Sorry for delay, I was offline. I'll try to return to this problem next
week, currently I can't even read this thread but at first glance the
proposed patch(es) do not look right...

On 09/21, Michal Hocko wrote:
>
> The further investigation shown that the tracer (strace) is stuck
> waiting for cred_guard_mutex
> [<0000000000000000>] mm_access+0x22/0xa0
> [<0000000000000000>] process_vm_rw_core.isra.1+0x112/0x6c0
> [<0000000000000000>] process_vm_rw+0xab/0x110
> [<0000000000000000>] SyS_process_vm_readv+0x15/0x20
> [<0000000000000000>] system_call_fastpath+0x16/0x1b
> [<0000000000000000>] 0x7f186f031c3a
> [<0000000000000000>] 0xffffffffffffffff
...
> this however doesn't happen because both threads which are dead
> are zombies waiting to be reaped by the parent and to call their
> release_task->__exit_signal.

Yes, I know, this was already reported. And so far I do not see any
reasonable fix. I _think_ that the "real" fix should rework the
security_bprm_* helpers, but unlikely this is possible.

The trivial test-case:

        void *thread(void *arg)
        {
                ptrace(PTRACE_TRACEME, 0,0,0);
                return NULL;
        }

        int main(void)
        {
                int pid = fork();

                if (!pid) {
                        pthread_t pt;
                        pthread_create(&pt, NULL, thread, NULL);
                        pthread_join(pt, NULL);
                        execlp("echo", "echo", "passed", NULL);
                }

                sleep(1);
                // or anything else which needs ->cred_guard_mutex,
                // say open(/proc/$pid/mem)
                ptrace(PTRACE_ATTACH, pid, 0,0);
                kill(pid, SIGCONT);

                return 0;
        }

Oleg.


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