Re: [PATCH v2 1/8] exec: introduce cred_guard_light

[Oleg Nesterov <oleg@redhat.com>]

On 09/23, Jann Horn wrote:
>
> This is a new per-threadgroup lock that can often be taken instead of
> cred_guard_mutex and has less deadlock potential.

Oh, please don't.

> I'm doing this because
> Oleg Nesterov mentioned the potential for deadlocks, in particular if a
> debugged task is stuck in execve, trying to get rid of a ptrace-stopped
> thread, and the debugger attempts to inspect procfs files of the debugged
> task.

Yes, but we need to fix this anyway. And I am not sure the new mutex can
actually help.

And I think that cred_guard_mutex is already over-used in fs/proc. Say,
I think lock_trace() must die, I simply can't understand why it is useful.

Suppose we modify, say, proc_pid_stack() to do

	save_stack_trace_tsk(task, &trace);
	if (!ptrace_may_access(task, ...))
		goto return -EPERM;

	for (i = 0; i < trace.nr_entries; i++)
		seq_printf(...);

	return 0;

is there any problem if it shows some trace before setuid exec does
install_exec_creds() ?

Oleg.