Re: [PATCH] ethtool: avoiding integer overflow in ethtool_phys_id()

[Maxim Korotkov]

Ok, I'll replace cast to macro in patch V2

On 22.11.2022 00:30, Keller, Jacob E wrote:
> > -----Original Message-----
> > From: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx>
> > Sent: Monday, November 21, 2022 7:03 AM
> > To: Andrew Lunn <andrew@xxxxxxx>
> > Cc: Lobakin, Alexandr <alexandr.lobakin@xxxxxxxxx>; Maxim Korotkov
> > <korotkov.maxim.s@xxxxxxxxx>; David S. Miller <davem@xxxxxxxxxxxxx>; Eric
> > Dumazet <edumazet@xxxxxxxxxx>; Jakub Kicinski <kuba@xxxxxxxxxx>; Paolo
> > Abeni <pabeni@xxxxxxxxxx>; Guangbin Huang
> > <huangguangbin2@xxxxxxxxxx>; Tom Rix <trix@xxxxxxxxxx>; Marco Bonelli
> > <marco@xxxxxxxxxx>; Edward Cree <ecree@xxxxxxxxxxxxxx>;
> > netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; lvc-
> > project@xxxxxxxxxxxxxxxx
> > Subject: Re: [PATCH] ethtool: avoiding integer overflow in ethtool_phys_id()
> >
> > From: Andrew Lunn <andrew@xxxxxxx>
> > Date: Mon, 21 Nov 2022 15:10:18 +0100
> >
> > > 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 data
> > > > 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@xxxxxxxxx>
> > > > ---
> > > >   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 *dev,
> > void __user *useraddr)
> > > >    } else {
> > > >            /* Driver expects to be called at twice the frequency in rc */
> > > >            int n = rc * 2, interval = HZ / n;
> > > > -         u64 count = n * id.data, i = 0;
> > > > +         u64 count = (u64)n * id.data, i = 0;
> > >
> > >
> > > How about moving the code around a bit, change n to a u64 and drop the
> > > cast? Does this look correct?
> > >
> > >              int interval = HZ / rc / 2;
> > >              u64 n = rc * 2;
> > >              u64 count = n * id.data;
> > >
> > >              i = 0;
> > >
> > > I just don't like casts, they suggest the underlying types are wrong,
> > > so should fix that, not add a cast.
> >
> > 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].
>
> Why not just use mul_u32_u32 then?
>
> Thanks,
> Jake
>
> > >      Andrew
> >
> > [0] https://elixir.bootlin.com/linux/v6.1-
> > rc6/source/include/linux/math64.h#L153
> >
> > Thanks,
> > Olek