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

[Richard Guy Briggs]

On 16/04/21, Paul Moore wrote:
> 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 did reflect on that initially and decided this was the least
disruptive way to implement it.  There are others similar around it and
when I started it wasn't as involved, but now it is starting to push the
limits with the kref bits...

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

Can't say I remember...  I was tempted to put it in as a define, but
decided to stick with the existing style, right?  :-)

> > 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.

Well, I did try to put that keyword consistently with where it was
inserted in other messages.  My understanding was that adding an extra
in the middle wouldn't cause a problem because the keyword scanner
looked ahead until it found the keyword it sought.  This way, older
scanners will just hop over this keyword it wasn't seeking.

> paul moore

- RGB

--
Richard Guy Briggs <rgb@xxxxxxxxxx>
Kernel Security Engineering, Base Operating Systems, Red Hat
Remote, Ottawa, Canada
Voice: +1.647.777.2635, Internal: (81) 32635