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

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Message-ID: <547629B0.9020406@oracle.com>
Date:	Wed, 26 Nov 2014 14:27:44 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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 11/26/2014 02:12 PM, Linus Torvalds wrote:
>> 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".

I guess, though it wouldn't be clear to the user why it's broken since he passed a
seemingly valid looking value for txc->freq.


Thanks,
Sasha
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