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

Eric W. Biederman
2020-03-06T22:01:13Z

Bernd Edlinger <bernd.edlinger@hotmail.de> writes:

> 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.

Good catch. I really appreciate your close look at the details.

I am wondering if process_vm_readv and process_vm_writev could be
safely changed to use mmgrab and mmdrop, instead of mmget and mmput.

That would resolve the potential issue you have pointed out. I just
haven't figured out if it is safe. The mm code has been seriously
refactored since I knew how it all worked.

Eric