Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755719AbbLHAWc (ORCPT ); Mon, 7 Dec 2015 19:22:32 -0500 Received: from aserp1040.oracle.com ([141.146.126.69]:29512 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751881AbbLHAWb (ORCPT ); Mon, 7 Dec 2015 19:22:31 -0500 Subject: Re: [PATCH] ntp: verify offset doesn't overflow in ntp_update_offset To: John Stultz References: <1449175608-1146-1-git-send-email-sasha.levin@oracle.com> Cc: Thomas Gleixner , lkml From: Sasha Levin Message-ID: <566622BB.4050304@oracle.com> Date: Mon, 7 Dec 2015 19:22:19 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Source-IP: aserv0022.oracle.com [141.146.126.234] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1524 Lines: 41 On 12/07/2015 07:02 PM, John Stultz wrote: > On Thu, Dec 3, 2015 at 12:46 PM, Sasha Levin wrote: >> > We need to make sure that the offset is valid before manipulating it, >> > otherwise it might overflow on the multiplication. >> > >> > Signed-off-by: Sasha Levin >> > --- >> > kernel/time/ntp.c | 6 ++++-- >> > 1 file changed, 4 insertions(+), 2 deletions(-) >> > >> > diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c >> > index 149cc80..36616c3 100644 >> > --- a/kernel/time/ntp.c >> > +++ b/kernel/time/ntp.c >> > @@ -297,6 +297,9 @@ static void ntp_update_offset(long offset) >> > if (!(time_status & STA_PLL)) >> > return; >> > >> > + /* Make sure the multiplication below won't overflow */ >> > + offset = clamp(offset, -MAXPHASE, MAXPHASE); >> > + >> > if (!(time_status & STA_NANO)) >> > offset *= NSEC_PER_USEC; > So looking at this a bit closer, this bit looks sort of crazy since we > clam the offset, do the multiply and then do the exact same clamp. > > I'd much rather do a more logical clamp(offset, -USEC_PER_SEC, > USEC_PER_SEC), but only in the case where we do the multiply. > > Any objection to that? Nope. Sounds right. Thanks, Sasha -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/