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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Mon, 14 Feb 2022 13:38:18 -0800
Message-ID: <CABCJKuciRBnz4JxBDJC=+kuJn4pU2uBkWPBov7-VL2o2j0F4SA@mail.gmail.com>
Subject: Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Joao Moreira <joao@overdrivepizza.com>,
        Kees Cook <keescook@chromium.org>, X86 ML <x86@kernel.org>,
        hjl.tools@gmail.com, Josh Poimboeuf <jpoimboe@redhat.com>,
        andrew.cooper3@citrix.com, LKML <linux-kernel@vger.kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        llvm@lists.linux.dev
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On Fri, Feb 11, 2022 at 5:38 AM Peter Zijlstra <peterz@infradead.org> wrote:
> I think we'll end up with something related to KCFI, but with distinct
> differences:
>
>  - 32bit immediates for smaller code

Sure, I don't see issues with that. Based on a quick test with
defconfig, this reduces vmlinux size by 0.30%.

>  - __kcfi_check_fail() is out for smaller code

I'm fine with adding a trap mode that's used by default, but having
more helpful diagnostics when something fails is useful even in
production systems in my experience. This change results in a vmlinux
that's another 0.92% smaller.

> Which then yields:
>
> caller:
>         cmpl    $0xdeadbeef, -0x4(%rax)         # 7 bytes
>         je      1f                              # 2 bytes
>         ud2                                     # 2 bytes
> 1:      call    __x86_indirect_thunk_rax        # 5 bytes

Note that the compiler might not emit this *exact* sequence of
instructions. For example, Clang generates this for events_sysfs_show
with the modified KCFI patch:

2274:       cmpl   $0x4d7bed9e,-0x4(%r11)
227c:       jne    22c0 <events_sysfs_show+0x6c>
227e:       call   2283 <events_sysfs_show+0x2f>
                    227f: R_X86_64_PLT32    __x86_indirect_thunk_r11-0x4
...
22c0:       ud2

In this case the function has two indirect calls and Clang seems to
prefer to emit just one ud2.

>         .align 16
>         .byte 0xef, 0xbe, 0xad, 0xde            # 4 bytes
> func:
>         endbr                                   # 4 bytes

Here func is no longer aligned to 16 bytes, in case that's important.

> Further, Andrew put in the request for __attribute__((cfi_seed(blah)))
> to allow distinguishing indirect functions with otherwise identical
> signature; eg. cookie = hash32(blah##signature).

Sounds reasonable.

> Did I miss anything? Got anything wrong?

How would you like to deal with the 4-byte hashes in objtool? We
either need to annotate all function symbols in the kernel, or we need
a way to distinguish the hashes from random instructions, so we can
also have functions that don't have a type hash.

Sami