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ptrace: Document that wait_task_inactive can't fail

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After ptrace_freeze_traced succeeds it is known that 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.

Tested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lkml.kernel.org/r/20220505182645.497868-8-ebiederm@xmission.com
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
This commit is contained in:
Eric W. Biederman 2022-05-05 12:25:36 -05:00
parent 6a2d90ba02
commit 7b0fe1367e
1 changed files with 3 additions and 11 deletions
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14
kernel/ptrace.c
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@ -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;
}