Re: [PATCH 21/35] arm64: mte: Add in-kernel tag fault handler

[Evgenii Stepanov]

On Thu, Aug 27, 2020 at 7:56 AM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>
> On Thu, Aug 27, 2020 at 03:34:42PM +0200, Andrey Konovalov wrote:
> > On Thu, Aug 27, 2020 at 3:10 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
> > > On Thu, Aug 27, 2020 at 02:31:23PM +0200, Andrey Konovalov wrote:
> > > > On Thu, Aug 27, 2020 at 11:54 AM Catalin Marinas
> > > > <catalin.marinas@xxxxxxx> wrote:
> > > > > On Fri, Aug 14, 2020 at 07:27:03PM +0200, Andrey Konovalov wrote:
> > > > > > +static int do_tag_recovery(unsigned long addr, unsigned int esr,
> > > > > > +                        struct pt_regs *regs)
> > > > > > +{
> > > > > > +     report_tag_fault(addr, esr, regs);
> > > > > > +
> > > > > > +     /* Skip over the faulting instruction and continue: */
> > > > > > +     arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);
> > > > >
> > > > > Ooooh, do we expect the kernel to still behave correctly after this? I
> > > > > thought the recovery means disabling tag checking altogether and
> > > > > restarting the instruction rather than skipping over it.
> [...]
> > > > Can we disable MTE, reexecute the instruction, and then reenable MTE,
> > > > or something like that?
> > >
> > > If you want to preserve the MTE enabled, you could single-step the
> > > instruction or execute it out of line, though it's a bit more convoluted
> > > (we have a similar mechanism for kprobes/uprobes).
> > >
> > > Another option would be to attempt to set the matching tag in memory,
> > > under the assumption that it is writable (if it's not, maybe it's fine
> > > to panic). Not sure how this interacts with the slub allocator since,
> > > presumably, the logical tag in the pointer is wrong rather than the
> > > allocation one.
> > >
> > > Yet another option would be to change the tag in the register and
> > > re-execute but this may confuse the compiler.
> >
> > Which one of these would be simpler to implement?
>
> Either 2 or 3 would be simpler (re-tag the memory location or the
> pointer) with the caveats I mentioned. Also, does the slab allocator
> need to touch the memory on free with a tagged pointer? Otherwise slab
> may hit an MTE fault itself.

Changing the memory tag can cause faults in other threads, and that
could be very confusing.
Probably the safest thing is to retag the register, single step and
then retag it back, but be careful with the instructions that change
the address register (like ldr x0, [x0]).

>
> > Perhaps we could somehow only skip faulting instructions that happen
> > in the KASAN test module?.. Decoding stack trace would be an option,
> > but that's a bit weird.
>
> If you want to restrict this to the KASAN tests, just add some
> MTE-specific accessors with a fixup entry similar to get_user/put_user.
> __do_kernel_fault() (if actually called) will invoke the fixup code
> which skips the access and returns an error. This way KASAN tests can
> actually verify that tag checking works, I'd find this a lot more
> useful.
>
> --
> Catalin