On Wed, Feb 08, 2023 at 02:03:45PM +0100, Ard Biesheuvel wrote: > On Wed, 8 Feb 2023 at 14:00, Will Deacon <will@xxxxxxxxxx> 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@xxxxxxxxxx> > > > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> > > > --- > > > 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.