Re: [PATCH V4] audit: add tty field to LOGIN event

[Paul Moore]

On Thu, Apr 21, 2016 at 2:14 PM, Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
> The tty field was missing from AUDIT_LOGIN events.
>
> Refactor code to create a new function audit_get_tty(), using it to
> replace the call in audit_log_task_info() and to add it to
> audit_log_set_loginuid().  Lock and bump the kref to protect it, adding
> audit_put_tty() alias to decrement it.
>
> Signed-off-by: Richard Guy Briggs <rgb@xxxxxxxxxx>
> ---
>
> V4: Add missing prototype for audit_put_tty() when audit syscall is not
>     enabled (MIPS).
>
> V3: Introduce audit_put_tty() alias to decrement kref.
>
> V2: Use kref to protect tty signal struct while in use.
>
> ---
>
>  include/linux/audit.h |   24 ++++++++++++++++++++++++
>  kernel/audit.c        |   18 +++++-------------
>  kernel/auditsc.c      |    8 ++++++--
>  3 files changed, 35 insertions(+), 15 deletions(-)
>
> diff --git a/include/linux/audit.h b/include/linux/audit.h
> index b40ed5d..32cdafb 100644
> --- a/include/linux/audit.h
> +++ b/include/linux/audit.h
> @@ -26,6 +26,7 @@
>  #include <linux/sched.h>
>  #include <linux/ptrace.h>
>  #include <uapi/linux/audit.h>
> +#include <linux/tty.h>
>
>  #define AUDIT_INO_UNSET ((unsigned long)-1)
>  #define AUDIT_DEV_UNSET ((dev_t)-1)
> @@ -343,6 +344,23 @@ static inline unsigned int audit_get_sessionid(struct task_struct *tsk)
>         return tsk->sessionid;
>  }
>
> +static inline struct tty_struct *audit_get_tty(struct task_struct *tsk)
> +{
> +       struct tty_struct *tty = NULL;
> +       unsigned long flags;
> +
> +       spin_lock_irqsave(&tsk->sighand->siglock, flags);
> +       if (tsk->signal)
> +               tty = tty_kref_get(tsk->signal->tty);
> +       spin_unlock_irqrestore(&tsk->sighand->siglock, flags);
> +       return tty;
> +}
> +
> +static inline void audit_put_tty(struct tty_struct *tty)
> +{
> +       tty_kref_put(tty);
> +}

I'm generally not a big fan of defining functions as inlines in header
files, with the general exception of dummy functions that will get
compiled out.  Although I guess there might be some performance
argument to be made wrt audit_log_task_info().

I guess I'm fine with this, but what was the idea behind the static
inlines in audit.h?  Performance, or something else?

> diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> index 195ffae..71e14d8 100644
> --- a/kernel/auditsc.c
> +++ b/kernel/auditsc.c
> @@ -1980,6 +1980,7 @@ static void audit_log_set_loginuid(kuid_t koldloginuid, kuid_t kloginuid,
>  {
>         struct audit_buffer *ab;
>         uid_t uid, oldloginuid, loginuid;
> +       struct tty_struct *tty;
>
>         if (!audit_enabled)
>                 return;
> @@ -1987,14 +1988,17 @@ static void audit_log_set_loginuid(kuid_t koldloginuid, kuid_t kloginuid,
>         uid = from_kuid(&init_user_ns, task_uid(current));
>         oldloginuid = from_kuid(&init_user_ns, koldloginuid);
>         loginuid = from_kuid(&init_user_ns, kloginuid),
> +       tty = audit_get_tty(current);
>
>         ab = audit_log_start(NULL, GFP_KERNEL, AUDIT_LOGIN);
>         if (!ab)
>                 return;
>         audit_log_format(ab, "pid=%d uid=%u", task_pid_nr(current), uid);
>         audit_log_task_context(ab);
> -       audit_log_format(ab, " old-auid=%u auid=%u old-ses=%u ses=%u res=%d",
> -                        oldloginuid, loginuid, oldsessionid, sessionid, !rc);
> +       audit_log_format(ab, " old-auid=%u auid=%u tty=%s old-ses=%u ses=%u res=%d",
> +                        oldloginuid, loginuid, tty ? tty_name(tty) : "(none)",
> +                        oldsessionid, sessionid, !rc);
> +       audit_put_tty(tty);
>         audit_log_end(ab);
>  }

Because we are still using the crappy fixed string format for
kernel<->userspace communication, this patch will likely "break the
world" ... let's check with Steve but I believe the way to handle this
is to add the tty information to the end of the record.

-- 
paul moore
www.paul-moore.com