linux-kernel - Re: [PATCH v9 2/4] seccomp: switch system call argument type to void *

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Date:   Sun, 2 Dec 2018 23:01:08 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Tycho Andersen <tycho@...ho.ws>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Paul Moore <paul@...l-moore.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Christian Brauner <christian@...uner.io>,
        Tyler Hicks <tyhicks@...onical.com>,
        Akihiro Suda <suda.akihiro@....ntt.co.jp>,
        Aleksa Sarai <asarai@...e.de>, Jann Horn <jannh@...gle.com>,
        linux-kernel@...r.kernel.org,
        containers@...ts.linux-foundation.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v9 2/4] seccomp: switch system call argument type to void
 *

On Sun, Dec 02, 2018 at 08:28:25PM -0700, Tycho Andersen wrote:
> The const qualifier causes problems for any code that wants to write to the
> third argument of the seccomp syscall, as we will do in a future patch in
> this series.
> 
> The third argument to the seccomp syscall is documented as void *, so
> rather than just dropping the const, let's switch everything to use void *
> as well.
> 
> I believe this is safe because of 1. the documentation above, 2. there's no
> real type information exported about syscalls anywhere besides the man
> pages.
> 
> Signed-off-by: Tycho Andersen <tycho@...ho.ws>
> CC: Kees Cook <keescook@...omium.org>
> CC: Andy Lutomirski <luto@...capital.net>
> CC: Oleg Nesterov <oleg@...hat.com>
> CC: Eric W. Biederman <ebiederm@...ssion.com>
> CC: "Serge E. Hallyn" <serge@...lyn.com>

Acked-by: Serge Hallyn <serge@...lyn.com>

Though I'm not entirely convinced there will be no ill effects of changing
the argument type.  I'll feel comfortable when Michael and Paul say it's
fine :)

> CC: Christian Brauner <christian@...uner.io>
> CC: Tyler Hicks <tyhicks@...onical.com>
> CC: Akihiro Suda <suda.akihiro@....ntt.co.jp>
> ---
>  include/linux/seccomp.h | 2 +-
>  kernel/seccomp.c        | 8 ++++----
>  2 files changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
> index e5320f6c8654..b5103c019cf4 100644
> --- a/include/linux/seccomp.h
> +++ b/include/linux/seccomp.h
> @@ -43,7 +43,7 @@ extern void secure_computing_strict(int this_syscall);
>  #endif
>  
>  extern long prctl_get_seccomp(void);
> -extern long prctl_set_seccomp(unsigned long, char __user *);
> +extern long prctl_set_seccomp(unsigned long, void __user *);
>  
>  static inline int seccomp_mode(struct seccomp *s)
>  {
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 96afc32e041d..393e029f778a 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -924,7 +924,7 @@ static long seccomp_get_action_avail(const char __user *uaction)
>  
>  /* Common entry point for both prctl and syscall. */
>  static long do_seccomp(unsigned int op, unsigned int flags,
> -		       const char __user *uargs)
> +		       void __user *uargs)
>  {
>  	switch (op) {
>  	case SECCOMP_SET_MODE_STRICT:
> @@ -944,7 +944,7 @@ static long do_seccomp(unsigned int op, unsigned int flags,
>  }
>  
>  SYSCALL_DEFINE3(seccomp, unsigned int, op, unsigned int, flags,
> -			 const char __user *, uargs)
> +			 void __user *, uargs)
>  {
>  	return do_seccomp(op, flags, uargs);
>  }
> @@ -956,10 +956,10 @@ SYSCALL_DEFINE3(seccomp, unsigned int, op, unsigned int, flags,
>   *
>   * Returns 0 on success or -EINVAL on failure.
>   */
> -long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter)
> +long prctl_set_seccomp(unsigned long seccomp_mode, void __user *filter)
>  {
>  	unsigned int op;
> -	char __user *uargs;
> +	void __user *uargs;
>  
>  	switch (seccomp_mode) {
>  	case SECCOMP_MODE_STRICT:
> -- 
> 2.19.1