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

[John Stultz]

On Fri, Aug 24, 2018 at 5:00 AM, Ondrej Mosnacek <omosnace@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
>  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.  :)

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.

thanks
-john