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

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Date:	Wed, 27 Feb 2013 15:22:33 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	nhorman@...driver.com
Cc:	linux@...ck-us.net, netdev@...r.kernel.org,
	linux-sctp@...r.kernel.org, vyasevich@...il.com, sri@...ibm.com
Subject: Re: [PATCH] net/sctp: Validate parameter size for
 SCTP_GET_ASSOC_STATS control message

From: Neil Horman <nhorman@...driver.com>
Date: Wed, 27 Feb 2013 15:09:31 -0500

> On Wed, Feb 27, 2013 at 11:43:51AM -0800, Guenter Roeck wrote:
>> 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>
>> ---
>>  net/sctp/socket.c |    2 ++
>>  1 file changed, 2 insertions(+)
>> 
>> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
>> index cedd9bf..0a5f2bf 100644
>> --- a/net/sctp/socket.c
>> +++ b/net/sctp/socket.c
>> @@ -5652,6 +5652,8 @@ static int sctp_getsockopt_assoc_stats(struct sock *sk, int len,
>>  	/* User must provide at least the assoc id */
>>  	if (len < sizeof(sctp_assoc_t))
>>  		return -EINVAL;
>> +	if (len > sizeof(struct sctp_assoc_stats))
>> +		len = sizeof(struct sctp_assoc_stats);
>>  
>>  	if (copy_from_user(&sas, optval, len))
>>  		return -EFAULT;
>> -- 
>> 1.7.9.7
>> 
>> 
> 
> Theres more than that going on here.  This will fix the warning, but the
> function is written such that, if you pass in a size that is greater than the
> size of a struct sctp_association, but less than a struct sctp_assoc_stats.  I'm
> not sure that a partial stat struct is really that useful to people.  What if
> you were to check for max(struct sctp_association, struct sctp_assoc_stats) as
> your minimum length check, then just did a copy_from_user of that length.  It
> would save you having to compute two lengths separately, since you could then
> just do a copy_to_user(...,sizeof(struct sctp_assoc_stats), at the bottom of
> that function.

Genreally, getsockopt() implementations happily give partial return values
when the user gives a too small length.