netdev - Re: [PATCH] net/sctp: Validate parameter size for SCTP_GET_ASSOC

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Date:	Wed, 27 Feb 2013 15:58:04 -0500
From:	Vlad Yasevich <vyasevic@...hat.com>
To:	David Miller <davem@...emloft.net>
CC:	linux@...ck-us.net, netdev@...r.kernel.org,
	linux-sctp@...r.kernel.org, vyasevich@...il.com, sri@...ibm.com,
	nhorman@...driver.com
Subject: Re: [PATCH] net/sctp: Validate parameter size for SCTP_GET_ASSOC_STATS
 control message

On 02/27/2013 03:33 PM, David Miller wrote:
> From: Guenter Roeck <linux@...ck-us.net>
> Date: Wed, 27 Feb 2013 11:43:51 -0800
>
>> Building sctp may fail with:
>>
>> In function ‘copy_from_user’,
>>      inlined from ‘sctp_getsockopt_assoc_stats’ at
>>      net/sctp/socket.c:5656:20:
>> arch/x86/include/asm/uaccess_32.h:211:26: error: call to
>>      ‘copy_from_user_overflow’ declared with attribute error: copy_from_user()
>>      buffer size is not provably correct
>>
>> if built with W=1 due to a missing parameter size validation.
>>
>> Signed-off-by: Guenter Roeck <linux@...ck-us.net>
>
> This change is correct, but please fix this by simply moving the:
>
> 	/* Allow the struct to grow and fill in as much as possible */
> 	len = min_t(size_t, len, sizeof(sas));
>
> line higher up in the function.
>
> And I also prefer this because:
>
> 	something testing sizeof(foo);
> 	if (copy_from_user(..., ..., sizeof(foo)))
>
> must easier to audit and validate, especially in patch form.
>
> Otherwise I have to bring the code into an editor and read the whole
> function just to make sure you got the type correct.

Right.  In this particular case, we are after a very small portion of 
user data (just the association id) and copying the entire structure is
rather pointless.

A nicer solution here would be to do this:
	 if (copy_from_user(&sas, optval, sizeof(sctp_assoc_t))

We are already guaranteed that much space and we make sure that we don't 
overwrite the kernel stack.

-vlad
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