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

Ondrej Mosnacek
2018-09-17T12:33:24Z

On Thu, Sep 13, 2018 at 5:59 PM Richard Guy Briggs <rgb@redhat.com> wrote:
> On 2018-08-27 10:28, Ondrej Mosnacek wrote:
> > 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.
>
> It looks like this should be safe already since if there is no context,
> it won't call into the real audit logging function and as you point out,
> this should never happen outside of a syscall.
>
> > > 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?
>
> The other way to address this would be to have your timekeeping audit
> logging functions save the parameters you want to log somewhere in the
> struct audit_context union and have it print the record on syscall exit
> based on the presence of this type and applicable filters.

Yes, I thought about that, but it doesn't seem to be worth the hassle.
I think I'll just add the paranoid WARN_ON_ONCE(...) unless there are
strong objections to that.

>
> > Ondrej Mosnacek <omosnace at redhat dot com>
>
> - RGB
>
> --
> Richard Guy Briggs <rgb@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

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