fork: do not invoke uffd on fork if error occurs

In the Linux kernel, the following vulnerability has been resolved:

fork: do not invoke uffd on fork if error occurs

Patch series "fork: do not expose incomplete mm on fork".

During fork we may place the virtual memory address space into an

inconsistent state before the fork operation is complete.

In addition, we may encounter an error during the fork operation that

indicates that the virtual memory address space is invalidated.

As a result, we should not be exposing it in any way to external machinery

that might interact with the mm or VMAs, machinery that is not designed to

deal with incomplete state.

We specifically update the fork logic to defer khugepaged and ksm to the

end of the operation and only to be invoked if no error arose, and

disallow uffd from observing fork events should an error have occurred.

This patch (of 2):

Currently on fork we expose the virtual address space of a process to

userland unconditionally if uffd is registered in VMAs, regardless of

whether an error arose in the fork.

This is performed in dup_userfaultfd_complete() which is invoked

unconditionally, and performs two duties - invoking registered handlers

for the UFFD_EVENT_FORK event via dup_fctx(), and clearing down

userfaultfd_fork_ctx objects established in dup_userfaultfd().

This is problematic, because the virtual address space may not yet be

correctly initialised if an error arose.

The change in commit d24062914837 ("fork: use __mt_dup() to duplicate

maple tree in dup_mmap()") makes this more pertinent as we may be in a

state where entries in the maple tree are not yet consistent.

We address this by, on fork error, ensuring that we roll back state that

we would otherwise expect to clean up through the event being handled by

userland and perform the memory freeing duty otherwise performed by

dup_userfaultfd_complete().

We do this by implementing a new function, dup_userfaultfd_fail(), which

performs the same loop, only decrementing reference counts.

Note that we perform mmgrab() on the parent and child mm's, however

userfaultfd_ctx_put() will mmdrop() this once the reference count drops to

zero, so we will avoid memory leaks correctly here.