Fix a race on architectures with prioritized interrupts (such as m68k)

which can causes crashes in drivers/char/random.c:get_reg().
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Merge tag 'random_for_linus_stable' of git://git.kernel.org/pub/scm/linux/kernel/git/tytso/random

Pull /dev/random bug fix from Ted Ts'o:
 "Fix a race on architectures with prioritized interrupts (such as m68k)
  which can causes crashes in drivers/char/random.c:get_reg()"

* tag 'random_for_linus_stable' of git://git.kernel.org/pub/scm/linux/kernel/git/tytso/random:
  fix race in drivers/char/random.c:get_reg()

drivers/char/random.c

@@ -1097,12 +1097,16 @@ static void add_interrupt_bench(cycles_t start)
 static __u32 get_reg(struct fast_pool *f, struct pt_regs *regs)
 {
 	__u32 *ptr = (__u32 *) regs;
+	unsigned long flags;
 
 	if (regs == NULL)
 		return 0;
+	local_irq_save(flags);
 	if (f->reg_idx >= sizeof(struct pt_regs) / sizeof(__u32))
 		f->reg_idx = 0;
-	return *(ptr + f->reg_idx++);
+	ptr += f->reg_idx++;
+	local_irq_restore(flags);
+	return *ptr;
 }
 
 void add_interrupt_randomness(int irq, int irq_flags)