI. Are the usual LTP,
> syskaller sufficient? Better static analysis would definitely help.
> > > 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?
> That's another aspect to figure out when we add the MTE support. I don't
> think we'd be able to do this without an explicit opt-in by the user.
> Or, if we ever want MTE to be turned on by default (i.e. tag checking),
> even if everything is tagged with 0, we have to disallow TBI for user
> and this includes hwasan. There were a small number of programs using
> the TBI (I think some JavaScript compilers tried this). But now we are
> bringing in the hwasan support and this can be a large user base. Shall
> we add an ELF note for such binaries that use TBI/hwasan?
> This series is still required for MTE but we may decide not to relax the
> ABI blindly, therefore the opt-in (prctl) or personality idea.
> > I feel like we got into the weeds about ioctl()s and one-off bugs...)
> This needs solving as well. Most driver developers won't know why
> untagged_addr() is needed unless we have more rigorous types or type
> annotations and a tool to check them (we should probably revive the old
> sparse thread).
> > 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)
> (I'd say tolerating rather than wiping since get_user still uses the tag
> in the current series)
> The current series does not allow any choice between 1 and 2, the
> default ABI basically becomes option 2.
> > 3- wiping the top bits for most things, but retaining them for MTE as
> >    needed (the future)
> 2 and 3 are not entirely compatible as a tagged pointer may be checked
> against the memory colour by the hardware. So you can't have hwasan
> binary with MTE enabled.
> > 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).
> The kernel handling of tagged pointers is indeed a prerequisite. The ABI
> distinction between the above 2 and 3 needs to be solved.
> > 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?
> Yes, mostly (for the kernel). If MTE is enabled by default for a hwasan
> binary, it will SEGFAULT (either in user space or in kernel uaccess).
> How does the kernel choose between 2 and 3?
> > 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?)
> Possibly, though not necessarily per process. For testing or if
> something goes wrong during boot, a command line option with a static
> label would do. The AT_FLAGS bit needs to be checked by user space. My
> preference would be per-process.
> > To choose between "2" and "3", it seems we need a per-process flag to
> > opt into TBI (and out of MTE).
> Or leave option 2 the default and get it to opt in to 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.
> The current plan is that a future binary issues a prctl(), after
> checking the HWCAP_MTE bit (as I replied to Elliot, the MTE instructions
> are not in the current NOP space). I'd expect this to be done by the
> libc or dynamic loader under the assumption that the binaries it loads
> do _not_ use the top pointer byte for anything else.

yeah, it sounds like to support hwasan and MTE, the dynamic linker
will need to not use either itself.

> With hwasan
> compiled objects this gets more confusing (any ELF note to identify
> them?).

no, at the moment code that wants to know checks for the presence of
__hwasan_init. (and bionic doesn't actually look at any ELF notes
right now.) but we can always add something if we need to.

> (there is also the risk of existing applications using TBI already but
> I'm not aware of any still using this feature other than hwasan)
> --
> Catalin

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