ebiederm@xmission.com (Eric W. Biederman) writes:
ebiederm@xmission.com (Eric W. Biederman) writes:
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.
Nope, mmget can not be replaced by mmgrab.
It might be possible to do something creative like store a cred in place of the userns on the mm and use that for mm_access permission checks. Still we are talking a pretty narrow window, and a case that no one has figured out how to trigger yet. So I will leave that corner case as something for future improvements.
My brain is restless and keep looking at it.
The worst case is processes created with CLONE_VM|CLONE_CHILD_CLEARTID but not CLONE_THREAD. For those that put_user will occur ever time in exec_mmap.
The only solution that I can see is to move taking the new mutex after exec_mm_release. Which may be feasible given how close exec_mmap follows de_thread.
I am going to sleep on that and perhaps I will be able to see how to move taking the mutex lower.
It would be very nice not to have a known issue going into this set of changes.
Eric