[RFC PATCH ghak10 v2 2/4] audit: Add the audit_adjtime() function

[Richard Guy Briggs]

On 2018-06-19 15:58, Ondrej Mosnacek wrote:
> This patch adds a new function that shall be used to log any
> modification of the system's clock (via the adjtimex() syscall).
> 
> The function logs an audit record of type AUDIT_TIME_ADJUSTED with the
> following fields:
> * txmodes  (the 'modes' field of struct timex; this is the only
>   mandatory field)
> * txsec, txusec (the 'tv_sec' and 'tv_usec' fields of the 'time' field
>   of struct timex (respectively))
> * txoffset (the 'offset' field of struct timex)
> * txfreq   (the 'freq' field of struct timex)
> * txmaxerr (the 'maxerror' field of struct timex)
> * txesterr (the 'esterror' field of struct timex)
> * txstatus (the 'status' field of struct timex)
> * txconst  (the 'constant' field of struct timex)
> * txtick   (the 'tick' field of struct timex)

I see below that you conditionally log these fields if the flag is
present.  There has been concern of fields coming and going
(disappearing) so this could cause problems for parsers looking for
specific fields that aren't present.  The first version of the patch
listed them all, which may be overkill.  As has been suggested, limiting
it to security-affecting fields would be a first step.

Another possibility would be to list one changed field per record.  This
could work if multiple flags are infrequent in one call.  A format I'd
suggest might be txfield=mode old=<value> new=<value>.  (Note only the
single flag value rather than all modes flags.)  There would be more
overhead if there was more than one field present in a call, but would
address the disappearing fields issue.

> These fields allow to reconstruct what exactly was changed by the
> adjtimex syscall and to what value(s). Note that the values reported are
> taken directly from the structure as received from userspace. The
> syscall handling code may do some clamping of the values internally
> before actually changing the kernel variables. Also, the fact that this
> record has been logged does not necessarily mean that some variable was
> changed (it may have been set to the same value as the old value).

If the value is the same, that may still be relevant for reporting that
someone tried to set/change it.

> Quick overview of the 'struct timex' semantics:
>   The 'modes' field is a bitmask specifying which time variables (if
>   any) should be adjusted. The other fields (of those listed above)
>   contain the values of the respective variables that should be set. If
>   the corresponding bit is not set in the 'modes' field, then a field's
>   value is not significant (it may be some garbage value passed from
>   userspace).
> 
>   Note that after processing the input values from userspace, the
>   handler writes (all) the current (new) internal values into the struct
>   and hands it back to userspace. These values are not logged.
> 
>   Also note that 'txusec' may actually mean nanoseconds, not
>   microseconds, depending on whether ADJ_NSEC is set in 'modes'.
> 
> Possible improvements:
> * move the conditional for logging/not logging into audit_adjtime()
>   - (see also next patch in series)
>   - may not be desirable due to reasons above
>   - we should probably either do both this and above or neither
> * log also the old values + the actual values that get set
>   - this would be rather difficult to do, probably not worth it
> 
> Signed-off-by: Ondrej Mosnacek <omosnace at redhat.com>
> ---
>  include/linux/audit.h | 11 +++++++++++
>  kernel/auditsc.c      | 38 ++++++++++++++++++++++++++++++++++++++
>  2 files changed, 49 insertions(+)
> 
> diff --git a/include/linux/audit.h b/include/linux/audit.h
> index 69c78477590b..26e9db46293c 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)
> @@ -353,6 +354,7 @@ 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_adjtime(const struct timex *txc);
>  
>  static inline void audit_ipc_obj(struct kern_ipc_perm *ipcp)
>  {
> @@ -455,6 +457,12 @@ static inline void audit_fanotify(unsigned int response)
>  		__audit_fanotify(response);
>  }
>  
> +static inline void audit_adjtime(const struct timex *txc)
> +{
> +	if (!audit_dummy_context())
> +		__audit_adjtime(txc);
> +}
> +
>  extern int audit_n_rules;
>  extern int audit_signals;
>  #else /* CONFIG_AUDITSYSCALL */
> @@ -581,6 +589,9 @@ static inline void audit_log_kern_module(char *name)
>  static inline void audit_fanotify(unsigned int response)
>  { }
>  
> +static inline void audit_adjtime(const struct timex *txc)
> +{ }
> +
>  static inline void audit_ptrace(struct task_struct *t)
>  { }
>  #define audit_n_rules 0
> diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> index ceb1c4596c51..9a1f3fcc377a 100644
> --- a/kernel/auditsc.c
> +++ b/kernel/auditsc.c
> @@ -2422,6 +2422,44 @@ void __audit_fanotify(unsigned int response)
>  		AUDIT_FANOTIFY,	"resp=%u", response);
>  }
>  
> +void __audit_adjtime(const struct timex *txc)
> +{
> +	struct audit_buffer *ab;
> +
> +	ab = audit_log_start(audit_context(), GFP_KERNEL, AUDIT_TIME_ADJUSTED);
> +	if (unlikely(!ab))
> +		return;
> +
> +	audit_log_format(ab, "txmodes=%u", txc->modes);
> +
> +	if (txc->modes & ADJ_SETOFFSET)
> +		audit_log_format(ab, " txsec=%li txusec=%li",
> +			(long)txc->time.tv_sec, (long)txc->time.tv_usec);
> +
> +	if (txc->modes & ADJ_OFFSET)
> +		audit_log_format(ab, " txoffset=%li", txc->offset);
> +
> +	if (txc->modes & ADJ_FREQUENCY)
> +		audit_log_format(ab, " txfreq=%li", txc->freq);
> +
> +	if (txc->modes & ADJ_MAXERROR)
> +		audit_log_format(ab, " txmaxerr=%li", txc->maxerror);
> +
> +	if (txc->modes & ADJ_ESTERROR)
> +		audit_log_format(ab, " txesterr=%li", txc->esterror);
> +
> +	if (txc->modes & ADJ_STATUS)
> +		audit_log_format(ab, " txstatus=%li", txc->status);
> +
> +	if (txc->modes & (ADJ_TIMECONST | ADJ_TAI))
> +		audit_log_format(ab, " txconst=%li", txc->constant);
> +
> +	if (txc->modes & ADJ_TICK)
> +		audit_log_format(ab, " txtick=%li", txc->tick);
> +
> +	audit_log_end(ab);
> +}
> +
>  static void audit_log_task(struct audit_buffer *ab)
>  {
>  	kuid_t auid, uid;
> -- 
> 2.17.1
> 

- RGB

--
Richard Guy Briggs <rgb at redhat.com>
Sr. S/W Engineer, Kernel Security, Base Operating Systems
Remote, Ottawa, Red Hat Canada
IRC: rgb, SunRaycer
Voice: +1.647.777.2635, Internal: (81) 32635