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

[Ard Biesheuvel]

On Wed, 8 Feb 2023 at 14:00, Will Deacon <will at 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 at kernel.org>
> > Reviewed-by: Kees Cook <keescook at 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?