linux-hardening - Re: [PATCH v5 05/15] tracepoint: Exclude tp_stub

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Date:   Wed, 13 Oct 2021 15:20:07 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     x86@...nel.org, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v5 05/15] tracepoint: Exclude tp_stub_func from CFI
 checking

On Wed, 13 Oct 2021 11:16:48 -0700
Sami Tolvanen <samitolvanen@...gle.com> wrote:

> If allocate_probes fails, func_remove replaces the old function
> with a pointer to tp_stub_func, which is called using a mismatching
> function pointer that will always trip indirect call checks with
> CONFIG_CFI_CLANG. Use DEFINE_CFI_IMMEDATE_RETURN_STUB to define
> tp_stub_func to allow it to pass CFI checking.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Tested-by: Sedat Dilek <sedat.dilek@...il.com>
> ---
>  kernel/tracepoint.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> index 64ea283f2f86..8a0d463c8507 100644
> --- a/kernel/tracepoint.c
> +++ b/kernel/tracepoint.c
> @@ -9,6 +9,7 @@
>  #include <linux/list.h>
>  #include <linux/rcupdate.h>
>  #include <linux/tracepoint.h>
> +#include <linux/cfi.h>
>  #include <linux/err.h>
>  #include <linux/slab.h>
>  #include <linux/sched/signal.h>
> @@ -99,10 +100,7 @@ struct tp_probes {
>  };
>  
>  /* Called in removal of a func but failed to allocate a new tp_funcs */
> -static void tp_stub_func(void)
> -{
> -	return;
> -}
> +static DEFINE_CFI_IMMEDIATE_RETURN_STUB(tp_stub_func);

Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

-- Steve

>  
>  static inline void *allocate_probes(int count)
>  {