Re: KASAN: use-after-free Read in __do_page_fault

[Andrea Arcangeli]

On Tue, Oct 31, 2017 at 08:37:47AM -0700, Linus Torvalds wrote:
> Yes. Accessing "vma" after calling "handle_mm_fault()" is a bug. An
> unfortunate issue with userfaultfd.
> 
> The suggested fix to simply look up pkey beforehand seems sane and simple.

Agreed.

> 
> But sadly, from a quick check, it looks like arch/um/ has the same
> bug, but even worse. It will do
> 
>  (a) handle_mm_fault() in a loop without re-calculating vma. Don't ask me why.
> 
>  (b) flush_tlb_page(vma, address); afterwards

Yes, that flush_tlb_page is unsafe. Luckily it's only using it for
vma->vm_mm so it doesn't sound major issue to fix it.

> 
> but much more importantly, I think __get_user_pages() is broken in two ways:
> 
>  - faultin_page() does:
> 
>         ret = handle_mm_fault(vma, address, fault_flags);
>         ...
>         if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE))
> 
>    (easily fixed the same way)
> 
>  - more annoyingly and harder to fix: the retry case in
> __get_user_pages(), and the VMA saving there.
> 
> Ho humm.
> 
> Andrea, looking at that get_user_pages() case, I really think it's
> userfaultfd that is broken.
> 
> Could we perhaps limit userfaultfd to _only_ do the VM_FAULT_RETRY,
> and simply fail for non-retry faults?

In the get_user_pages case we already limit it to do only
VM_FAULT_RETRY so no use after free should materialize whenever gup is
involved.

The problematic path for the return to userland (get_user_pages
returns to kernel) is this one:

	if (return_to_userland) {
		if (signal_pending(current) &&
		    !fatal_signal_pending(current)) {
			/*
			 * If we got a SIGSTOP or SIGCONT and this is
			 * a normal userland page fault, just let
			 * userland return so the signal will be
			 * handled and gdb debugging works.  The page
			 * fault code immediately after we return from
			 * this function is going to release the
			 * mmap_sem and it's not depending on it
			 * (unlike gup would if we were not to return
			 * VM_FAULT_RETRY).
			 *
			 * If a fatal signal is pending we still take
			 * the streamlined VM_FAULT_RETRY failure path
			 * and there's no need to retake the mmap_sem
			 * in such case.
			 */
			down_read(&mm->mmap_sem);
			ret = VM_FAULT_NOPAGE;
		}
	}

We could remove the above branch all together and then
handle_userfault() would always return VM_FAULT_RETRY whenever it
decides to release the mmap_sem. The above makes debugging with gdb
more user friendly and it potentially lowers the latency of signals as
signals can unblock handle_userfault. The downside is that the return
to userland cannot dereference the vma after calling handle_mm_fault.

Thanks,
Andrea