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:

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-----Original Message-----
From: Alexander Lobakin <redacted>
Sent: Monday, November 21, 2022 7:03 AM
To: Andrew Lunn <andrew@lunn.ch>
Cc: Lobakin, Alexandr <redacted>; Maxim Korotkov
[off-list ref]; David S. Miller [off-list ref]; Eric
Dumazet [off-list ref]; Jakub Kicinski [off-list ref]; Paolo
Abeni [off-list ref]; Guangbin Huang
[off-list ref]; Tom Rix [off-list ref]; Marco Bonelli
[off-list ref]; Edward Cree [off-list ref];
netdev@vger.kernel.org; linux-kernel@vger.kernel.org; lvc-
project@linuxtesting.org
Subject: Re: [PATCH] ethtool: avoiding integer overflow in ethtool_phys_id()

From: Andrew Lunn <andrew@lunn.ch>
Date: Mon, 21 Nov 2022 15:10:18 +0100

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On Mon, Nov 21, 2022 at 10:56:18AM +0300, Maxim Korotkov wrote:

quoted

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 <redacted>
---
  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)

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   } 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

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     Andrew

[0] https://elixir.bootlin.com/linux/v6.1-
rc6/source/include/linux/math64.h#L153

Thanks,
Olek