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

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Message-ID: <4bad1923-354d-3858-0339-82df8c090c3f@oracle.com>
Date:   Tue, 22 Feb 2022 13:53:56 -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 2/21/22 23:47, Michal Hocko wrote:
> On Mon 21-02-22 12:24:25, Mike Kravetz wrote:
>> On 2/21/22 00:42, Michal Hocko wrote:
>>> On Fri 18-02-22 13:29:46, Mike Kravetz wrote:
>>> [...]
>>>> @@ -4161,7 +4162,7 @@ static int __init hugepages_setup(char *s)
>>>>  			}
>>>>  			if (tmp >= nr_online_nodes)
>>>>  				goto invalid;
>>>> -			node = tmp;
>>>> +			node = array_index_nospec(tmp, nr_online_nodes);
>>>>  			p += count + 1;
>>>>  			/* Parse hugepages */
>>>>  			if (sscanf(p, "%lu%n", &tmp, &count) != 1)
>>>> @@ -6889,9 +6890,9 @@ static int __init cmdline_parse_hugetlb_cma(char *p)
>>>>  			break;
>>>>  
>>>>  		if (s[count] == ':') {
>>>> -			nid = tmp;
>>>> -			if (nid < 0 || nid >= MAX_NUMNODES)
>>>> +			if (tmp >= MAX_NUMNODES)
>>>>  				break;
>>>> +			nid = array_index_nospec(tmp, MAX_NUMNODES);
>>>>  
>>>>  			s += count + 1;
>>>>  			tmp = memparse(s, &s);
>>>
>>> This is an early boot code, how is this supposed to be used as a side
>>> channel?
>>
>> I do not have an evil hacker mind, but I can not think of a way this one time
>> use of a user specified index could be an issue.  It does add noise to the
>> BUILD REGRESSION emails sent to Andrew.
> 
> Maybe Smack can be taught to ignore __init and other early boot
> functions.
> 
> I do not have any strong objections to using array_index_nospec because
> it won't do any harm. Except that it makes a security measure a normal
> comodity so any future changes to array_index_nospec and its users will
> have to consult additional callers. Whether that is something we should
> deeply care about, I don't know.
> 
> At minimum make sure to be explicit that this can hardly be a Spectre
> gadget as it is a _one_ time early boot call. If there is a scenario
> where this could be really abused then it should be mentioned
> explicitly.

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.

Andrew, would you like me to send a v3?
-- 
Mike Kravetz