linux-kernel - Re: [PATCH] ethtool: avoiding integer overflow in ethtool_phys

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Date:   Tue, 22 Nov 2022 09:53:48 +0300
From:   Maxim Korotkov <korotkov.maxim.s@...il.com>
To:     "Keller, Jacob E" <jacob.e.keller@...el.com>,
        "Lobakin, Alexandr" <alexandr.lobakin@...el.com>,
        Andrew Lunn <andrew@...n.ch>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Guangbin Huang <huangguangbin2@...wei.com>,
        Tom Rix <trix@...hat.com>, Marco Bonelli <marco@...eim.net>,
        Edward Cree <ecree@...arflare.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "lvc-project@...uxtesting.org" <lvc-project@...uxtesting.org>
Subject: Re: [PATCH] ethtool: avoiding integer overflow in ethtool_phys_id()

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@...el.com>
>> Sent: Monday, November 21, 2022 7:03 AM
>> To: Andrew Lunn <andrew@...n.ch>
>> Cc: Lobakin, Alexandr <alexandr.lobakin@...el.com>; Maxim Korotkov
>> <korotkov.maxim.s@...il.com>; David S. Miller <davem@...emloft.net>; Eric
>> Dumazet <edumazet@...gle.com>; Jakub Kicinski <kuba@...nel.org>; Paolo
>> Abeni <pabeni@...hat.com>; Guangbin Huang
>> <huangguangbin2@...wei.com>; Tom Rix <trix@...hat.com>; Marco Bonelli
>> <marco@...eim.net>; Edward Cree <ecree@...arflare.com>;
>> netdev@...r.kernel.org; linux-kernel@...r.kernel.org; lvc-
>> project@...uxtesting.org
>> Subject: Re: [PATCH] ethtool: avoiding integer overflow in ethtool_phys_id()
>>
>> From: Andrew Lunn <andrew@...n.ch>
>> 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@...il.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 *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