cross memory attach && security check

[Oleg Nesterov <oleg@redhat.com>]

Hello,

Just noticed the new file in mm/ ;) A couple of questions.

process_vm_rw_core() does

	task_lock(task);
	if (__ptrace_may_access(task, PTRACE_MODE_ATTACH)) {
		task_unlock(task);
		rc = -EPERM;
		goto put_task_struct;
	}
	mm = task->mm;

this is racy, task_lock() can't help. And I don't think you should
use it directly.

execve() does exec_mmap() first, this switches to the new ->mm.
After that install_exec_creds() changes task->cred. The window
is not that small.

I guess you need ->cred_guard_mutex, please look at mm_for_maps().




Another question, process_vm_rw_pages() does get_user_pages() without
FOLL_FORCE. Is this on purpose? This limits the usage of the new
syscalls.




Hmm. And could you please explain the change in rw_copy_check_uvector()?
Why process_vm_rw() does
rw_copy_check_uvector(READ, rvec, check_access => 0) ?

Oleg.