CVE-2025-68258: comedi: multiq3: sanitize config options in multiq3_attach()

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

comedi: multiq3: sanitize config options in multiq3attach()

Syzbot identified an issue [1] in multiq3attach() that induces a task timeout due to open() or COMEDIDEVCONFIG ioctl operations, specifically, in the case of multiq3 driver.

This problem arose when syzkaller managed to craft weird configuration options used to specify the number of channels in encoder subdevice. If a particularly great number is passed to s->nchan in multiq3attach() via it->options[2], then multiple calls to multiq3encoderreset() at the end of driver-specific attach() method will be running for minutes, thus blocking tasks and affected devices as well.

While this issue is most likely not too dangerous for real-life devices, it still makes sense to sanitize configuration inputs. Enable a sensible limit on the number of encoder chips (4 chips max, each with 2 channels) to stop this behaviour from manifesting.

[1] Syzbot crash: INFO: task syz.2.19:6067 blocked for more than 143 seconds. ... Call Trace: <TASK> contextswitch kernel/sched/core.c:5254 [inline] schedule+0x17c4/0x4d60 kernel/sched/core.c:6862 scheduleloop kernel/sched/core.c:6944 [inline] schedule+0x165/0x360 kernel/sched/core.c:6959 schedulepreemptdisabled+0x13/0x30 kernel/sched/core.c:7016 mutexlockcommon kernel/locking/mutex.c:676 [inline] mutexlock+0x7e6/0x1350 kernel/locking/mutex.c:760 comediopen+0xc0/0x590 drivers/comedi/comedifops.c:2868 chrdevopen+0x4cc/0x5e0 fs/chardev.c:414 dodentryopen+0x953/0x13f0 fs/open.c:965 vfsopen+0x3b/0x340 fs/open.c:1097 ...