gpib: fix use-after-free in IO ioctl handlers

In the Linux kernel, the following vulnerability has been resolved:

gpib: fix use-after-free in IO ioctl handlers

The IBRD, IBWRT, IBCMD, and IBWAIT ioctl handlers use a gpib_descriptor

pointer after board->big_gpib_mutex has been released. A concurrent

IBCLOSEDEV ioctl can free the descriptor via close_dev_ioctl() during

this window, causing a use-after-free.

The IO handlers (read_ioctl, write_ioctl, command_ioctl) explicitly

release big_gpib_mutex before calling their handler. wait_ioctl() is

called with big_gpib_mutex held, but ibwait() releases it internally

when wait_mask is non-zero. In all four cases, the descriptor pointer

obtained from handle_to_descriptor() becomes unprotected.

Fix this by introducing a kernel-only descriptor_busy reference count

in struct gpib_descriptor. Each handler atomically increments

descriptor_busy under file_priv->descriptors_mutex before releasing the

lock, and decrements it when done. close_dev_ioctl() checks

descriptor_busy under the same lock and rejects the close with -EBUSY

if the count is non-zero.

A reference count rather than a simple flag is necessary because

multiple handlers can operate on the same descriptor concurrently

(e.g. IBRD and IBWAIT on the same handle from different threads).

A separate counter is needed because io_in_progress can be cleared from

unprivileged userspace via the IBWAIT ioctl (through general_ibstatus()

with set_mask containing CMPL), which would allow an attacker to bypass

a check based solely on io_in_progress. The new descriptor_busy

counter is only modified by the kernel IO paths.

The lock ordering is consistent (big_gpib_mutex -> descriptors_mutex)

and the handlers only hold descriptors_mutex briefly during the lookup,

so there is no deadlock risk and no impact on IO throughput.