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From:   Brian Gerst <brgerst@gmail.com>
Date:   Wed, 21 Jun 2023 17:07:11 -0400
Message-ID: <CAMzpN2j2Okm1BPrQcrd7=Duo286RqZ-AKvy=NoJOBQ0sLJLhAw@mail.gmail.com>
Subject: Re: [PATCH 1/2] x86/cfi: Fix ret_from_fork indirect calls
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     x86@kernel.org, alyssa.milburn@linux.intel.com,
        linux-kernel@vger.kernel.org, samitolvanen@google.com,
        keescook@chromium.org, jpoimboe@kernel.org,
        joao@overdrivepizza.com, tim.c.chen@linux.intel.com
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On Thu, Jun 15, 2023 at 3:56=E2=80=AFPM Peter Zijlstra <peterz@infradead.or=
g> wrote:
>
> The ret_from_fork stub does an indirect call to the kthread function,
> but only knows about Retpolines. Instead of making the asm more
> complicated, punt to C and let the compiler figure it out.
>
> Specifically, this makes it a proper kCFI indirect call when needed (in
> fact, it is nearly impossible to code a kCFI indirect call in asm).
>
> This was the only callsite that was still calling func()+0 on regular
> indirect functions.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  arch/x86/entry/entry_64.S        |    6 ++++--
>  arch/x86/include/asm/switch_to.h |    2 ++
>  arch/x86/kernel/process_64.c     |    5 +++++
>  3 files changed, 11 insertions(+), 2 deletions(-)
>
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -304,8 +304,10 @@ SYM_CODE_START_NOALIGN(ret_from_fork)
>  1:
>         /* kernel thread */
>         UNWIND_HINT_END_OF_STACK
> -       movq    %r12, %rdi
> -       CALL_NOSPEC rbx
> +       movq    %rbx, %rdi
> +       movq    %r12, %rsi
> +       call    kthread_from_fork
> +
>         /*
>          * A kernel thread is allowed to return here after successfully
>          * calling kernel_execve().  Exit to userspace to complete the ex=
ecve()
> --- a/arch/x86/include/asm/switch_to.h
> +++ b/arch/x86/include/asm/switch_to.h
> @@ -74,6 +74,8 @@ static inline void update_task_stack(str
>  #endif
>  }
>
> +extern void kthread_from_fork(int (*fn)(void *), void *);
> +
>  static inline void kthread_frame_init(struct inactive_task_frame *frame,
>                                       int (*fun)(void *), void *arg)
>  {
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -544,6 +544,11 @@ void compat_start_thread(struct pt_regs
>  }
>  #endif
>
> +__visible noinstr void kthread_from_fork(int (*fn)(void *), void *arg)
> +{
> +       fn(arg);
> +}
> +
>  /*
>   *     switch_to(x,y) should switch tasks from x to y.
>   *

I think this makes a case for converting all of ret_from_fork() to C
(other than some minimal asm glue).  Patches coming soon.

Brian Gerst