linux-kernel - Re: [PATCH 1/1] cfi: Use rcu_read_{un}lock_sched

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Date:   Wed, 11 Aug 2021 09:12:11 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Elliot Berman <quic_eberman@...cinc.com>
Cc:     Kees Cook <keescook@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jinlong Mao <jinlmao@...eaurora.org>
Subject: Re: [PATCH 1/1] cfi: Use rcu_read_{un}lock_sched_notrace

Hi Elliot,

On Wed, Aug 11, 2021 at 9:00 AM Elliot Berman <quic_eberman@...cinc.com> wrote:
>
> If rcu_read_lock_sched tracing is enabled, the tracing subsystem can
> perform a jump which needs to be checked by CFI. For example, stm_ftrace
> source is enabled as a module and hooks into enabled ftrace events. This
> can cause an recursive loop where find_shadow_check_fn ->
> rcu_read_lock_sched -> (call to stm_ftrace generates cfi slowpath) ->
> find_shadow_check_fn -> rcu_read_lock_sched -> ...
>
> To avoid the recursion, either the ftrace codes needs to be marked with
> __no_cfi or CFI should not trace. Use the "_notrace" in CFI to avoid
> tracing so that CFI can guard ftrace.
>
> Signed-off-by: Elliot Berman <quic_eberman@...cinc.com>
> ---
>  kernel/cfi.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/cfi.c b/kernel/cfi.c
> index e17a56639766..9594cfd1cf2c 100644
> --- a/kernel/cfi.c
> +++ b/kernel/cfi.c
> @@ -248,9 +248,9 @@ static inline cfi_check_fn find_shadow_check_fn(unsigned long ptr)
>  {
>         cfi_check_fn fn;
>
> -       rcu_read_lock_sched();
> +       rcu_read_lock_sched_notrace();
>         fn = ptr_to_check_fn(rcu_dereference_sched(cfi_shadow), ptr);
> -       rcu_read_unlock_sched();
> +       rcu_read_unlock_sched_notrace();
>
>         return fn;
>  }
> @@ -269,11 +269,11 @@ static inline cfi_check_fn find_module_check_fn(unsigned long ptr)
>         cfi_check_fn fn = NULL;
>         struct module *mod;
>
> -       rcu_read_lock_sched();
> +       rcu_read_lock_sched_notrace();
>         mod = __module_address(ptr);
>         if (mod)
>                 fn = mod->cfi_check;
> -       rcu_read_unlock_sched();
> +       rcu_read_unlock_sched_notrace();
>
>         return fn;
>  }

Thanks for the patch! I agree, this looks like a better solution than
littering tracing code with __nocfi annotations.

Reviewed-by: Sami Tolvanen <samitolvanen@...gle.com>

Sami