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

enh enh at google.com
Wed May 22 16:58:22 UTC 2019

On Wed, May 22, 2019 at 9:35 AM Catalin Marinas <catalin.marinas at arm.com> wrote:
> On Wed, May 22, 2019 at 08:30:21AM -0700, enh wrote:
> > On Wed, May 22, 2019 at 3:11 AM Catalin Marinas <catalin.marinas at arm.com> wrote:
> > > On Tue, May 21, 2019 at 05:04:39PM -0700, Kees Cook wrote:
> > > > I just want to make sure I fully understand your concern about this
> > > > being an ABI break, and I work best with examples. The closest situation
> > > > I can see would be:
> > > >
> > > > - some program has no idea about MTE
> > >
> > > Apart from some libraries like libc (and maybe those that handle
> > > specific device ioctls), I think most programs should have no idea about
> > > MTE. I wouldn't expect programmers to have to change their app just
> > > because we have a new feature that colours heap allocations.
> >
> > obviously i'm biased as a libc maintainer, but...
> >
> > i don't think it helps to move this to libc --- now you just have an
> > extra dependency where to have a guaranteed working system you need to
> > update your kernel and libc together. (or at least update your libc to
> > understand new ioctls etc _before_ you can update your kernel.)
> That's not what I meant (or I misunderstood you). If we have a relaxed
> ABI in the kernel and a libc that returns tagged pointers on malloc() I
> wouldn't expect the programmer to do anything different in the
> application code like explicit untagging. Basically the program would
> continue to run unmodified irrespective of whether you use an old libc
> without tagged pointers or a new one which tags heap allocations.
> What I do expect is that the libc checks for the presence of the relaxed
> ABI, currently proposed as an AT_FLAGS bit (for MTE we'd have a
> HWCAP_MTE), and only tag the malloc() pointers if the kernel supports
> the relaxed ABI. As you said, you shouldn't expect that the C library
> and kernel are upgraded together, so they should be able to work in any
> new/old version combination.

yes, that part makes sense. i do think we'd use the AT_FLAGS bit, for
exactly this.

i was questioning the argument about the ioctl issues, and saying that
from my perspective, untagging bugs are not really any different than
any other kind of kernel bug.

> > > > The trouble I see with this is that it is largely theoretical and
> > > > requires part of userspace to collude to start using a new CPU feature
> > > > that tickles a bug in the kernel. As I understand the golden rule,
> > > > this is a bug in the kernel (a missed ioctl() or such) to be fixed,
> > > > not a global breaking of some userspace behavior.
> > >
> > > Yes, we should follow the rule that it's a kernel bug but it doesn't
> > > help the user that a newly installed kernel causes user space to no
> > > longer reach a prompt. Hence the proposal of an opt-in via personality
> > > (for MTE we would need an explicit opt-in by the user anyway since the
> > > top byte is no longer ignored but checked against the allocation tag).
> >
> > but realistically would this actually get used in this way? or would
> > any given system either be MTE or non-MTE. in which case a kernel
> > configuration option would seem to make more sense. (because either
> > way, the hypothetical user basically needs to recompile the kernel to
> > get back on their feet. or all of userspace.)
> 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.
> The distro user space usually has a hard requirement that it continues
> to run on (certain) old hardware. We can't enforce this in the kernel
> but we offer the option to user space developers of checking feature
> availability through HWCAP bits.
> The Android story may be different as you have more control about which
> kernel configurations are deployed on specific SoCs. I'm looking more
> from a Linux distro angle where you just get an off-the-shelf OS image
> and install it on your hardware, either taking advantage of new features
> or just not using them if the software was not updated. Or, if updated
> software is installed on old hardware, it would just run.
> 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.
> 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.

(a bit off-topic, but i thought the MTE instructions were encoded in
the no-op space, to avoid this?)

> > i'm not sure i see this new way for a kernel update to break my system
> > and need to be fixed forward/rolled back as any different from any of
> > the existing ways in which this can happen :-) as an end-user i have
> > to rely on whoever's sending me software updates to test adequately
> > enough that they find the problems. as an end user, there isn't any
> > difference between "my phone rebooted when i tried to take a photo
> > because of a kernel/driver leak", say, and "my phone rebooted when i
> > tried to take a photo because of missing untagging of a pointer passed
> > via ioctl".
> >
> > i suspect you and i have very different people in mind when we say "user" :-)
> Indeed, I think we have different users in mind. I didn't mean the end
> user who doesn't really care which C library version it's running on
> their phone but rather advanced users (not necessarily kernel
> developers) that prefer to build their own kernels with every release.
> We could extend this to kernel developers who don't have time to track
> down why a new kernel triggers lots of SIGSEGVs during boot.

i still don't see how this isn't just a regular testing/CI issue, the
same as any other kind of kernel bug. it's already the case that i can
get a bad kernel...

> --
> Catalin

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