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From:   "Keller, Jacob E" <jacob.e.keller@intel.com>
To:     "Lobakin, Alexandr" <alexandr.lobakin@intel.com>,
        Andrew Lunn <andrew@lunn.ch>
CC:     "Lobakin, Alexandr" <alexandr.lobakin@intel.com>,
        Maxim Korotkov <korotkov.maxim.s@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        "Eric Dumazet" <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Guangbin Huang <huangguangbin2@huawei.com>,
        Tom Rix <trix@redhat.com>, Marco Bonelli <marco@mebeim.net>,
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        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "lvc-project@linuxtesting.org" <lvc-project@linuxtesting.org>
Subject: RE: [PATCH] ethtool: avoiding integer overflow in ethtool_phys_id()
Thread-Topic: [PATCH] ethtool: avoiding integer overflow in ethtool_phys_id()
Thread-Index: AQHY/X7qeqK51hvWY0SWiEWUHH/t+a5JavoAgAAOygCAAGv28A==
Date:   Mon, 21 Nov 2022 21:30:10 +0000
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References: <20221121075618.15877-1-korotkov.maxim.s@gmail.com>
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 <20221121150314.393682-1-alexandr.lobakin@intel.com>
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> -----Original Message-----
> From: Alexander Lobakin <alexandr.lobakin@intel.com>
> Sent: Monday, November 21, 2022 7:03 AM
> To: Andrew Lunn <andrew@lunn.ch>
> Cc: Lobakin, Alexandr <alexandr.lobakin@intel.com>; Maxim Korotkov
> <korotkov.maxim.s@gmail.com>; David S. Miller <davem@davemloft.net>; Eric
> Dumazet <edumazet@google.com>; Jakub Kicinski <kuba@kernel.org>; Paolo
> Abeni <pabeni@redhat.com>; Guangbin Huang
> <huangguangbin2@huawei.com>; Tom Rix <trix@redhat.com>; Marco Bonelli
> <marco@mebeim.net>; Edward Cree <ecree@solarflare.com>;
> netdev@vger.kernel.org; linux-kernel@vger.kernel.org; lvc-
> project@linuxtesting.org
> Subject: Re: [PATCH] ethtool: avoiding integer overflow in ethtool_phys_i=
d()
>=20
> From: Andrew Lunn <andrew@lunn.ch>
> Date: Mon, 21 Nov 2022 15:10:18 +0100
>=20
> > On Mon, Nov 21, 2022 at 10:56:18AM +0300, Maxim Korotkov wrote:
> > > The value of an arithmetic expression "n * id.data" is subject
> > > to possible overflow due to a failure to cast operands to a larger da=
ta
> > > type before performing arithmetic. Added cast of first operand to u64
> > > for avoiding overflow.
> > >
> > > Found by Linux Verification Center (linuxtesting.org) with SVACE.
> > >
> > > Fixes: 2adc6edcaec0 ("ethtool: fix error handling in ethtool_phys_id"=
)
> > > Signed-off-by: Maxim Korotkov <korotkov.maxim.s@gmail.com>
> > > ---
> > >  net/ethtool/ioctl.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/net/ethtool/ioctl.c b/net/ethtool/ioctl.c
> > > index 6a7308de192d..cf87e53c2e74 100644
> > > --- a/net/ethtool/ioctl.c
> > > +++ b/net/ethtool/ioctl.c
> > > @@ -2007,7 +2007,7 @@ static int ethtool_phys_id(struct net_device *d=
ev,
> void __user *useraddr)
> > >  	} else {
> > >  		/* Driver expects to be called at twice the frequency in rc */
> > >  		int n =3D rc * 2, interval =3D HZ / n;
> > > -		u64 count =3D n * id.data, i =3D 0;
> > > +		u64 count =3D (u64)n * id.data, i =3D 0;
> >
> >
> > How about moving the code around a bit, change n to a u64 and drop the
> > cast? Does this look correct?
> >
> > 		int interval =3D HZ / rc / 2;
> > 		u64 n =3D rc * 2;
> > 		u64 count =3D n * id.data;
> >
> > 		i =3D 0;
> >
> > I just don't like casts, they suggest the underlying types are wrong,
> > so should fix that, not add a cast.
>=20
> This particular one is absolutely fine. When you want to multiply
> u32 by u32, you always need a cast, otherwise the result will be
> truncated. mul_u32_u32() does it the very same way[0].
>=20

Why not just use mul_u32_u32 then?

Thanks,
Jake

> >
> > 	Andrew
> >
>=20
> [0] https://elixir.bootlin.com/linux/v6.1-
> rc6/source/include/linux/math64.h#L153
>=20
> Thanks,
> Olek