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

[piaojun]

Hi Tomas,

I found some other subtraction between pdu->size and pdu->offset such as:
p9pdu_vreadf()
--min_t(uint32_t, *count, pdu->size - pdu->offset);
p9_check_zc_errors()
--len = req->rc->size - req->rc->offset;

I wonder if there are underflow risks?

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;
>  }
>