[PATCH 1/2] ptrace: ptrace_resume() shouldn't wake up !TASK_TRACEDthread

From: Oleg Nesterov
Date: Mon May 23 2011 - 13:06:55 EST

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 to
not use ->exit_code.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>

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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