Re: [PATCH] s390/qeth: fix a missing-check bug

[Heiko Carstens <heiko.carstens@de.ibm.com>]

On Sat, Oct 06, 2018 at 11:08:23AM -0500, Wenwen Wang wrote:
> In qeth_snmp_command(), the length of the user request is firstly copied
> from the user-space buffer 'udata' to the kernel variable 'req_len' and
> checked to see whether it is too large. If the check fails, an error code
> EINVAL is returned. Otherwise, the execution continues and the whole buffer
> is copied again from 'udata' and saved to the kernel buffer 'ureq'.
> However, after the second copy, no re-check is enforced on the newly-copied
> request length. Given that the buffer 'udata' is in the user space, a
> malicious user can race to change the request length between the two
> copies. In this way, the attacker can supply malicious data to the kernel
> and cause undefined behavior.
> 
> This patch adds a re-check on the request length after the second copy from
> the buffer 'udata'. If the newly-copied value is different from the value
> obtained in the first copy, i.e., 'req_len', an error code EINVAL will be
> returned after the buffer 'ureq' is freed.
> 
> Signed-off-by: Wenwen Wang <wang6495@umn.edu>
> ---
>  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 de82824..6199743 100644
> --- a/drivers/s390/net/qeth_core_main.c
> +++ b/drivers/s390/net/qeth_core_main.c
> @@ -4613,6 +4613,10 @@ static int qeth_snmp_command(struct qeth_card *card, char __user *udata)
>  		QETH_CARD_TEXT(card, 2, "snmpnome");
>  		return PTR_ERR(ureq);
>  	}
> +	if (ureq->hdr.req_len != req_len) {
> +		kfree(ureq);
> +		return -EINVAL;
> +	}

ureq->hdr.req_len is not used anywhere in the code, so could you please
explain what the undefined behavior is?

You could argue that adding a second sanity check may help to avoid
potential future bugs, but currently the code looks sane to me.