Re: [RFC][PATCH 0/3] arm64 relaxed ABI

From: Kevin Brodsky
Date: Wed Feb 13 2019 - 11:42:21 EST

(+Cc other people with MTE experience: Branislav, Ruben)

On 13/02/2019 14:58, Dave Martin wrote:
On Tue, Feb 12, 2019 at 06:02:24PM +0000, Catalin Marinas wrote:
On Mon, Feb 11, 2019 at 12:32:55PM -0800, Evgenii Stepanov wrote:
On Mon, Feb 11, 2019 at 9:28 AM Kevin Brodsky <kevin.brodsky@xxxxxxx> wrote:
On 19/12/2018 12:52, Dave Martin wrote:

* A single C object should be accessed using a single, fixed
pointer tag throughout its entire lifetime.
Agreed. Allocators themselves may need to be excluded though,
depending on how they represent their managed memory.

* Tags can be changed only when there are no outstanding pointers to
the affected object or region that may be used to access the object
or region (i.e., if the object were allocated from the C heap and
is it safe to realloc() it, then it is safe to change the tag; for
other types of allocation, analogous arguments can be applied).
Tags can only be changed at the point of deallocation/
reallocation. Pointers to the object become invalid and cannot
be used after it has been deallocated; memory tagging allows to
catch such invalid usage.
All the above sound well but that's mostly a guideline on what a C
library can do. It doesn't help much with defining the kernel ABI.
Anyway, it's good to clarify the use-cases.
My aim was to clarify the use case in userspace, since I wasn't directly
involved in that. The kernel ABI needs to be compatible with the the
use case, but doesn't need to specify must of it.

I'm wondering whether we can piggy-back on existing concepts.

We could say that recolouring memory is safe when and only when
unmapping of the page or removing permissions on the page (via
munmap/mremap/mprotect) would be safe. Otherwise, the resulting
behaviour of the process is undefined.

Is that a sufficient requirement? I don't think that anything prevents you from using mprotect() on say [vvar], but we don't necessarily want to map [vvar] as tagged. I'm not sure it's easy to define what "safe" would mean here.

Hopefully there are friendly fuzzers testing this kind of thing.


It would also be valuable to narrow down the set of "relaxed" (i.e.
not tag-checking) syscalls as reasonably possible. We would want to
provide tag-checking userspace wrappers for any important calls that
are not checked in the kernel. Is it correct to assume that anything
that goes through copy_from_user / copy_to_user is checked?
I lost track of the context of this thread but if it's just about
relaxing the ABI for hwasan, the kernel has no idea of the compiler
generated structures in user space, so nothing is checked.

If we talk about tags in the context of MTE, than yes, with the current
proposal the tag would be checked by copy_*_user() functions.
Also put_user() and friends?

It might be reasonable to do the check in access_ok() and skip it in
__put_user() etc.

(I seem to remember some separate discussion about abolishing
__put_user() and friends though, due to the accident risk they pose.)

Keep in mind that with MTE, there is no need to do any explicit check when accessing user memory via a user-provided pointer. The tagged user pointer is directly passed to copy_*_user() or put_user(). If the load/store causes a tag fault, then it is handled just like a page fault (i.e. invoking the fixup handler). As far as I can tell, there's no need to do anything special in access_ok() in that case.

[The above applies to precise mode. In imprecise mode, some more work will be needed after the load/store to check whether a tag fault happened.]

For aio* operations it would be nice if the tag was checked at the
time of the actual userspace read/write, either instead of or in
addition to at the time of the system call.
With aio* (and synchronous iovec-based syscalls), the kernel may access
the memory while the corresponding user process is scheduled out. Given
that such access is not done in the context of the user process (and
using the user VA like copy_*_user), the kernel cannot handle potential
tag faults. Moreover, the transfer may be done by DMA and the device
does not understand tags.

I'd like to keep tags as a property of the pointer in a specific virtual
address space. The moment you convert it to a different address space
(e.g. kernel linear map, physical address), the tag property is stripped
and I don't think we should re-build it (and have it checked).
This is probably reasonable.

Ideally we would check the tag at the point of stripping it off, but
most likely it's going to be rather best-effort.

If memory tagging is essential a debugging feature then this seems
an acceptable compromise.

There are many possible ways to deploy MTE, and debugging is just one of them. For instance, you may want to turn on heap colouring for some processes in the system, including in production.

Regarding those cases where it is impossible to check tags at the point of accessing user memory, it is indeed possible to check the memory tags at the point of stripping the tag from the user pointer. Given that some MTE use-cases favour performance over tag check coverage, the ideal approach would be to make these checks configurable (e.g. check one granule, check all of them, or check none). I don't know how feasible this is in practice.


* For purposes other than dereference, the kernel shall accept any
legitimately tagged pointer (according to the above rules) as
identifying the associated memory location.

So, mprotect(some_page_aligned_object, ...); is valid irrespective
of where page_aligned_object() came from. There is no implicit
derefence by the kernel here, hence no tag check.

The kernel does not guarantee to work correctly if the wrong tag
is used, but there is not always a well-defined "right" tag, so
we can't really guarantee to check it. So a pointer derived by
any reasonable means by userspace has to be treated as equally
This is a disputed point :) In my opinion, this is the the most
reasonable approach.
Yes, it would be nice if the kernel explicitly promised, ex.
mprotect() over a range of differently tagged pages to be allowed
(i.e. address tag should be unchecked).
I don't think mprotect() over differently tagged pages was ever a
problem. I originally asked that mprotect() and friends do not accept
tagged pointers since these functions deal with memory ranges rather
than dereferencing such pointer (the reason being minimal kernel
changes). However, given how complicated it is to specify an ABI, I came
to the conclusion that a pointer passed to such function should be
allowed to have non-zero top byte. It would be the kernel's
responsibility to strip it out as appropriate.
I think that if the page range is all the same colour then it should be
legitimate to pass a matching tag.

But it doesn't seem reasonable for the kernel to require this. If
free() calls munmap(), the page(s) will contain possibly randomly-
coloured garbage. There's no correct tag to pass in such a case.

The most obvious solution is just to ignore the tags passed by userspace
to such syscalls. This would imply that the kernel must explicitly
strip it out, as you suggest.

The number of affected syscalls is relatively small though.