Subject: Re: [V9fs-developer] [PATCH] Integer underflow in pdu_read()

From: piaojun <[email-masked]>
Date: Tue, 10 Jul 2018 19:06:17 +0800

Hi Tomas,

Thanks for your explaination, and I get your point.

On 2018/7/10 16:27, Tomas Bortoli wrote:
> Hi Jun,
> 
> Intuitively, if you have a packet of size x and you read at an offset y,
> when y>x you are off the packet. That's an out out bound read.
> 
> In this specific code when offset > size, the available length
> estimation will fail as there will be an underflow resulting from
> offset-size (it'll give a big big number) that breaks the out-of-bound
> control put in place (if offset-size is a big big number, the asked size
> to read will be probably smaller and therefore allowed).
> 
> These definitions might help:
> https://cwe.mitre.org/data/definitions/787.html
> https://cwe.mitre.org/data/definitions/125.html
> 
> Tomas
>> Hi Tomas,
>>
>> It looks like pdu->size should always be greater than pdu->offset, right?
>> My question may be very easy for you, please help explaining.
>>
>> Thanks,
>> Jun
>>
>> On 2018/7/10 3:26, Tomas Bortoli wrote:
>>> The pdu_read() function suffers from an integer underflow.
>>> When pdu->offset is greater than pdu->size, the length calculation will have
>>> a wrong result, resulting in an out-of-bound read.
>>> This patch modifies also pdu_write() in the same way to prevent the same
>>> issue from happening there and for consistency.
>>>
>>> Signed-off-by: Tomas Bortoli <[email-masked]>
>>> Reported-by: [email-masked]
>>> ---
>>>  net/9p/protocol.c | 12 ++++++++----
>>>  1 file changed, 8 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/net/9p/protocol.c b/net/9p/protocol.c
>>> index 931ea00c4fed..f1e2425f920b 100644
>>> --- a/net/9p/protocol.c
>>> +++ b/net/9p/protocol.c
>>> @@ -55,16 +55,20 @@ EXPORT_SYMBOL(p9stat_free);
>>>  
>>>  size_t pdu_read(struct p9_fcall *pdu, void *data, size_t size)
>>>  {
>>> -	size_t len = min(pdu->size - pdu->offset, size);
>>> -	memcpy(data, &pdu->sdata[pdu->offset], len);
>>> +	size_t len = pdu->offset > pdu->size ? 0 :
>>> +	 min(pdu->size - pdu->offset, size);
>>> +	if (len != 0)
>>> +		memcpy(data, &pdu->sdata[pdu->offset], len);
>>>  	pdu->offset += len;
>>>  	return size - len;
>>>  }
>>>  
>>>  static size_t pdu_write(struct p9_fcall *pdu, const void *data, size_t size)
>>>  {
>>> -	size_t len = min(pdu->capacity - pdu->size, size);
>>> -	memcpy(&pdu->sdata[pdu->size], data, len);
>>> +	size_t len = pdu->size > pdu->capacity ? 0 :
>>> +	 min(pdu->capacity - pdu->size, size);
>>> +	if (len != 0)
>>> +		memcpy(&pdu->sdata[pdu->size], data, len);
>>>  	pdu->size += len;
>>>  	return size - len;
>>>  }
>>>
> 
> 
>