Input: uinput - reject requests with unreasonable number of slots

From: Dmitry Torokhov <dmitry.torokhov@gmail.com>

When exercising uinput interface syzkaller may try setting up device
with a really large number of slots, which causes memory allocation
failure in input_mt_init_slots(). While this allocation failure is
handled properly and request is rejected, it results in syzkaller
reports. Additionally, such request may put undue burden on the
system which will try to free a lot of memory for a bogus request.

Fix it by limiting allowed number of slots to 100. This can easily
be extended if we see devices that can track more than 100 contacts.
Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+0122fa359a69694395d5@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=0122fa359a69694395d5
Link: https://lore.kernel.org/r/Zqgi7NYEbpRsJfa2@google.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/misc/uinput.c

@@ -417,6 +417,20 @@ static int uinput_validate_absinfo(struct input_dev *dev, unsigned int code,
 		return -EINVAL;
 	}
 
+	/*
+	 * Limit number of contacts to a reasonable value (100). This
+	 * ensures that we need less than 2 pages for struct input_mt
+	 * (we are not using in-kernel slot assignment so not going to
+	 * allocate memory for the "red" table), and we should have no
+	 * trouble getting this much memory.
+	 */
+	if (code == ABS_MT_SLOT && max > 99) {
+		printk(KERN_DEBUG
+		       "%s: unreasonably large number of slots requested: %d\n",
+		       UINPUT_NAME, max);
+		return -EINVAL;
+	}
+
 	return 0;
 }