[PATCH v2 3/4] seccomp: Create an action to log before allowing

[Kees Cook]

On Thu, Feb 2, 2017 at 9:37 PM, Tyler Hicks <tyhicks at canonical.com> wrote:
> Add a new action, SECCOMP_RET_LOG, that logs a syscall before allowing
> the syscall. At the implementation level, this action is identical to
> the existing SECCOMP_RET_ALLOW action. However, it can be very useful when
> initially developing a seccomp filter for an application. The developer
> can set the default action to be SECCOMP_RET_LOG, maybe mark any
> obviously needed syscalls with SECCOMP_RET_ALLOW, and then put the
> application through its paces. A list of syscalls that triggered the
> default action (SECCOMP_RET_LOG) can be easily gleaned from the logs and
> that list can be used to build the syscall whitelist. Finally, the
> developer can change the default action to the desired value.
>
> This provides a more friendly experience than seeing the application get
> killed, then updating the filter and rebuilding the app, seeing the
> application get killed due to a different syscall, then updating the
> filter and rebuilding the app, etc.
>
> The functionality is similar to what's supported by the various LSMs.
> SELinux has permissive mode, AppArmor has complain mode, SMACK has
> bring-up mode, etc.
>
> SECCOMP_RET_LOG is given a lower value than SECCOMP_RET_ALLOW so that
> "allow" can be written to the max_action_to_log sysctl in order to get a
> list of logged actions without the, potentially larger, set of allowed
> actions.
>
> Signed-off-by: Tyler Hicks <tyhicks at canonical.com>
> ---
>  Documentation/prctl/seccomp_filter.txt | 6 ++++++
>  include/uapi/linux/seccomp.h           | 1 +
>  kernel/seccomp.c                       | 4 ++++
>  3 files changed, 11 insertions(+)
>
> diff --git a/Documentation/prctl/seccomp_filter.txt b/Documentation/prctl/seccomp_filter.txt
> index 1e469ef..ba55a91 100644
> --- a/Documentation/prctl/seccomp_filter.txt
> +++ b/Documentation/prctl/seccomp_filter.txt
> @@ -138,6 +138,12 @@ SECCOMP_RET_TRACE:
>         allow use of ptrace, even of other sandboxed processes, without
>         extreme care; ptracers can use this mechanism to escape.)
>
> +SECCOMP_RET_LOG:
> +       Results in the system call being executed after it is logged. This
> +       should be used by application developers to learn which syscalls their
> +       application needs without having to iterate through multiple test and
> +       development cycles to build the list.
> +
>  SECCOMP_RET_ALLOW:
>         Results in the system call being executed.
>
> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> index 0f238a4..67f72cd 100644
> --- a/include/uapi/linux/seccomp.h
> +++ b/include/uapi/linux/seccomp.h
> @@ -29,6 +29,7 @@
>  #define SECCOMP_RET_TRAP       0x00030000U /* disallow and force a SIGSYS */
>  #define SECCOMP_RET_ERRNO      0x00050000U /* returns an errno */
>  #define SECCOMP_RET_TRACE      0x7ff00000U /* pass to a tracer or disallow */
> +#define SECCOMP_RET_LOG                0x7ffe0000U /* allow after logging */

This adds to UAPI, so it'd be good to think for a moment about how
this would work on older kernels: right now, if someone tried to use
this RET_LOG on an old kernel, it'll get treated like RET_KILL. Is
this sane?

I'm also trying to figure out if there is some other solution to this,
but they all involve tests against an otherwise RET_ALLOW case, which
I want to avoid. :)

So, I think, for now, this looks good, but I'd prefer this be
0x7ffc0000U, just to make sure we have not painted ourselves into a
numerical corner if we for some reason ever need to put something
between RET_ALLOW and RET_LOG.

>  #define SECCOMP_RET_ALLOW      0x7fff0000U /* allow */
>
>  /* Masks for the return value sections. */
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 548fb89..8627481 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -650,6 +650,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
>
>                 return 0;
>
> +       case SECCOMP_RET_LOG:

Given my protective feelings about the RET_ALLOW case, can you make
this a fully separate case statement? I'd rather have RET_ALLOW be
distinctly separate.

>         case SECCOMP_RET_ALLOW:
>                 seccomp_log(this_syscall, 0, action);
>                 return 0;
> @@ -934,6 +935,7 @@ long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
>  #define SECCOMP_RET_TRAP_NAME          "trap"
>  #define SECCOMP_RET_ERRNO_NAME         "errno"
>  #define SECCOMP_RET_TRACE_NAME         "trace"
> +#define SECCOMP_RET_LOG_NAME           "log"
>  #define SECCOMP_RET_ALLOW_NAME         "allow"
>
>  /* Largest strlen() of all action names */
> @@ -943,6 +945,7 @@ static char seccomp_actions_avail[] = SECCOMP_RET_KILL_NAME " "
>                                       SECCOMP_RET_TRAP_NAME     " "
>                                       SECCOMP_RET_ERRNO_NAME    " "
>                                       SECCOMP_RET_TRACE_NAME    " "
> +                                     SECCOMP_RET_LOG_NAME      " "
>                                       SECCOMP_RET_ALLOW_NAME;
>
>  struct seccomp_action_name {
> @@ -955,6 +958,7 @@ static struct seccomp_action_name seccomp_action_names[] = {
>         { SECCOMP_RET_TRAP, SECCOMP_RET_TRAP_NAME },
>         { SECCOMP_RET_ERRNO, SECCOMP_RET_ERRNO_NAME },
>         { SECCOMP_RET_TRACE, SECCOMP_RET_TRACE_NAME },
> +       { SECCOMP_RET_LOG, SECCOMP_RET_LOG_NAME },
>         { SECCOMP_RET_ALLOW, SECCOMP_RET_ALLOW_NAME },
>         { }
>  };
> --
> 2.7.4
>

-Kees

-- 
Kees Cook
Pixel Security