lib/syscall: Pin the task stack in collect_syscall()

This will avoid a potential read-after-free if collect_syscall()
(e.g. /proc/PID/syscall) is called on an exiting task.
Reported-by: Jann Horn <jann@thejh.net>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/0bfd8e6d4729c97745d3781a29610a33d0a8091d.1474003868.git.luto@kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

lib/syscall.c

@@ -7,9 +7,19 @@ static int collect_syscall(struct task_struct *target, long *callno,
 			   unsigned long args[6], unsigned int maxargs,
 			   unsigned long *sp, unsigned long *pc)
 {
-	struct pt_regs *regs = task_pt_regs(target);
-	if (unlikely(!regs))
+	struct pt_regs *regs;
+
+	if (!try_get_task_stack(target)) {
+		/* Task has no stack, so the task isn't in a syscall. */
+		*callno = -1;
+		return 0;
+	}
+
+	regs = task_pt_regs(target);
+	if (unlikely(!regs)) {
+		put_task_stack(target);
 		return -EAGAIN;
+	}
 
 	*sp = user_stack_pointer(regs);
 	*pc = instruction_pointer(regs);
@@ -18,6 +28,7 @@ static int collect_syscall(struct task_struct *target, long *callno,
 	if (*callno != -1L && maxargs > 0)
 		syscall_get_arguments(target, regs, 0, maxargs, args);
 
+	put_task_stack(target);
 	return 0;
 }