[PATCH 24/35] arm64: mte: Switch GCR_EL1 in kernel entry and exit

[Andrey Konovalov]

On Tue, Sep 8, 2020 at 4:02 PM Andrey Konovalov <andreyknvl at google.com> wrote:
>
> On Thu, Aug 27, 2020 at 2:16 PM Catalin Marinas <catalin.marinas at arm.com> wrote:
> >
> > On Thu, Aug 27, 2020 at 11:56:49AM +0100, Vincenzo Frascino wrote:
> > > On 8/27/20 11:38 AM, Catalin Marinas wrote:
> > > > On Fri, Aug 14, 2020 at 07:27:06PM +0200, Andrey Konovalov wrote:
> > > >> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> > > >> index 7717ea9bc2a7..cfac7d02f032 100644
> > > >> --- a/arch/arm64/kernel/mte.c
> > > >> +++ b/arch/arm64/kernel/mte.c
> > > >> @@ -18,10 +18,14 @@
> > > >>
> > > >>  #include <asm/barrier.h>
> > > >>  #include <asm/cpufeature.h>
> > > >> +#include <asm/kasan.h>
> > > >> +#include <asm/kprobes.h>
> > > >>  #include <asm/mte.h>
> > > >>  #include <asm/ptrace.h>
> > > >>  #include <asm/sysreg.h>
> > > >>
> > > >> +u64 gcr_kernel_excl __read_mostly;
> > > >
> > > > Could we make this __ro_after_init?
> > >
> > > Yes, it makes sense, it should be updated only once through mte_init_tags().
> > >
> > > Something to consider though here is that this might not be the right approach
> > > if in future we want to add stack tagging. In such a case we need to know the
> > > kernel exclude mask before any C code is executed. Initializing the mask via
> > > mte_init_tags() it is too late.
> >
> > It depends on how stack tagging ends up in the kernel, whether it uses
> > ADDG/SUBG or not. If it's only IRG, I think it can cope with changing
> > the GCR_EL1.Excl in the middle of a function.
> >
> > > I was thinking to add a compilation define instead of having gcr_kernel_excl in
> > > place. This might not work if the kernel excl mask is meant to change during the
> > > execution.
> >
> > A macro with the default value works for me. That's what it basically is
> > currently, only that it ends up in a variable.
>
> Some thoughts on the topic: gcr_kernel_excl is currently initialized
> in mte_init_tags() and depends on the max_tag value dynamically
> provided to it, so it's not something that can be expressed with a
> define. In the case of KASAN the max_tag value is static, but if we
> rely on that we make core MTE code depend on KASAN, which doesn't seem
> right from the design perspective.

Thinking more about this, I think we've actually discussed moving
KASAN_MAX_TAG to somewhere in low-level headers, so I guess we can
reuse that and make gcr_kernel_excl a define. I'll look into this.