Re: [PATCH v10 2/4] seccomp: switch system call argument type to void *

[Kees Cook]

On Sun, Dec 9, 2018 at 10:24 AM Tycho Andersen <tycho@xxxxxxxx> 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@xxxxxxxx>

Yeah, this can be void *, as you've found. I think it was just an
artifact of the old filter code to use const char *.

Applied for -next.

-Kees

> CC: Kees Cook <keescook@xxxxxxxxxxxx>
> CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> CC: Oleg Nesterov <oleg@xxxxxxxxxx>
> CC: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> CC: "Serge E. Hallyn" <serge@xxxxxxxxxx>
> Acked-by: Serge Hallyn <serge@xxxxxxxxxx>
> CC: Christian Brauner <christian@xxxxxxxxxx>
> CC: Tyler Hicks <tyhicks@xxxxxxxxxxxxx>
> CC: Akihiro Suda <suda.akihiro@xxxxxxxxxxxxx>
> ---
> v10: change type in include/linux/syscalls.h too to avoid compilation error
>      in the !CONFIG_ARCH_HAS_SYSCALL_WRAPPER case
> ---
>  include/linux/seccomp.h  | 2 +-
>  include/linux/syscalls.h | 2 +-
>  kernel/seccomp.c         | 8 ++++----
>  3 files changed, 6 insertions(+), 6 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/include/linux/syscalls.h b/include/linux/syscalls.h
> index 2ac3d13a915b..a60694fb0f58 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -879,7 +879,7 @@ asmlinkage long sys_renameat2(int olddfd, const char __user *oldname,
>                               int newdfd, const char __user *newname,
>                               unsigned int flags);
>  asmlinkage long sys_seccomp(unsigned int op, unsigned int flags,
> -                           const char __user *uargs);
> +                           void __user *uargs);
>  asmlinkage long sys_getrandom(char __user *buf, size_t count,
>                               unsigned int flags);
>  asmlinkage long sys_memfd_create(const char __user *uname_ptr, unsigned int flags);
> 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
>


-- 
Kees Cook