linux-kernel - Re: [PATCH] Pass correct length to strnlen

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Date:	Fri, 09 Sep 2011 10:02:12 +1000
From:	Ryan Mallon <rmallon@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Mark Salter <msalter@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Pass correct length to strnlen_user in fs/exec.c

On 09/09/11 09:52, Andrew Morton wrote:
> On Thu, 08 Sep 2011 10:39:24 +1000
> Ryan Mallon <rmallon@...il.com> wrote:
>
>> Replace valid_arg_len function in fs/exec.c with max_arg_len function
>> and pass the correct length to strnlen_user.
>>
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -296,9 +296,9 @@ err:
>>  	return err;
>>  }
>>  
>> -static bool valid_arg_len(struct linux_binprm *bprm, long len)
>> +static long max_arg_len(struct linux_binprm *bprm)
>>  {
>> -	return len <= MAX_ARG_STRLEN;
>> +	return MAX_ARG_STRLEN;
>>  }
>>  
>>  #else
>> @@ -354,9 +354,9 @@ static int __bprm_mm_init(struct linux_binprm *bprm)
>>  	return 0;
>>  }
>>  
>> -static bool valid_arg_len(struct linux_binprm *bprm, long len)
>> +static long max_arg_len(struct linux_binprm *bprm)
>>  {
>> -	return len <= bprm->p;
>> +	return bprm->p;
>>  }
>>  
>>  #endif /* CONFIG_MMU */
>> @@ -474,18 +474,19 @@ static int copy_strings(int argc, struct user_arg_ptr argv,
>>  		const char __user *str;
>>  		int len;
>>  		unsigned long pos;
>> +		long max_len = max_arg_len(bprm);
>>  
>>  		ret = -EFAULT;
>>  		str = get_user_arg_ptr(argv, argc);
>>  		if (IS_ERR(str))
>>  			goto out;
>>  
>> -		len = strnlen_user(str, MAX_ARG_STRLEN);
>> -		if (!len || len > MAX_ARG_STRLEN)
>> +		len = strnlen_user(str, max_len);
>> +		if (!len)
>>  			goto out;
>>  
>>  		ret = -E2BIG;
>> -		if (!valid_arg_len(bprm, len))
>> +		if (len > max_len)
>>  			goto out;
>>  
>>  		/* We're going to work our way backwords. */
> I'm struggling to find a reason to merge this - it churns code around
> rather pointlessly?
>
That's fine. I originally went looking after a discussion with Mark
about the weird strnlen_user semantics  and this usage looked incorrect
to me because it wasn't obviously checking >= MAX_ARG_STRLEN.

The rework I think makes it a bit more clear and passes the correct max
length to strnlen_user. Its a bit odd to pass MAX_ARG_STRLEN and then
check if it is longer than bprm->len, and I guess assumes that bprm->len
is less than MAX_ARG_STRLEN.

Feel free to drop the patch if you think it is just churn.

~Ryan


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