Re: [PATCH ghak10 v5 1/2] audit: Add functions to log time adjustments

[Ondrej Mosnacek <omosnace@redhat.com>]

On Fri, Aug 24, 2018 at 8:33 PM John Stultz <john.stultz@linaro.org> wrote:
> On Fri, Aug 24, 2018 at 5:00 AM, Ondrej Mosnacek <omosnace@redhat.com> wrote:
> > This patch adds two auxiliary record types that will be used to annotate
> > the adjtimex SYSCALL records with the NTP/timekeeping values that have
> > been changed.
> >
> > Next, it adds two functions to the audit interface:
> >  - audit_tk_injoffset(), which will be called whenever a timekeeping
> >    offset is injected by a syscall from userspace,
> >  - audit_ntp_adjust(), which will be called whenever an NTP internal
> >    variable is changed by a syscall from userspace.
> >
> > Quick reference for the fields of the new records:
> >     AUDIT_TIME_INJOFFSET
> >         sec - the 'seconds' part of the offset
> >         nsec - the 'nanoseconds' part of the offset
> >     AUDIT_TIME_ADJNTPVAL
> >         op - which value was adjusted:
> >             offset - corresponding to the time_offset variable
> >             freq   - corresponding to the time_freq variable
> >             status - corresponding to the time_status variable
> >             adjust - corresponding to the time_adjust variable
> >             tick   - corresponding to the tick_usec variable
> >             tai    - corresponding to the timekeeping's TAI offset
> >         old - the old value
> >         new - the new value
> >
> > Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
> > ---
> >  include/linux/audit.h      | 21 +++++++++++++++++++++
> >  include/uapi/linux/audit.h |  2 ++
> >  kernel/auditsc.c           | 15 +++++++++++++++
> >  3 files changed, 38 insertions(+)
> >
> > diff --git a/include/linux/audit.h b/include/linux/audit.h
> > index 9334fbef7bae..0d084d4b4042 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 <uapi/linux/timex.h>
> >
> >  #define AUDIT_INO_UNSET ((unsigned long)-1)
> >  #define AUDIT_DEV_UNSET ((dev_t)-1)
> > @@ -356,6 +357,8 @@ extern void __audit_log_capset(const struct cred *new, const struct cred *old);
> >  extern void __audit_mmap_fd(int fd, int flags);
> >  extern void __audit_log_kern_module(char *name);
> >  extern void __audit_fanotify(unsigned int response);
> > +extern void __audit_tk_injoffset(struct timespec64 offset);
> > +extern void __audit_ntp_adjust(const char *type, s64 oldval, s64 newval);
> >
> >  static inline void audit_ipc_obj(struct kern_ipc_perm *ipcp)
> >  {
> > @@ -458,6 +461,18 @@ static inline void audit_fanotify(unsigned int response)
> >                 __audit_fanotify(response);
> >  }
> >
> > +static inline void audit_tk_injoffset(struct timespec64 offset)
> > +{
> > +       if (!audit_dummy_context())
> > +               __audit_tk_injoffset(offset);
> > +}
> > +
> > +static inline void audit_ntp_adjust(const char *type, s64 oldval, s64 newval)
> > +{
> > +       if (!audit_dummy_context())
> > +               __audit_ntp_adjust(type, oldval, newval);
> > +}
> > +
> >  extern int audit_n_rules;
> >  extern int audit_signals;
> >  #else /* CONFIG_AUDITSYSCALL */
> > @@ -584,6 +599,12 @@ static inline void audit_log_kern_module(char *name)
> >  static inline void audit_fanotify(unsigned int response)
> >  { }
> >
> > +static inline void audit_tk_injoffset(struct timespec64 offset)
> > +{ }
> > +
> > +static inline void audit_ntp_adjust(const char *type, s64 oldval, s64 newval)
> > +{ }
> > +
> >  static inline void audit_ptrace(struct task_struct *t)
> >  { }
> >  #define audit_n_rules 0
> > diff --git a/include/uapi/linux/audit.h b/include/uapi/linux/audit.h
> > index 4e3eaba84175..242ce562b41a 100644
> > --- a/include/uapi/linux/audit.h
> > +++ b/include/uapi/linux/audit.h
> > @@ -114,6 +114,8 @@
> >  #define AUDIT_REPLACE          1329    /* Replace auditd if this packet unanswerd */
> >  #define AUDIT_KERN_MODULE      1330    /* Kernel Module events */
> >  #define AUDIT_FANOTIFY         1331    /* Fanotify access decision */
> > +#define AUDIT_TIME_INJOFFSET   1332    /* Timekeeping offset injected */
> > +#define AUDIT_TIME_ADJNTPVAL   1333    /* NTP value adjustment */
> >
> >  #define AUDIT_AVC              1400    /* SE Linux avc denial or grant */
> >  #define AUDIT_SELINUX_ERR      1401    /* Internal SE Linux Errors */
> > diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> > index fb207466e99b..d355d32d9765 100644
> > --- a/kernel/auditsc.c
> > +++ b/kernel/auditsc.c
> > @@ -2422,6 +2422,21 @@ void __audit_fanotify(unsigned int response)
> >                 AUDIT_FANOTIFY, "resp=%u", response);
> >  }
> >
> > +/* We need to allocate with GFP_ATOMIC here, since these two functions will be
> > + * called while holding the timekeeping lock: */
> > +void __audit_tk_injoffset(struct timespec64 offset)
> > +{
> > +       audit_log(audit_context(), GFP_ATOMIC, AUDIT_TIME_INJOFFSET,
> > +                 "sec=%lli nsec=%li", (long long)offset.tv_sec, offset.tv_nsec);
> > +}
> > +
> > +void __audit_ntp_adjust(const char *type, s64 oldval, s64 newval)
> > +{
> > +       audit_log(audit_context(), GFP_ATOMIC, AUDIT_TIME_ADJNTPVAL,
> > +                 "op=%s old=%lli new=%lli", type,
> > +                 (long long)oldval, (long long)newval);
> > +}
>
> So it looks like you've been careful here, but just want to make sure,
> nothing in the audit_log path calls anything that might possibly call
> back into timekeeping code? We've had a number of issues over time
> where debug calls out to other subsystems end up getting tweaked to
> add some timestamping and we deadlock.  :)

Hm, that's a good point... AFAIK, the only place where audit_log()
might call into timekeeping is when it captures the timestamp for the
record (via ktime_get_coarse_real_ts64()), but this is only called
when the functions are called outside of a syscall and that should
never happen here. I'm thinking I could harden this more by returning
early from these functions if WARN_ON_ONCE(!audit_context() ||
!audit_context()->in_syscall)... It is not a perfect solution, but at
least there will be something in the code reminding us about this
issue.

>
> One approach we've done to make sure we don't create a trap for future
> changes in other subsystems, is avoid calling into other subsystems
> until later when we've dropped the locks, (see clock_was_set).  Its a
> little rough for some of the things done deep in the ntp code, but
> might be an extra cautious approach to try.

I'm afraid that delaying the audit_* calls would complicate things too
much here... Paul/Richard, any thoughts?

--
Ondrej Mosnacek <omosnace at redhat dot com>
Associate Software Engineer, Security Technologies
Red Hat, Inc.