linux-kernel - Re: [PATCH 3/4] security/selinux: decrement sizeof size in strncmp

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Date:	Thu, 12 Nov 2009 08:21:52 -0800
From:	Casey Schaufler <casey@...aufler-ca.com>
To:	Julia Lawall <julia@...u.dk>
CC:	"Serge E. Hallyn" <serue@...ibm.com>,
	James Morris <jmorris@...ei.org>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	Eric Paris <eparis@...isplace.org>,
	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	kernel-janitors@...r.kernel.org
Subject: Re: [PATCH 3/4] security/selinux: decrement sizeof size in strncmp

Julia Lawall wrote:
> On Thu, 12 Nov 2009, Serge E. Hallyn wrote:
>
>   
>> Quoting James Morris (jmorris@...ei.org):
>>     
>>> On Thu, 12 Nov 2009, Julia Lawall wrote:
>>>
>>>       
>>>> From: Julia Lawall <julia@...u.dk>
>>>>
>>>> As observed by Joe Perches, sizeof of a constant string includes the
>>>> trailing 0.  If what is wanted is to check the initial characters of
>>>> another string, this trailing 0 should not be taken into account.  If an
>>>> exact match is wanted, strcmp should be used instead.
>>>>         
>>>> --- a/security/selinux/hooks.c
>>>> +++ b/security/selinux/hooks.c
>>>> @@ -448,7 +448,7 @@ static int sb_finish_set_opts(struct sup
>>>>  		sbsec->flags &= ~SE_SBLABELSUPP;
>>>>  
>>>>  	/* Special handling for sysfs. Is genfs but also has setxattr handler*/
>>>> -	if (strncmp(sb->s_type->name, "sysfs", sizeof("sysfs")) == 0)
>>>> +	if (strncmp(sb->s_type->name, "sysfs", sizeof("sysfs") - 1) == 0)
>>>>  		sbsec->flags |= SE_SBLABELSUPP;
>>>>         
>>> Shouldn't this be a simple strcmp() ?
>>>       
>> Yes I think so.
>>
>> Julia seems to be arguing that if a module introduces a new fs with
>> name 'sysfs_foo' then this check should match that fs too (since
>> for sysfs, sb->s_type->name = "sysfs" which also has a trailing \0,
>> so for the regular sysfs her patch makes no practical difference).
>>     
>
> If strcmp is what is wanted, I can make a patch for that.
>   

I strongly suggest that this is not what is wanted.
    strcmp(x,y)
and
    strncmp(x,y,sizeof(y))

are functionally equivalent and strcmp has a bad reputation in
the security community because it is associated with potential
buffer overrun issues.

    strncmp(x,y,sizeof(y)-1)

is not the same and is more cluttered as well. This code
does not need to be changed. It does what it is intended
to do in a strait-forward manner.
> julia
> --
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>
>   

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