linux-kernel - Re: [PATCH v2] hugetlb: clean up potential spectre issue warnings

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Date:   Wed, 23 Feb 2022 10:36:55 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Zhenguo Yao <yaozhenguo1@...il.com>,
        Liu Yuntao <liuyuntao10@...wei.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] hugetlb: clean up potential spectre issue warnings

On 2/23/22 00:33, Michal Hocko wrote:
> On Tue 22-02-22 13:53:56, Mike Kravetz wrote:
>> On 2/21/22 23:47, Michal Hocko wrote:
>> How about adding this note to the commit message?
>>
>> Note: these routines take a user specified value used as an index ONCE
>> during the boot process.  As a result, they can not be used as a general
>> method of exploitation.  Code changes are being made to eliminate warnings.
> 
> This would help but the question whether the change is worth remains.
> Does this change have any other advantage than silencing the warning?
> 

Silencing the warnings was the primary motivation for the change.  If Dan
has a plan to change smatch so that they are silenced for __init functions,
then it would be better to not make the changes to use array_index_nospec.

While making the changes, I shuffled the code a little and did not immediately
notice that it also 'fixes' an overflow/truncation issue when assigning an
unsigned long to int as addressed in [1].  We should probably make this change
whether or not we use array_index_nospec to silence warnings.

[1] https://lore.kernel.org/linux-mm/20220209134018.8242-1-liuyuntao10@huawei.com/

Thanks for your comments and suggestions!
-- 
Mike Kravetz