isdn: eicon: fix a missing-check bug

isdn: eicon: fix a missing-check bug

In divasmain.c, the function divas_write() firstly invokes the function
diva_xdi_open_adapter() to open the adapter that matches with the adapter
number provided by the user, and then invokes the function diva_xdi_write()
to perform the write operation using the matched adapter. The two functions
diva_xdi_open_adapter() and diva_xdi_write() are located in diva.c.

In diva_xdi_open_adapter(), the user command is copied to the object 'msg'
from the userspace pointer 'src' through the function pointer 'cp_fn',
which eventually calls copy_from_user() to do the copy. Then, the adapter
number 'msg.adapter' is used to find out a matched adapter from the
'adapter_queue'. A matched adapter will be returned if it is found.
Otherwise, NULL is returned to indicate the failure of the verification on
the adapter number.

As mentioned above, if a matched adapter is returned, the function
diva_xdi_write() is invoked to perform the write operation. In this
function, the user command is copied once again from the userspace pointer
'src', which is the same as the 'src' pointer in diva_xdi_open_adapter() as
both of them are from the 'buf' pointer in divas_write(). Similarly, the
copy is achieved through the function pointer 'cp_fn', which finally calls
copy_from_user(). After the successful copy, the corresponding command
processing handler of the matched adapter is invoked to perform the write
operation.

It is obvious that there are two copies here from userspace, one is in
diva_xdi_open_adapter(), and one is in diva_xdi_write(). Plus, both of
these two copies share the same source userspace pointer, i.e., the 'buf'
pointer in divas_write(). Given that a malicious userspace process can race
to change the content pointed by the 'buf' pointer, this can pose potential
security issues. For example, in the first copy, the user provides a valid
adapter number to pass the verification process and a valid adapter can be
found. Then the user can modify the adapter number to an invalid number.
This way, the user can bypass the verification process of the adapter
number and inject inconsistent data.

This patch reuses the data copied in
diva_xdi_open_adapter() and passes it to diva_xdi_write(). This way, the
above issues can be avoided.

Signed-off-by: Wenwen Wang <wang6495@umn.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>