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

[Keller, Jacob E]

> -----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