comedi: check device's attached status in compat ioctls

Syzbot identified an issue [1] that crashes kernel, seemingly due to
unexistent callback dev->get_valid_routes(). By all means, this should
not occur as said callback must always be set to
get_zero_valid_routes() in __comedi_device_postconfig().

As the crash seems to appear exclusively in i386 kernels, at least,
judging from [1] reports, the blame lies with compat versions
of standard IOCTL handlers. Several of them are modified and
do not use comedi_unlocked_ioctl(). While functionality of these
ioctls essentially copy their original versions, they do not
have required sanity check for device's attached status. This,
in turn, leads to a possibility of calling select IOCTLs on a
device that has not been properly setup, even via COMEDI_DEVCONFIG.

Doing so on unconfigured devices means that several crucial steps
are missed, for instance, specifying dev->get_valid_routes()
callback.

Fix this somewhat crudely by ensuring device's attached status before
performing any ioctls, improving logic consistency between modern
and compat functions.

[1] Syzbot report:
BUG: kernel NULL pointer dereference, address: 0000000000000000
...
CR2: ffffffffffffffd6 CR3: 000000006c717000 CR4: 0000000000352ef0
Call Trace:
<TASK>
get_valid_routes drivers/comedi/comedi_fops.c:1322 [inline]
parse_insn+0x78c/0x1970 drivers/comedi/comedi_fops.c:1401
do_insnlist_ioctl+0x272/0x700 drivers/comedi/comedi_fops.c:1594
compat_insnlist drivers/comedi/comedi_fops.c:3208 [inline]
comedi_compat_ioctl+0x810/0x990 drivers/comedi/comedi_fops.c:3273
__do_compat_sys_ioctl fs/ioctl.c:695 [inline]
__se_compat_sys_ioctl fs/ioctl.c:638 [inline]
__ia32_compat_sys_ioctl+0x242/0x370 fs/ioctl.c:638
do_syscall_32_irqs_on arch/x86/entry/syscall_32.c:83 [inline]
...

Reported-by: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=ab8008c24e84adee93ff
Fixes: 3fbfd2223a27 ("comedi: get rid of compat_alloc_user_space() mess in COMEDI_CHANINFO compat")
Cc: stable <[e-mail]>
Reviewed-by: Ian Abbott <[e-mail]>
Signed-off-by: Nikita Zhandarovich <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/comedi/comedi_fops.c

···
 	chaninfo.rangelist = compat_ptr(chaninfo32.rangelist);
 
 	mutex_lock(&dev->mutex);
-	err = do_chaninfo_ioctl(dev, &chaninfo);
+	if (!dev->attached) {
+		dev_dbg(dev->class_dev, "no driver attached\n");
+		err = -ENODEV;
+	} else {
+		err = do_chaninfo_ioctl(dev, &chaninfo);
+	}
 	mutex_unlock(&dev->mutex);
 	return err;
 }
···
 	rangeinfo.range_ptr = compat_ptr(rangeinfo32.range_ptr);
 
 	mutex_lock(&dev->mutex);
-	err = do_rangeinfo_ioctl(dev, &rangeinfo);
+	if (!dev->attached) {
+		dev_dbg(dev->class_dev, "no driver attached\n");
+		err = -ENODEV;
+	} else {
+		err = do_rangeinfo_ioctl(dev, &rangeinfo);
+	}
 	mutex_unlock(&dev->mutex);
 	return err;
 }
···
 		return rc;
 
 	mutex_lock(&dev->mutex);
-	rc = do_cmd_ioctl(dev, &cmd, &copy, file);
+	if (!dev->attached) {
+		dev_dbg(dev->class_dev, "no driver attached\n");
+		rc = -ENODEV;
+	} else {
+		rc = do_cmd_ioctl(dev, &cmd, &copy, file);
+	}
 	mutex_unlock(&dev->mutex);
 	if (copy) {
 		/* Special case: copy cmd back to user. */
···
 		return rc;
 
 	mutex_lock(&dev->mutex);
-	rc = do_cmdtest_ioctl(dev, &cmd, &copy, file);
+	if (!dev->attached) {
+		dev_dbg(dev->class_dev, "no driver attached\n");
+		rc = -ENODEV;
+	} else {
+		rc = do_cmdtest_ioctl(dev, &cmd, &copy, file);
+	}
 	mutex_unlock(&dev->mutex);
 	if (copy) {
 		err = put_compat_cmd(compat_ptr(arg), &cmd);
···
 	}
 
 	mutex_lock(&dev->mutex);
-	rc = do_insnlist_ioctl(dev, insns, insnlist32.n_insns, file);
+	if (!dev->attached) {
+		dev_dbg(dev->class_dev, "no driver attached\n");
+		rc = -ENODEV;
+	} else {
+		rc = do_insnlist_ioctl(dev, insns, insnlist32.n_insns, file);
+	}
 	mutex_unlock(&dev->mutex);
 	kfree(insns);
 	return rc;
···
 		return rc;
 
 	mutex_lock(&dev->mutex);
-	rc = do_insn_ioctl(dev, &insn, file);
+	if (!dev->attached) {
+		dev_dbg(dev->class_dev, "no driver attached\n");
+		rc = -ENODEV;
+	} else {
+		rc = do_insn_ioctl(dev, &insn, file);
+	}
 	mutex_unlock(&dev->mutex);
 	return rc;
 }