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From: enh <enh@google.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Kees Cook <keescook@chromium.org>,
	Evgenii Stepanov <eugenis@google.com>,
	Andrey Konovalov <andreyknvl@google.com>,
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	Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
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	Kevin Brodsky <kevin.brodsky@arm.com>,
	Szabolcs Nagy <Szabolcs.Nagy@arm.com>
Subject: Re: [PATCH v15 00/17] arm64: untag user pointers passed to the kernel
Date: Thu, 23 May 2019 08:44:12 -0700
Message-ID: <CAJgzZoqX--Kd9=Kjpnfz-5cjVJ=TdsXM5dJM_EjLFKniVbny2w@mail.gmail.com> (raw)
In-Reply-To: <20190523144449.waam2mkyzhjpqpur@mbp>

On Thu, May 23, 2019 at 7:45 AM Catalin Marinas <catalin.marinas@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

  reply index

Thread overview: 99+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-06 16:30 Andrey Konovalov
2019-05-06 16:30 ` [PATCH v15 01/17] uaccess: add untagged_addr definition for other arches Andrey Konovalov
2019-05-29 14:49   ` Khalid Aziz
2019-05-06 16:30 ` [PATCH v15 02/17] arm64: untag user pointers in access_ok and __uaccess_mask_ptr Andrey Konovalov
2019-05-06 16:30 ` [PATCH v15 03/17] lib, arm64: untag user pointers in strn*_user Andrey Konovalov
2019-05-22 10:41   ` Catalin Marinas
2019-05-06 16:30 ` [PATCH v15 04/17] mm: add ksys_ wrappers to memory syscalls Andrey Konovalov
2019-05-22 10:56   ` Catalin Marinas
2019-05-06 16:30 ` [PATCH v15 05/17] arms64: untag user pointers passed " Andrey Konovalov
2019-05-22 11:49   ` Catalin Marinas
2019-05-22 21:16     ` Evgenii Stepanov
2019-05-23  9:04       ` Catalin Marinas
2019-05-24  4:23         ` Evgenii Stepanov
2019-05-24 15:41   ` Andrew Murray
2019-05-25  9:57   ` Catalin Marinas
2019-05-27  9:42   ` Catalin Marinas
2019-05-27 14:37   ` Catalin Marinas
2019-05-28 14:54     ` Andrew Murray
2019-05-28 15:40       ` Catalin Marinas
2019-05-28 15:56         ` Dave Martin
2019-05-28 16:34           ` Catalin Marinas
2019-05-29 12:42             ` Dave Martin
2019-05-29 13:23               ` Catalin Marinas
2019-05-29 15:18                 ` Dave Martin
2019-05-28 23:33         ` Khalid Aziz
2019-05-29 14:20           ` Catalin Marinas
2019-05-29 19:16             ` Khalid Aziz
2019-05-30 15:11               ` Catalin Marinas
2019-05-30 16:05                 ` Khalid Aziz
2019-05-30 16:57                   ` Catalin Marinas
2019-05-28 13:05   ` Catalin Marinas
2019-05-06 16:30 ` [PATCH v15 06/17] mm: untag user pointers in do_pages_move Andrey Konovalov
2019-05-22 11:51   ` Catalin Marinas
2019-05-06 16:30 ` [PATCH v15 07/17] mm, arm64: untag user pointers in mm/gup.c Andrey Konovalov
2019-05-22 11:56   ` Catalin Marinas
2019-05-06 16:30 ` [PATCH v15 08/17] mm, arm64: untag user pointers in get_vaddr_frames Andrey Konovalov
2019-05-06 16:30 ` [PATCH v15 09/17] fs, arm64: untag user pointers in copy_mount_options Andrey Konovalov
2019-05-22 12:09   ` Catalin Marinas
2019-05-06 16:30 ` [PATCH v15 10/17] fs, arm64: untag user pointers in fs/userfaultfd.c Andrey Konovalov
2019-05-06 16:30 ` [PATCH v15 11/17] drm/amdgpu, arm64: untag user pointers Andrey Konovalov
