Re: [PATCH] time: verify time values in adjtimex ADJ_SETOFFSET to avoid overflow

Richard Cochran
2015-12-06T22:11:56Z

On Sun, Dec 06, 2015 at 10:07:01AM +0100, Thomas Gleixner wrote:
> On Sat, 5 Dec 2015, Sasha Levin wrote:
> > On 12/05/2015 12:10 PM, Thomas Gleixner wrote:
> > > That's not a canonical timeval. timeval_valid() is what you want to
> > > check it. Or has adjtimex some magic exception here?

You can't use timeval_valid for ADJ_SETOFFSET, because the seconds
field can also be negative (when setting the time back).

> > Nope, it looks like timeval_valid() is indeed what I've needed to use.
> >
> > Is there a reason ntp_validate_timex() doesn't do timeval_valid() too
> > for at least the ADJ_SETOFFSET case? If not, I'll add it in.

So you should not use timeval_valid, and the original patch is also
not right. The rule is:

The value of a timeval is the sum of its fields, but the
field tv_usec must always be non-negative.

We had a discussion about this a year or two ago. Maybe I can find it
again.

The overflow is a latent problem, and the patch should:

1. return error in case (txc->time.tv_usec >= USEC_PER_SEC)
2. remove the redundant test in timekeeping_inject_offset.

Thanks,
Richard