linux-kernel - Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI

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Date:   Mon, 16 May 2022 11:54:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joao Moreira <joao@...rdrivepizza.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, llvm@...ts.linux.dev
Subject: Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG

On Fri, May 13, 2022 at 01:21:58PM -0700, Sami Tolvanen wrote:
> With CONFIG_CFI_CLANG, the compiler injects a type preamble
> immediately before each function and a check to validate the target
> function type before indirect calls:
> 
>   ; type preamble
>   __cfi_function:
>     int3
>     int3
>     mov <id>, %eax
>     int3
>     int3
>   function:
>     ...

When I enable CFI_CLANG and X86_KERNEL_IBT I get:

0000000000000c80 <__cfi_io_schedule_timeout>:
c80:   cc                      int3
c81:   cc                      int3
c82:   b8 b5 b1 39 b3          mov    $0xb339b1b5,%eax
c87:   cc                      int3
c88:   cc                      int3

0000000000000c89 <io_schedule_timeout>:
c89:   f3 0f 1e fa             endbr64


That seems unfortunate. Would it be possible to get an additional
compiler option to suppress the endbr for all symbols that get a __cfi_
preaamble?

Also, perhaps s/CFI_CLANG/KERNEL_CFI/ or somesuch, so that GCC might
also implement this same scheme (in time)?

>   ; indirect call check
>     cmpl    <id>, -6(%r11)
>     je      .Ltmp1
>     ud2
>   .Ltmp1:
>     call    __x86_indirect_thunk_r11

The first one I try and find looks like:

26:       41 81 7b fa a6 96 9e 38         cmpl   $0x389e96a6,-0x6(%r11)
2e:       74 02                   je     32 <__traceiter_sched_kthread_stop+0x29>
30:       0f 0b                   ud2
32:       4c 89 f6                mov    %r14,%rsi
35:       e8 00 00 00 00          call   3a <__traceiter_sched_kthread_stop+0x31> 36: R_X86_64_PLT32      __x86_indirect_thunk_r11-0x4

This must not be. If I'm to rewrite that lot to:

  movl	$\hash, %r10d
  sub	$9, %r11
  call	*%r11
  .nop  4

Then there must not be spurious instruction in between the ud2 and the
indirect call/retpoline thing.