svn commit: r347063 - head/sys/kern

[Mark Johnston]

On Mon, May 06, 2019 at 01:40:19PM -0700, John Baldwin wrote:
> On 5/6/19 11:45 AM, Mark Johnston wrote:
> > On Mon, May 06, 2019 at 11:07:18AM -0700, John Baldwin wrote:
> >> On 5/3/19 2:26 PM, Mark Johnston wrote:
> >>> Author: markj
> >>> Date: Fri May  3 21:26:44 2019
> >>> New Revision: 347063
> >>> URL: https://svnweb.freebsd.org/changeset/base/347063
> >>>
> >>> Log:
> >>>   Disallow excessively small times of day in clock_settime(2).
> >>>   
> >>>   Reported by:	syzkaller
> >>>   Reviewed by:	cem, kib
> >>>   MFC after:	1 week
> >>>   Sponsored by:	The FreeBSD Foundation
> >>>   Differential Revision:	https://reviews.freebsd.org/D20151
> >>>
> >>> Modified:
> >>>   head/sys/kern/kern_time.c
> >>>
> >>> Modified: head/sys/kern/kern_time.c
> >>> ==============================================================================
> >>> --- head/sys/kern/kern_time.c	Fri May  3 21:13:09 2019	(r347062)
> >>> +++ head/sys/kern/kern_time.c	Fri May  3 21:26:44 2019	(r347063)
> >>> @@ -412,7 +412,9 @@ kern_clock_settime(struct thread *td, clockid_t clock_
> >>>  	if (ats->tv_nsec < 0 || ats->tv_nsec >= 1000000000 ||
> >>>  	    ats->tv_sec < 0)
> >>>  		return (EINVAL);
> >>> -	if (!allow_insane_settime && ats->tv_sec > 8000ULL * 365 * 24 * 60 * 60)
> >>> +	if (!allow_insane_settime &&
> >>> +	    (ats->tv_sec > 8000ULL * 365 * 24 * 60 * 60 ||
> >>> +	    ats->tv_sec < utc_offset()))
> >>>  		return (EINVAL);
> >>>  	/* XXX Don't convert nsec->usec and back */
> >>>  	TIMESPEC_TO_TIMEVAL(&atv, ats);
> >>
> >> Pardon my ignorance, but I can't see why you are checking against utc_offset()
> >> vs some small constant?  None of the discussion in the review mentioned the
> >> reason for using this particular value, and I didn't see any comparisons
> >> against utc_offset or kernadjtz in kern_clock_setttime() or settime() that
> >> would have underflowed or panicked.  Can you give a bit more detail on why
> >> utc_offset() is the lower bound?  Thanks.
> > 
> > I chose it because we subtract utc_offset() from the time passed in to
> > clock_settime(); see settime_task_func().  That subtraction caused the
> > underflow that later caused the observed panics.
> 
> Ok, thanks.  A few things I didn't see anyone else note in the review then:
> 
> 1) This subtraction is actually not done for all rtc drivers, so it seems
>    like we might block small times for RTC clocks that set
>    CLOCKF_GETTIME_NO_ADJ.

The drivers that set NO_ADJ still account for the offset in their
individual settime methods.  I don't see how it can be correct for any
driver to ignore adjkerntz?

> 2) utc_offset can be negative for machines using local time in timezones
>    "before" UTC.

Hmm, I believe the patch still handles this case?

> I suppose we don't think any FreeBSD machines actually need to set the
> running clock to 0 anyway so fixing it here rather than rejecting invalid
> values only for RTCs that can't handle it is probably ok, but the
> connection doesn't feel obvious that we are rejecting times that might
> be non-representable in RTCs.

I can add a comment explaining where the comment comes from, assuming
there are no objections to keeping the existing change.  The placement
of the check was motivated by the placement of the pre-existing bounds
check, and the fact that we have no good way to signal an error after
setting the clock.