Re: [PATCH 2/2] exec: Add a exec_update_mutex to replace cred_guard_mutex

Bernd Edlinger
2020-03-06T19:16:12Z

On 3/6/20 6:17 AM, Eric W. Biederman wrote:
> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
>
>> On 3/5/20 10:16 PM, Eric W. Biederman wrote:
>>>
>>> The cred_guard_mutex is problematic. The cred_guard_mutex is held
>>> over the userspace accesses as the arguments from userspace are read.
>>> The cred_guard_mutex is held of PTRACE_EVENT_EXIT as the the other
>>> threads are killed. The cred_guard_mutex is held over
>>> "put_user(0, tsk->clear_child_tid)" in exit_mm().
>>>

I am all for this patch, and the direction it is heading, Eric.

I just wanted to add a note that I think it is
possible that exec_mm_release can also invoke put_user(0, tsk->clear_child_tid),
under the new exec_update_mutex, since vm_access increments the
mm->mm_users, under the cred_update_mutex, but releases the mutex,
and the caller can hold the reference for a while and then exec_mmap is not
releasing the last reference.

Bernd.