Re: [PATCH net] s390/qeth: reject oversized SNMP requests

[Julian Wiedmann <jwi@linux.ibm.com>]

+cc Dan

On 23.08.19 11:29, Julian Wiedmann wrote:
> Commit d4c08afafa04 ("s390/qeth: streamline SNMP cmd code") removed
> the bounds checking for req_len, under the assumption that the check in
> qeth_alloc_cmd() would suffice.
> 
> But that code path isn't sufficiently robust to handle a user-provided
> data_length, which could overflow (when adding the cmd header overhead)
> before being checked against QETH_BUFSIZE. We end up allocating just a
> tiny iob, and the subsequent copy_from_user() writes past the end of
> that iob.
> 
> Special-case this path and add a coarse bounds check, to protect against
> maliciuous requests. This let's the subsequent code flow do its normal
> job and precise checking, without risk of overflow.
> 
> Fixes: d4c08afafa04 ("s390/qeth: streamline SNMP cmd code")
> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
> Reviewed-by: Ursula Braun <ubraun@linux.ibm.com>
> ---
>  drivers/s390/net/qeth_core_main.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/s390/net/qeth_core_main.c b/drivers/s390/net/qeth_core_main.c
> index 9c3310c4d61d..6502b148541e 100644
> --- a/drivers/s390/net/qeth_core_main.c
> +++ b/drivers/s390/net/qeth_core_main.c
> @@ -4374,6 +4374,10 @@ static int qeth_snmp_command(struct qeth_card *card, char __user *udata)
>  	    get_user(req_len, &ureq->hdr.req_len))
>  		return -EFAULT;
>  
> +	/* Sanitize user input, to avoid overflows in iob size calculation: */
> +	if (req_len > QETH_BUFSIZE)
> +		return -EINVAL;
> +
>  	iob = qeth_get_adapter_cmd(card, IPA_SETADP_SET_SNMP_CONTROL, req_len);
>  	if (!iob)
>  		return -ENOMEM;
>