Re: [PATCH v5 14/18] arm64: add __nocfi to functions that jump to a physical address

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

[adding Ard for EFI runtime services bits]

On Thu, Apr 01, 2021 at 04:32:12PM -0700, Sami Tolvanen wrote:
> Disable CFI checking for functions that switch to linear mapping and
> make an indirect call to a physical address, since the compiler only
> understands virtual addresses and the CFI check for such indirect calls
> would always fail.

What does physical vs virtual have to do with this? Does the address
actually matter, or is this just a general thing that when calling an
assembly function we won't have a trampoline that the caller expects?

I wonder if we need to do something with asmlinkage here, perhaps?

I didn't spot anything in the seriues handling EFI runtime services
calls, and I strongly suspect we need to do something for those, unless
they're handled implicitly by something else.

> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/arm64/include/asm/mmu_context.h | 2 +-
>  arch/arm64/kernel/cpu-reset.h        | 8 ++++----
>  arch/arm64/kernel/cpufeature.c       | 2 +-
>  3 files changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
> index 386b96400a57..d3cef9133539 100644
> --- a/arch/arm64/include/asm/mmu_context.h
> +++ b/arch/arm64/include/asm/mmu_context.h
> @@ -119,7 +119,7 @@ static inline void cpu_install_idmap(void)
>   * Atomically replaces the active TTBR1_EL1 PGD with a new VA-compatible PGD,
>   * avoiding the possibility of conflicting TLB entries being allocated.
>   */
> -static inline void cpu_replace_ttbr1(pgd_t *pgdp)
> +static inline void __nocfi cpu_replace_ttbr1(pgd_t *pgdp)

Given these are inlines, what's the effect when these are inlined into a
function that would normally use CFI? Does CFI get supressed for the
whole function, or just the bit that got inlined?

Is there an attribute that we could place on a function pointer to tell
the compiler to not check calls via that pointer? If that existed we'd
be able to scope this much more tightly.

Thanks,
Mark.

>  {
>  	typedef void (ttbr_replace_func)(phys_addr_t);
>  	extern ttbr_replace_func idmap_cpu_replace_ttbr1;
> diff --git a/arch/arm64/kernel/cpu-reset.h b/arch/arm64/kernel/cpu-reset.h
> index f3adc574f969..9a7b1262ef17 100644
> --- a/arch/arm64/kernel/cpu-reset.h
> +++ b/arch/arm64/kernel/cpu-reset.h
> @@ -13,10 +13,10 @@
>  void __cpu_soft_restart(unsigned long el2_switch, unsigned long entry,
>  	unsigned long arg0, unsigned long arg1, unsigned long arg2);
>  
> -static inline void __noreturn cpu_soft_restart(unsigned long entry,
> -					       unsigned long arg0,
> -					       unsigned long arg1,
> -					       unsigned long arg2)
> +static inline void __noreturn __nocfi cpu_soft_restart(unsigned long entry,
> +						       unsigned long arg0,
> +						       unsigned long arg1,
> +						       unsigned long arg2)
>  {
>  	typeof(__cpu_soft_restart) *restart;
>  
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 0b2e0d7b13ec..c2f94a5206e0 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1445,7 +1445,7 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>  }
>  
>  #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> -static void
> +static void __nocfi
>  kpti_install_ng_mappings(const struct arm64_cpu_capabilities *__unused)
>  {
>  	typedef void (kpti_remap_fn)(int, int, phys_addr_t);
> -- 
> 2.31.0.208.g409f899ff0-goog
>