Re: [PATCH v2] netfilter: ebtables: fix fortify warnings

[Kees Cook <kees@kernel.org>]

On August 8, 2023 9:32:50 AM PDT, "Gustavo A. R. Silva" <gustavoars@kernel.org> wrote:
>On Tue, Aug 08, 2023 at 09:30:38PM +0800, GONG, Ruiqi wrote:
>> From: "GONG, Ruiqi" <gongruiqi1@huawei.com>
>> 
>> When compiling with gcc 13 and CONFIG_FORTIFY_SOURCE=y, the following
>> warning appears:
>> 
>> In function ‘fortify_memcpy_chk’,
>>     inlined from ‘size_entry_mwt’ at net/bridge/netfilter/ebtables.c:2118:2:
>> ./include/linux/fortify-string.h:592:25: error: call to ‘__read_overflow2_field’
>> declared with attribute warning: detected read beyond size of field (2nd parameter);
>> maybe use struct_group()? [-Werror=attribute-warning]
>>   592 |                         __read_overflow2_field(q_size_field, size);
>>       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> 
>> The compiler is complaining:
>> 
>> memcpy(&offsets[1], &entry->watchers_offset,
>>                        sizeof(offsets) - sizeof(offsets[0]));
>> 
>> where memcpy reads beyong &entry->watchers_offset to copy
>> {watchers,target,next}_offset altogether into offsets[]. Silence the
>> warning by wrapping these three up via struct_group().
>> 
>> Signed-off-by: GONG, Ruiqi <gongruiqi1@huawei.com>
>> ---
>> 
>> v2: fix HDRTEST error by replacing struct_group() with __struct_group(),
>> since it's a uapi header.
>> 
>>  include/uapi/linux/netfilter_bridge/ebtables.h | 14 ++++++++------
>>  net/bridge/netfilter/ebtables.c                |  3 +--
>>  2 files changed, 9 insertions(+), 8 deletions(-)
>> 
>> diff --git a/include/uapi/linux/netfilter_bridge/ebtables.h b/include/uapi/linux/netfilter_bridge/ebtables.h
>> index a494cf43a755..b0caad82b693 100644
>> --- a/include/uapi/linux/netfilter_bridge/ebtables.h
>> +++ b/include/uapi/linux/netfilter_bridge/ebtables.h
>> @@ -182,12 +182,14 @@ struct ebt_entry {
>>  	unsigned char sourcemsk[ETH_ALEN];
>>  	unsigned char destmac[ETH_ALEN];
>>  	unsigned char destmsk[ETH_ALEN];
>> -	/* sizeof ebt_entry + matches */
>> -	unsigned int watchers_offset;
>> -	/* sizeof ebt_entry + matches + watchers */
>> -	unsigned int target_offset;
>> -	/* sizeof ebt_entry + matches + watchers + target */
>> -	unsigned int next_offset;
>> +	__struct_group(/* no tag */, offsets, /* no attrs */,
>> +		/* sizeof ebt_entry + matches */
>> +		unsigned int watchers_offset;
>> +		/* sizeof ebt_entry + matches + watchers */
>> +		unsigned int target_offset;
>> +		/* sizeof ebt_entry + matches + watchers + target */
>> +		unsigned int next_offset;
>> +	);
>>  	unsigned char elems[0] __attribute__ ((aligned (__alignof__(struct ebt_replace))));
>>  };

Actually, looking at what size_entry_mwt() is doing, I think you probably DO want a tag for this and to use a real structure for the manipulations instead of doing array indexing? I dunno. This is a weird function! :)

-Kees

>>  
>> diff --git a/net/bridge/netfilter/ebtables.c b/net/bridge/netfilter/ebtables.c
>> index 757ec46fc45a..5ec66b1ebb64 100644
>> --- a/net/bridge/netfilter/ebtables.c
>> +++ b/net/bridge/netfilter/ebtables.c
>> @@ -2115,8 +2115,7 @@ static int size_entry_mwt(const struct ebt_entry *entry, const unsigned char *ba
>>  		return ret;
>>  
>>  	offsets[0] = sizeof(struct ebt_entry); /* matches come first */
>> -	memcpy(&offsets[1], &entry->watchers_offset,
>> -			sizeof(offsets) - sizeof(offsets[0]));
>> +	memcpy(&offsets[1], &entry->offsets, sizeof(offsets) - sizeof(offsets[0]));
>							^^^^^^^^^^^^
>You now can replace this ____________________________________|
>with just `sizeof(entry->offsets)`
>
>With that change you can add my
>Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
>
>Thank you
>--
>Gustavo
>
>>  
>>  	if (state->buf_kern_start) {
>>  		buf_start = state->buf_kern_start + state->buf_kern_offset;
>> -- 
>> 2.41.0
>> 


-- 
Kees Cook