linux-kernel - Re: [PATCH] time: compat settimeofday: Validate the values of tv from user

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Message-ID: <39dd218b-4fc1-83bd-df2a-3cd69cc90c72@huawei.com>
Date:   Fri, 5 Jul 2019 20:32:55 +0800
From:   "zhengbin (A)" <zhengbin13@...wei.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     <john.stultz@...aro.org>, <sboyd@...nel.org>,
        <linux-kernel@...r.kernel.org>, <yi.zhang@...wei.com>,
        <zhangxiaoxu5@...wei.com>
Subject: Re: [PATCH] time: compat settimeofday: Validate the values of tv from
 user

On 2019/7/5 20:14, Thomas Gleixner wrote:
> Zhengbin,
>
> On Fri, 5 Jul 2019, zhengbin wrote:
>
>> Similar to commit 6ada1fc0e1c4
>> ("time: settimeofday: Validate the values of tv from user"),
>> an unvalidated user input is multiplied by a constant, which can result
>> in an undefined behaviour for large values. While this is validated
>> later, we should avoid triggering undefined behaviour.
> I surely agree with the patch, but the argument that this is validated
> later and we just should avoid UB in general is just wrong.
>
> For a wide range of negative tv_usec values the multiplication overflow
> turns them in positive numbers. So the 'validated later' is not catching
> the invalid input.
>
> So 'should avoid ....' is just the wrong argument here.
>
> Validation _is_ required before the multiplication so UB won't turn an
> invalid value into a valid one.
>
> Thanks,
>
> 	tglx
Strongly agree with this, I send a v2 patch, modify the comment?
>
> .
>