2019-05-07 16:43   ` Kuehling, Felix
2019-05-06 16:30 ` [PATCH v15 12/17] drm/radeon, arm64: untag user pointers in radeon_gem_userptr_ioctl Andrey Konovalov
2019-05-07 16:44   ` Kuehling, Felix
2019-05-06 16:30 ` [PATCH v15 13/17] IB, arm64: untag user pointers in ib_uverbs_(re)reg_mr() Andrey Konovalov
2019-05-06 19:50   ` Jason Gunthorpe
2019-05-07  6:33     ` Leon Romanovsky
2019-05-06 16:31 ` [PATCH v15 14/17] media/v4l2-core, arm64: untag user pointers in videobuf_dma_contig_user_get Andrey Konovalov
2019-05-24 13:13   ` Mauro Carvalho Chehab
2019-05-06 16:31 ` [PATCH v15 15/17] tee, arm64: untag user pointers in tee_shm_register Andrey Konovalov
2019-05-06 16:31 ` [PATCH v15 16/17] vfio/type1, arm64: untag user pointers in vaddr_get_pfn Andrey Konovalov
2019-05-06 16:31 ` [PATCH v15 17/17] selftests, arm64: add a selftest for passing tagged pointers to kernel Andrey Konovalov
2019-05-22 14:16   ` Catalin Marinas
2019-05-31 14:21     ` Andrey Konovalov
2019-05-31 16:22       ` Catalin Marinas
2019-05-17 14:49 ` [PATCH v15 00/17] arm64: untag user pointers passed to the kernel Catalin Marinas
2019-05-20 23:53   ` Evgenii Stepanov
2019-05-21 18:29     ` Catalin Marinas
2019-05-22  0:04       ` Kees Cook
2019-05-22 10:11         ` Catalin Marinas
2019-05-22 15:30           ` enh
2019-05-22 16:35             ` Catalin Marinas
2019-05-22 16:58               ` enh
2019-05-23 15:21                 ` Catalin Marinas
2019-05-22 20:47               ` Kees Cook
2019-05-22 23:03                 ` Evgenii Stepanov
2019-05-22 23:09                   ` enh
2019-05-23  7:34                     ` Catalin Marinas
2019-05-23 14:44                 ` Catalin Marinas
2019-05-23 15:44                   ` enh [this message]
2019-05-23 17:00                     ` Catalin Marinas
2019-05-23 16:38                   ` Kees Cook
2019-05-23 17:43                     ` Catalin Marinas
2019-05-23 21:31                       ` Kees Cook
2019-05-24 11:20                         ` Catalin Marinas
2019-05-28 17:02                         ` Catalin Marinas
2019-06-02  5:06                           ` Kees Cook
2019-05-22 19:21             ` Kees Cook
2019-05-22 20:15               ` enh
2019-05-23 15:08               ` Catalin Marinas
2019-05-23 17:51         ` Khalid Aziz
2019-05-23 20:11           ` Catalin Marinas
2019-05-23 21:42             ` Khalid Aziz
2019-05-23 21:49             ` Khalid Aziz
2019-05-24 10:11               ` Catalin Marinas
2019-05-24 14:25                 ` Khalid Aziz
2019-05-28 14:14                   ` Andrey Konovalov
2019-05-29  6:11                     ` Christoph Hellwig
2019-05-29 12:12                       ` Catalin Marinas
2019-05-30 17:15                     ` Catalin Marinas
2019-05-31 14:29                       ` Andrey Konovalov
2019-05-31 16:19                         ` Catalin Marinas
2019-05-31 16:24                           ` Andrey Konovalov
2019-05-31 16:46                             ` Catalin Marinas
2019-05-21 18:48   ` Jason Gunthorpe
2019-05-22 13:49     ` Dave Martin
2019-05-23  0:20       ` Jason Gunthorpe
2019-05-23 10:42         ` Dave Martin
2019-05-23 16:57           ` Catalin Marinas
2019-05-24 14:23             ` Dave Martin

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