Messages in this thread | | | From | (Eric W. Biederman) | Date | Fri, 06 Mar 2020 15:58:52 -0600 | Subject | Re: [PATCH 2/2] exec: Add a exec_update_mutex to replace cred_guard_mutex |
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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
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