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

From: Andrew Lunn <[email-masked]>
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 <[email-masked]>
> ---
>  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.

	Andrew