stacktrace: Force USER_DS for stack_trace_save_user()

When walking userspace stacks, USER_DS needs to be set, otherwise
access_ok() will not function as expected.

Reported-by: Vegard Nossum <vegard.nossum@oracle.com>
Reported-by: Eiichi Tsukata <devel@etsukata.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Vegard Nossum <vegard.nossum@oracle.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Link: https://lkml.kernel.org/r/20190718085754.GM3402@hirez.programming.kicks-ass.net

kernel/stacktrace.c

@@ -226,12 +226,17 @@ unsigned int stack_trace_save_user(unsigned long *store, unsigned int size)
 		.store	= store,
 		.size	= size,
 	};
+	mm_segment_t fs;
 
 	/* Trace user stack if not a kernel thread */
 	if (current->flags & PF_KTHREAD)
 		return 0;
 
+	fs = get_fs();
+	set_fs(USER_DS);
 	arch_stack_walk_user(consume_entry, &c, task_pt_regs(current));
+	set_fs(fs);
+
 	return c.len;
 }
 #endif