On Wed, Oct 13, 2021 at 11:16:52AM -0700, Sami Tolvanen wrote:

The kernel has several assembly functions that are not directly callable
from C. Use an opaque type for these function prototypes to make misuse
harder, and to avoid the need to annotate references to these functions
for Clang's Control-Flow Integrity (CFI).

Suggested-by: Andy Lutomirski <luto@amacapital.net>
Suggested-by: Alexander Lobakin <redacted>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <redacted>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Sedat Dilek <redacted>
---
 arch/x86/include/asm/ftrace.h         |  2 +-
 arch/x86/include/asm/idtentry.h       | 10 +++++-----
 arch/x86/include/asm/page_64.h        |  7 ++++---
 arch/x86/include/asm/paravirt_types.h |  3 ++-
 arch/x86/include/asm/processor.h      |  2 +-
 arch/x86/include/asm/proto.h          | 25 +++++++++++++------------
 arch/x86/include/asm/uaccess_64.h     |  9 +++------
 arch/x86/kernel/alternative.c         |  2 +-
 arch/x86/kernel/ftrace.c              |  2 +-
 arch/x86/kernel/paravirt.c            |  4 ++--
 arch/x86/kvm/emulate.c                |  4 ++--
 arch/x86/kvm/kvm_emulate.h            |  9 ++-------
 arch/x86/xen/enlighten_pv.c           |  6 +++---
 arch/x86/xen/xen-ops.h                | 10 +++++-----
 14 files changed, 45 insertions(+), 50 deletions(-)

No matter from which direction I look at it, wrapping some functions
which a crazy macro doesn't look good.

So what's the plan here?

Everytime someone adds an asm function which is not callable from C but
forgets to use that magic macro, someone from team CFI will send a patch
fixing that?

I.e., a whack-a-mole game?

If we're going to do that keep-CFI-working game, we might just as well
not do the macro but use the C code it evaluates to, so that at least it
looks ok-ish:

DECLARE_NOT_CALLED_FROM_C(int3_magic);

vs

extern const u8 int3_magic[];

(Just picked one at random).

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette