| Message ID | 20220505182645.497868-8-ebiederm@xmission.com (mailing list archive) |
|---|---|
| State | Handled Elsewhere, archived |
| Headers | show |
| Series | ptrace: cleaning up ptrace_stop | expand |
On 2022-05-05 13:26:41 [-0500], Eric W. Biederman wrote: > After ptrace_freeze_traced succeeds it is known that the the tracee the > has a __state value of __TASK_TRACED and that no __ptrace_unlink will > happen because the tracer is waiting for the tracee, and the tracee is > in ptrace_stop. Sebastian
diff --git a/kernel/ptrace.c b/kernel/ptrace.c index 7105821595bc..05953ac9f7bd 100644 --- a/kernel/ptrace.c +++ b/kernel/ptrace.c @@ -266,17 +266,9 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state) } read_unlock(&tasklist_lock); - if (!ret && !ignore_state) { - if (!wait_task_inactive(child, __TASK_TRACED)) { - /* - * This can only happen if may_ptrace_stop() fails and - * ptrace_stop() changes ->state back to TASK_RUNNING, - * so we should not worry about leaking __TASK_TRACED. - */ - WARN_ON(READ_ONCE(child->__state) == __TASK_TRACED); - ret = -ESRCH; - } - } + if (!ret && !ignore_state && + WARN_ON_ONCE(!wait_task_inactive(child, __TASK_TRACED))) + ret = -ESRCH; return ret; }
After ptrace_freeze_traced succeeds it is known that the the tracee has a __state value of __TASK_TRACED and that no __ptrace_unlink will happen because the tracer is waiting for the tracee, and the tracee is in ptrace_stop. The function ptrace_freeze_traced can succeed at any point after ptrace_stop has set TASK_TRACED and dropped siglock. The read_lock on tasklist_lock only excludes ptrace_attach. This means that the !current->ptrace which executes under a read_lock of tasklist_lock will never see a ptrace_freeze_trace as the tracer must have gone away before the tasklist_lock was taken and ptrace_attach can not occur until the read_lock is dropped. As ptrace_freeze_traced depends upon ptrace_attach running before it can run that excludes ptrace_freeze_traced until __state is set to TASK_RUNNING. This means that task_is_traced will fail in ptrace_freeze_attach and ptrace_freeze_attached will fail. On the current->ptrace branch of ptrace_stop which will be reached any time after ptrace_freeze_traced has succeed it is known that __state is __TASK_TRACED and schedule() will be called with that state. Use a WARN_ON_ONCE to document that wait_task_inactive(TASK_TRACED) should never fail. Remove the stale comment about may_ptrace_stop. Strictly speaking this is not true because if PREEMPT_RT is enabled wait_task_inactive can fail because __state can be changed. I don't see this as a problem as the ptrace code is currently broken on PREMPT_RT, and this is one of the issues. Failing and warning when the assumptions of the code are broken is good. Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> --- kernel/ptrace.c | 14 +++----------- 1 file changed, 3 insertions(+), 11 deletions(-)