[PATCH v5 2/6] seccomp: Sysctl to configure actions that are allowed to be logged

[Kees Cook]

On Fri, Jul 28, 2017 at 1:55 PM, Tyler Hicks <tyhicks at canonical.com> wrote:
> Adminstrators can write to this sysctl to set the seccomp actions that
> are allowed to be logged. Any actions not found in this sysctl will not
> be logged.
>
> For example, all SECCOMP_RET_KILL, SECCOMP_RET_TRAP, and
> SECCOMP_RET_ERRNO actions would be loggable if "kill trap errno" were
> written to the sysctl. SECCOMP_RET_TRACE actions would not be logged
> since its string representation ("trace") wasn't present in the sysctl
> value.

Just to make sure I'm clear on this, the key word above is "loggable",
in that filters requesting logging will be seen.

i.e. at the end of the series, the final state of "will it be logged?" is:

if action==RET_ALLOW:
  do not log
else if action==RET_KILL || audit-enabled:
  log
else if filter-requests-logging && action in actions_logged:
  log
else:
  do not log

> The path to the sysctl is:
>
>  /proc/sys/kernel/seccomp/actions_logged
>
> The actions_avail sysctl can be read to discover the valid action names
> that can be written to the actions_logged sysctl with the exception of
> SECCOMP_RET_ALLOW. It cannot be configured for logging.
>
> The default setting for the sysctl is to allow all actions to be logged
> except SECCOMP_RET_ALLOW.
>
> There's one important exception to this sysctl. If a task is
> specifically being audited, meaning that an audit context has been
> allocated for the task, seccomp will log all actions other than
> SECCOMP_RET_ALLOW despite the value of actions_logged. This exception
> preserves the existing auditing behavior of tasks with an allocated
> audit context.
>
> Signed-off-by: Tyler Hicks <tyhicks at canonical.com>
> ---
>
> * Changes since v4:
>   - the sysctl is now a list of actions that are allowed by the admin to be
>     logged rather than a list of actions that should be logged
>     + a follow up patch will let applications have a say in what should be
>       logged but the admin has the final say with this sysctl
>     + RET_ALLOW cannot be allowed to be logged
>   - fix comment style
>   - mark the seccomp_action_names array as const
>   - adjust for new reStructuredText format
>
>  Documentation/userspace-api/seccomp_filter.rst |  18 +++
>  include/linux/audit.h                          |   6 +-
>  kernel/seccomp.c                               | 180 ++++++++++++++++++++++++-
>  3 files changed, 196 insertions(+), 8 deletions(-)
>
> diff --git a/Documentation/userspace-api/seccomp_filter.rst b/Documentation/userspace-api/seccomp_filter.rst
> index 35fc7cb..2d1d8ab 100644
> --- a/Documentation/userspace-api/seccomp_filter.rst
> +++ b/Documentation/userspace-api/seccomp_filter.rst
> @@ -187,6 +187,24 @@ directory. Here's a description of each file in that directory:
>         program was built, differs from the set of actions actually
>         supported in the current running kernel.
>
> +``actions_logged``:
> +       A read-write ordered list of seccomp return values (refer to the
> +       ``SECCOMP_RET_*`` macros above) that are allowed to be logged. Writes
> +       to the file do not need to be in ordered form but reads from the file
> +       will be ordered in the same way as the actions_avail sysctl.
> +
> +       It is important to note that the value of ``actions_logged`` does not
> +       prevent certain actions from being logged when the audit subsystem is
> +       configured to audit a task. If the action is not found in
> +       ``actions_logged`` list, the final decision on whether to audit the
> +       action for that task is ultimately left up to the audit subsystem to
> +       decide for all seccomp return values other than ``SECCOMP_RET_ALLOW``.
> +
> +       The ``allow`` string is not accepted in the ``actions_logged`` sysctl
> +       as it is not possible to log ``SECCOMP_RET_ALLOW`` actions. Attempting
> +       to write ``allow`` to the sysctl will result in an EINVAL being
> +       returned.
> +
>  Adding architecture support
>  ===========================
>
> diff --git a/include/linux/audit.h b/include/linux/audit.h
> index 2150bdc..8c30f06 100644
> --- a/include/linux/audit.h
> +++ b/include/linux/audit.h
> @@ -314,11 +314,7 @@ void audit_core_dumps(long signr);
>
>  static inline void audit_seccomp(unsigned long syscall, long signr, int code)
>  {
> -       if (!audit_enabled)
> -               return;
> -
> -       /* Force a record to be reported if a signal was delivered. */
> -       if (signr || unlikely(!audit_dummy_context()))
> +       if (audit_enabled && unlikely(!audit_dummy_context()))
>                 __audit_seccomp(syscall, signr, code);
>  }
>
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 6bff068..87257f2 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -516,6 +516,52 @@ static void seccomp_send_sigsys(int syscall, int reason)
>  }
>  #endif /* CONFIG_SECCOMP_FILTER */
>
> +/* For use with seccomp_actions_logged */
> +#define SECCOMP_LOG_KILL               (1 << 0)
> +#define SECCOMP_LOG_TRAP               (1 << 2)
> +#define SECCOMP_LOG_ERRNO              (1 << 3)
> +#define SECCOMP_LOG_TRACE              (1 << 4)
> +#define SECCOMP_LOG_ALLOW              (1 << 5)
> +
> +static u32 seccomp_actions_logged = SECCOMP_LOG_KILL  | SECCOMP_LOG_TRAP  |
> +                                   SECCOMP_LOG_ERRNO | SECCOMP_LOG_TRACE;
> +
> +static inline void seccomp_log(unsigned long syscall, long signr, u32 action)
> +{
> +       bool log;
> +
> +       switch (action) {
> +       case SECCOMP_RET_TRAP:
> +               log = seccomp_actions_logged & SECCOMP_LOG_TRAP;
> +               break;
> +       case SECCOMP_RET_ERRNO:
> +               log = seccomp_actions_logged & SECCOMP_LOG_ERRNO;
> +               break;
> +       case SECCOMP_RET_TRACE:
> +               log = seccomp_actions_logged & SECCOMP_LOG_TRACE;
> +               break;
> +       case SECCOMP_RET_ALLOW:
> +               log = false;
> +               break;
> +       case SECCOMP_RET_KILL:
> +       default:
> +               log = seccomp_actions_logged & SECCOMP_LOG_KILL;
> +       }
> +
> +       /*
> +        * Force an audit message to be emitted when the action is allowed to
> +        * be logged by the admin.
> +        */
> +       if (log)
> +               return __audit_seccomp(syscall, signr, action);

At this point in the patch series, there's no filter-requested-logging
flag, so I think the above logic isn't needed until later in the
series (or rather, only RET_KILL should be checked).

> +
> +       /*
> +        * Let the audit subsystem decide if the action should be audited based
> +        * on whether the current task itself is being audited.
> +        */
> +       return audit_seccomp(syscall, signr, action);

With audit_seccomp() being a single if, maybe it should just be
collapsed into this function?

if (log || (audit_enabled && unlikely(!audit_dummy_context()))
    audit_seccomp(...)

I do like the change in name, though: this new function is correctly
named seccomp_log().

-Kees

-- 
Kees Cook
Pixel Security