futex: Handle invalid node numbers supplied by user

syzbot used a negative node number which was not rejected early and led
to invalid memory access in node_possible().

Reject negative node numbers except for FUTEX_NO_NODE.

[bigeasy: Keep the FUTEX_NO_NODE check]

Closes: https://lore.kernel.org/all/6835bfe3.a70a0220.253bc2.00b5.GAE@google.com/
Fixes: cec199c ("futex: Implement FUTEX2_NUMA")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reported-by: syzbot+9afaf6749e3a7aa1bdf3@syzkaller.appspotmail.com
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250528085521.1938355-4-bigeasy@linutronix.de

Author: Peter Zijlstra

kernel/futex/core.c

@@ -583,8 +583,8 @@ int get_futex_key(u32 __user *uaddr, unsigned int flags, union futex_key *key,
 		if (futex_get_value(&node, naddr))
 			return -EFAULT;
 
-		if (node != FUTEX_NO_NODE &&
-		    (node >= MAX_NUMNODES || !node_possible(node)))
+		if ((node != FUTEX_NO_NODE) &&
+		    ((unsigned int)node >= MAX_NUMNODES || !node_possible(node)))
 			return -EINVAL;
 	}