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Subject: Re: [V9fs-developer] [PATCH] Integer underflow in pdu_read()
To:     jiangyiwen <jiangyiwen@huawei.com>, ericvh@gmail.com,
        rminnich@sandia.gov, lucho@ionkov.net,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-kernel@vger.kernel.org, syzkaller@googlegroups.com,
        v9fs-developer@lists.sourceforge.net, davem@davemloft.net
References: <20180709192651.28095-1-tomasbortoli@gmail.com>
 <5B4565AA.6050309@huawei.com>
From:   Tomas Bortoli <tomasbortoli@gmail.com>
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Message-ID: <512b9cc7-3b7a-3ee4-5759-0a27144825a8@gmail.com>
Date:   Thu, 12 Jul 2018 13:05:31 +0200
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On 07/11/2018 04:04 AM, jiangyiwen wrote:
> 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.
> I guess this case may happened only when server send wrong size to
> the client and then cause size < offset, or else I think this case
> will not happen. Is it right? Or other cases?
>
> In addition, the email should also send to andrew morton, because
> 9p maintainer already don't maintain the project, andrew can help
> merge the patch.
>
>> Signed-off-by: Tomas Bortoli <tomasbortoli@gmail.com>
>> Reported-by: syzbot+65c6b72f284a39d416b4@syzkaller.appspotmail.com
>> ---
>>  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);
> I suggest this add two *Tab* lens.
>
>> +	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);
> The same as above.
>
>> +	if (len != 0)
>> +		memcpy(&pdu->sdata[pdu->size], data, len);
>>  	pdu->size += len;
>>  	return size - len;
>>  }
>>
>

This patch is not necessary anymore, the one I just sent fixes the
length validation issue.