linux-kernel - Re: [PATCH 17/23] y2038: time: avoid timespec usage in settimeofday()

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Date:   Thu, 14 Nov 2019 12:06:28 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     y2038 Mailman List <y2038@...ts.linaro.org>,
        John Stultz <john.stultz@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        Christian Brauner <christian@...uner.io>,
        Jens Axboe <axboe@...nel.dk>, Ingo Molnar <mingo@...nel.org>,
        Corey Minyard <cminyard@...sta.com>,
        zhengbin <zhengbin13@...wei.com>,
        Li RongQing <lirongqing@...du.com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH 17/23] y2038: time: avoid timespec usage in settimeofday()

On Wed, Nov 13, 2019 at 10:53 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Fri, 8 Nov 2019, Arnd Bergmann wrote:
> > -SYSCALL_DEFINE2(settimeofday, struct timeval __user *, tv,
> > +SYSCALL_DEFINE2(settimeofday, struct __kernel_old_timeval __user *, tv,
> >               struct timezone __user *, tz)
> >  {
> >       struct timespec64 new_ts;
> > -     struct timeval user_tv;
> >       struct timezone new_tz;
> >
> >       if (tv) {
> > -             if (copy_from_user(&user_tv, tv, sizeof(*tv)))
> > +             if (get_user(new_ts.tv_sec, &tv->tv_sec) ||
> > +                 get_user(new_ts.tv_nsec, &tv->tv_usec))
> >                       return -EFAULT;
>
> How is that supposed to be correct on a 32bit kernel?

I don't see the problem you are referring to. This should behave the
same way on a 32-bit kernel and on a 64-bit kernel, sign-extending
the tv_sec field, and copying the user tv_usec field into the
kernel tv_nsec, to be multiplied by 1000 a few lines later.

Am I missing something?

> > -             if (!timeval_valid(&user_tv))
> > +             if (tv->tv_usec > USEC_PER_SEC)
> >                       return -EINVAL;
>
> That's incomplete:
>
> static inline bool timeval_valid(const struct timeval *tv)
> {
>         /* Dates before 1970 are bogus */
>         if (tv->tv_sec < 0)
>                 return false;
>
>         /* Can't have more microseconds then a second */
>         if (tv->tv_usec < 0 || tv->tv_usec >= USEC_PER_SEC)
>                 return false;
>
>         return true;
> }

My idea was to not duplicate the range check that is done
in do_sys_settimeofday64() and again in do_settimeofday64:

        if (!timespec64_valid_settod(ts))
                return -EINVAL;

The only check we should need in addition to this is to ensure
that passing an invalid tv_usec number doesn't become an
unexpectedly valid tv_nsec after the multiplication.

I agree the patch looks like I'm missing a check here, but
the code after the patch appears clear enough to me.

          Arnd