futex: Ensure that futex address is aligned in handle_futex_death()

The futex code requires that the user space addresses of futexes are 32bit
aligned. sys_futex() checks this in futex_get_keys() but the robust list
code has no alignment check in place.

As a consequence the kernel crashes on architectures with strict alignment
requirements in handle_futex_death() when trying to cmpxchg() on an
unaligned futex address which was retrieved from the robust list.

[ tglx: Rewrote changelog, proper sizeof() based alignement check and add
  	comment ]

Fixes: 0771dfefc9 ("[PATCH] lightweight robust futexes: core")
Signed-off-by: Chen Jie <chenjie6@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: <dvhart@infradead.org>
Cc: <peterz@infradead.org>
Cc: <zengweilin@huawei.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/1552621478-119787-1-git-send-email-chenjie6@huawei.com

kernel/futex.c

@@ -3436,6 +3436,10 @@ static int handle_futex_death(u32 __user *uaddr, struct task_struct *curr, int p
 {
 	u32 uval, uninitialized_var(nval), mval;
 
+	/* Futex address must be 32bit aligned */
+	if ((((unsigned long)uaddr) % sizeof(*uaddr)) != 0)
+		return -1;
+
 retry:
 	if (get_user(uval, uaddr))
 		return -1;