[Lucid CVE 1/4] ptrace: ptrace_resume() shouldn't wake up !TASK_TRACED thread

Luis Henriques luis.henriques at canonical.com
Tue Feb 19 17:31:07 UTC 2013

From: Oleg Nesterov <oleg at redhat.com>


BugLink: http://bugs.launchpad.net/bugs/1129192

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;

		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

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 ptrace
to not use ->exit_code.

Signed-off-by: Oleg Nesterov <oleg at redhat.com>
(cherry picked from commit 0666fb51b1483f27506e212cc7f7b2645b5c7acc)

Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
 kernel/ptrace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 05625f6..d8184b5 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -506,7 +506,7 @@ static int ptrace_resume(struct task_struct *child, long request, long data)
 	child->exit_code = data;
-	wake_up_process(child);
+	wake_up_state(child, __TASK_TRACED);
 	return 0;

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