linux-kernel - Re: [RFC 2/2] kvm: eventfd: detect integer overflow using check_*

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Message-ID: <CA+55aFwCUyy3gF9M7JXuyRGgOzELVbPAe0T_7giqufgGCn3XKg@mail.gmail.com>
Date:	Wed, 26 Nov 2014 11:12:26 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Andrey Ryabinin <ryabinin.a.a@...il.com>, mingo <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 2/2] kvm: eventfd: detect integer overflow using check_*_overflow

On Wed, Nov 26, 2014 at 11:06 AM, Sasha Levin <sasha.levin@...cle.com> wrote:
>
> I'm pretty sure that this is something GCC will warn you about in the
> compilation stage.

It does? I've never seen it, but maybe it's a new thing.

The gcc signedness warnings have historically been so wretched that
it's just sad, and they have to be turned off.

> kernel/time/ntp.c:process_adjtimex_modes():
>
>         if (txc->modes & ADJ_FREQUENCY) {
>                 time_freq = txc->freq * PPM_SCALE;   <=== Undefined overflow
>                 time_freq = min(time_freq, MAXFREQ_SCALED);
>                 time_freq = max(time_freq, -MAXFREQ_SCALED);
>                 /* update pps_freq */
>                 pps_set_freq(time_freq);
>         }
>
> The multiplication is between signed integers, and it overflows (user triggerable).

Well, we check that the end result - overflowed or not - is in a sane
range. So this might fall under the heading of "user gets what he asks
for".

                         Linus
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