netdev - Re: [PATCH] tipc: Avoid copying bytes beyond the supplied data

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Date:   Sun, 5 May 2019 20:20:14 +0000
From:   Chris Packham <Chris.Packham@...iedtelesis.co.nz>
To:     David Miller <davem@...emloft.net>
CC:     "jon.maloy@...csson.com" <jon.maloy@...csson.com>,
        "ying.xue@...driver.com" <ying.xue@...driver.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "tipc-discussion@...ts.sourceforge.net" 
        <tipc-discussion@...ts.sourceforge.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tipc: Avoid copying bytes beyond the supplied data

On 4/05/19 4:45 PM, David Miller wrote:
> From: Chris Packham <chris.packham@...iedtelesis.co.nz>
> Date: Thu,  2 May 2019 15:10:04 +1200
> 
>> TLV_SET is called with a data pointer and a len parameter that tells us
>> how many bytes are pointed to by data. When invoking memcpy() we need
>> to careful to only copy len bytes.
>>
>> Previously we would copy TLV_LENGTH(len) bytes which would copy an extra
>> 4 bytes past the end of the data pointer which newer GCC versions
>> complain about.
>>
>>   In file included from test.c:17:
>>   In function 'TLV_SET',
>>       inlined from 'test' at test.c:186:5:
>>   /usr/include/linux/tipc_config.h:317:3:
>>   warning: 'memcpy' forming offset [33, 36] is out of the bounds [0, 32]
>>   of object 'bearer_name' with type 'char[32]' [-Warray-bounds]
>>       memcpy(TLV_DATA(tlv_ptr), data, tlv_len);
>>       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>   test.c: In function 'test':
>>   test.c::161:10: note:
>>   'bearer_name' declared here
>>       char bearer_name[TIPC_MAX_BEARER_NAME];
>>            ^~~~~~~~~~~
>>
>> Signed-off-by: Chris Packham <chris.packham@...iedtelesis.co.nz>
> 
> But now the pad bytes at the end are uninitialized.
> 
> The whole idea is that the encapsulating TLV object has to be rounded
> up in size based upon the given 'len' for the data.
> 

TLV_LENGTH() does not account for any padding bytes due to the 
alignment. TLV_SPACE() does but that wasn't used in the code before my 
change.

Are you suggesting something like this


-        if (len && data)
-               memcpy(TLV_DATA(tlv_ptr), data, tlv_len);
+        if (len && data) {
+               memcpy(TLV_DATA(tlv_ptr), data, len);
+               memset(TLV_DATA(tlv_ptr) + len, 0, TLV_SPACE(len) - 
TLV_LENGTH(len));
+        }