s390/qeth: reject oversized SNMP requests

s390/qeth: reject oversized SNMP requests

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>
Signed-off-by: David S. Miller <davem@davemloft.net>