linux-kernel - [PATCH 1/2] ptrace: ptrace_resume() shouldn't wake up !TASK

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Date:	Mon, 23 May 2011 19:05:28 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Denys Vlasenko <vda.linux@...glemail.com>,
	Tejun Heo <tj@...nel.org>
Cc:	jan.kratochvil@...hat.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	indan@....nu
Subject: [PATCH 1/2] ptrace: ptrace_resume() shouldn't wake up !TASK_TRACED
	thread

It is not clear why ptrace_resume() does wake_up_process(). Unless the
caller is PTRACE_KILL the tracee should be TASK_TRACED so we can use
wake_up_state(__TASK_TRACED). If sys_ptrace() races with SIGKILL we do
not need the extra and potentionally spurious wakeup.

If the caller is PTRACE_KILL, wake_up_process() is even more wrong.
The tracee can sleep in any state in any place, and if we have a buggy
code which doesn't handle a spurious wakeup correctly PTRACE_KILL can
be used to exploit it. For example:

	int main(void)
	{
		int child, status;

		child = fork();
		if (!child) {
			int ret;

			assert(ptrace(PTRACE_TRACEME, 0,0,0) == 0);

			ret = pause();
			printf("pause: %d %m\n", ret);

			return 0x23;
		}

		sleep(1);
		assert(ptrace(PTRACE_KILL, child, 0,0) == 0);

		assert(child == wait(&status));
		printf("wait: %x\n", status);

		return 0;
	}

prints "pause: -1 Unknown error 514", -ERESTARTNOHAND leaks to the
userland. In this case sys_pause() is buggy as well and should be
fixed.

I do not know what was the original rationality behind PTRACE_KILL.
The man page is simply wrong and afaics it was always wrong. Imho
it should be deprecated, or may be it should do send_sig(SIGKILL)
as Denys suggests, but in any case I do not think that the current
behaviour was intentional.

Note: there is another problem, ptrace_resume() changes ->exit_code
and this can race with SIGKILL too. Eventually we should change to
not use ->exit_code.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---

 kernel/ptrace.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- sigprocmask/kernel/ptrace.c~ptrace_resume_wakeup	2011-05-23 18:09:48.000000000 +0200
+++ sigprocmask/kernel/ptrace.c	2011-05-23 18:20:18.000000000 +0200
@@ -561,7 +561,7 @@ static int ptrace_resume(struct task_str
 	}

 	child->exit_code = data;
-	wake_up_process(child);
+	wake_up_state(child, __TASK_TRACED);

 	return 0;
 }

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