Re: [PATCH v2 2/3] efi: arm64: Wire up BTI annotation in memory attributes table

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Feb 08, 2023 at 02:03:45PM +0100, Ard Biesheuvel wrote:
> On Wed, 8 Feb 2023 at 14:00, Will Deacon <will@kernel.org> wrote:
> >
> > On Mon, Feb 06, 2023 at 01:49:37PM +0100, Ard Biesheuvel wrote:
> > > UEFI v2.10 extends the EFI memory attributes table with a flag that
> > > indicates whether or not all RuntimeServicesCode regions were
> > > constructed with BTI landing pads, permitting the OS to map these
> > > regions with BTI restrictions enabled.
> > >
> > > So let's take this into account on arm64.
> > >
> > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > > Reviewed-by: Kees Cook <keescook@chromium.org>
> > > ---
> > >  arch/arm64/kernel/efi.c   | 14 ++++++++++++--
> > >  arch/arm64/kernel/traps.c |  6 ++++++
> > >  2 files changed, 18 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/arm64/kernel/efi.c b/arch/arm64/kernel/efi.c
> > > index 78ffd5aaddcbbaee..99971cd349f36310 100644
> > > --- a/arch/arm64/kernel/efi.c
> > > +++ b/arch/arm64/kernel/efi.c
> > > @@ -96,15 +96,23 @@ int __init efi_create_mapping(struct mm_struct *mm, efi_memory_desc_t *md)
> > >       return 0;
> > >  }
> > >
> > > +struct set_perm_data {
> > > +     const efi_memory_desc_t *md;
> > > +     bool                    has_bti;
> > > +};
> > > +
> > >  static int __init set_permissions(pte_t *ptep, unsigned long addr, void *data)
> > >  {
> > > -     efi_memory_desc_t *md = data;
> > > +     struct set_perm_data *spd = data;
> > > +     const efi_memory_desc_t *md = spd->md;
> > >       pte_t pte = READ_ONCE(*ptep);
> > >
> > >       if (md->attribute & EFI_MEMORY_RO)
> > >               pte = set_pte_bit(pte, __pgprot(PTE_RDONLY));
> > >       if (md->attribute & EFI_MEMORY_XP)
> > >               pte = set_pte_bit(pte, __pgprot(PTE_PXN));
> > > +     else if (system_supports_bti() && spd->has_bti)
> >
> > system_supports_bti() seems to check CONFIG_ARM64_BTI rather than
> > CONFIG_ARM64_BTI_KERNEL. In theory, I think this means we could have
> > mismatched BTI support, so it might be slightly more robust to use the
> > latter option here even thought the runtime services aren't kernel code.
> >
> > What do you think?
> 
> v1 checked for CONFIG_ARM64_BTI_KERNEL as well, but I dropped it
> because we can do the enforcement even without it.
> 
> I'm not sure how mismatched BTI support factors into that, though,
> given that CONFIG_ARM64_BTI_KERNEL is set at compile time. You mean
> mismatched between cores, right?

I believe that there's no issue with mismatched CPUs, but there *might* might
be a different issue with the ordering of feature detection and usage of the
cap:

* If CONFIG_ARM64_BTI_KERNEL=y, then the ARM64_BTI cap is detected as a strict
  boot cpu feature, and secondaries without it will be rejected.

* If CONFIG_ARM64_BTI_KERNEL=n then the ARM64_BTI cap is detected as a system
  feature, and so we only set the cap bit after bringing all secondary CPUs
  online, and only when *all* CPUs support it.

  The happens under setup_cpu_features(), called from smp_cpus_done().

So there's no issue with mismatch, but if system_supports_bti is called before
smp_cpus_done() on a CONFIG_ARM64_BTI_KERNEL kernel it will return false. When
do we set up the EFI mappings relative to that?

Thanks,
Mark.