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

[Tomas Bortoli]

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 <tomasbortoli@xxxxxxxxx>
>> Reported-by: syzbot+65c6b72f284a39d416b4@xxxxxxxxxxxxxxxxxxxxxxxxx
>> ---
>>  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;
>>  }
>>