linux-kernel - Re: [PATCH] time: copy tai value (International Atomic Time, in seconds) to output __user struct in get_old

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Message-ID: <87a647s3cb.ffs@tglx>
Date:   Thu, 01 Dec 2022 13:41:56 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Arnd Bergmann <arnd@...db.de>, John Stultz <jstultz@...gle.com>,
        Jacob Macneal <jake.macneal@...il.com>
Cc:     linux-kernel@...r.kernel.org, Stephen Boyd <sboyd@...nel.org>
Subject: Re: [PATCH] time: copy tai value (International Atomic Time, in
 seconds) to output __user struct in get_old_timex32().

On Wed, Nov 23 2022 at 20:53, Arnd Bergmann wrote:
> On Wed, Nov 23, 2022, at 19:54, John Stultz wrote:
>>> --- a/kernel/time/time.c
>>> +++ b/kernel/time/time.c
>>> @@ -311,6 +311,7 @@ int get_old_timex32(struct __kernel_timex *txc, const struct old_timex32 __user
>>>         txc->calcnt = tx32.calcnt;
>>>         txc->errcnt = tx32.errcnt;
>>>         txc->stbcnt = tx32.stbcnt;
>>> +       txc->tai = tx32.tai;
>>>
>>
>> This does seem like something that was overlooked.
>>
>> Arnd: There isn't something more subtle I'm missing here, right?
>
> I agree. Looking at the git history, it seems that the tai field
> was added a long time ago in 153b5d054ac2 ("ntp: support for TAI").
> The commit correctly did the conversion for copying the data out
> of the kernel and did not copy the value in because it wasn't
> needed at the time.
>
> I don't see any user of the tai field that gets copied into
> the kernel, so the bug appears harmless, but Jacob's fix is
> nevertheless correct, as we should not use any uninitialized
> data in a structure that comes from userspace.

There is no uninitialized data. txc is zeroed at the beginning of that
function.

I agree that it's inconsistent vs. the regular adjtimex(), but as there
is no usage of the txc->tai value coming from user space it's pretty
much cosmetic.

Thanks,

        tglx