Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect

[Andy Lutomirski]

On Sat, Dec 19, 2020 at 1:34 PM Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
>
> [ cc’ing some more people who have experience with similar problems ]
>
> > On Dec 19, 2020, at 11:15 AM, Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote:
> >
> > Hello,
> >
> > On Fri, Dec 18, 2020 at 08:30:06PM -0800, Nadav Amit wrote:
> >> Analyzing this problem indicates that there is a real bug since
> >> mmap_lock is only taken for read in mwriteprotect_range(). This might
> >
> > Never having to take the mmap_sem for writing, and in turn never
> > blocking, in order to modify the pagetables is quite an important
> > feature in uffd that justifies uffd instead of mprotect. It's not the
> > most important reason to use uffd, but it'd be nice if that guarantee
> > would remain also for the UFFDIO_WRITEPROTECT API, not only for the
> > other pgtable manipulations.
> >
> >> Consider the following scenario with 3 CPUs (cpu2 is not shown):
> >>
> >> cpu0                         cpu1
> >> ----                         ----
> >> userfaultfd_writeprotect()
> >> [ write-protecting ]
> >> mwriteprotect_range()
> >> mmap_read_lock()
> >> change_protection()
> >>  change_protection_range()
> >>   ...
> >>   change_pte_range()
> >>   [ defer TLB flushes]
> >>                              userfaultfd_writeprotect()
> >>                               mmap_read_lock()
> >>                               change_protection()
> >>                               [ write-unprotect ]
> >>                               ...
> >>                                [ unprotect PTE logically ]
> >>                              ...
> >>                              [ page-fault]
> >>                              ...
> >>                              wp_page_copy()
> >>                              [ set new writable page in PTE]
> >
> > Can't we check mm_tlb_flush_pending(vma->vm_mm) if MM_CP_UFFD_WP_ALL
> > is set and do an explicit (potentially spurious) tlb flush before
> > write-unprotect?
>
> There is a concrete scenario that I actually encountered and then there is a
> general problem.
>
> In general, the kernel code assumes that PTEs that are read from the
> page-tables are coherent across all the TLBs, excluding permission promotion
> (i.e., the PTE may have higher permissions in the page-tables than those
> that are cached in the TLBs).
>
> We therefore need to both: (a) protect change_protection_range() from the
> changes of others who might defer TLB flushes without taking mmap_sem for
> write (e.g., try_to_unmap_one()); and (b) to protect others (e.g.,
> page-fault handlers) from concurrent changes of change_protection().
>
> We have already encountered several similar bugs, and debugging such issues
> s time consuming and these bugs impact is substantial (memory corruption,
> security). So I think we should only stick to general solutions.
>
> So perhaps your the approach of your proposed solution is feasible, but it
> would have to be applied all over the place: we will need to add a check for
> mm_tlb_flush_pending() and conditionally flush the TLB in every case in
> which PTEs are read and there might be an assumption that the
> access-permission reflect what the TLBs hold. This includes page-fault
> handlers, but also NUMA migration code in change_protection(), softdirty
> cleanup in clear_refs_write() and maybe others.

I missed the beginning of this thread, but it looks to me like
userfaultfd changes PTEs with not locking except mmap_read_lock().  It
also calls inc_tlb_flush_pending(), which is very explicitly
documented as requiring the pagetable lock.  Those docs must be wrong,
because mprotect() uses the mmap_sem write lock, which is just fine,
but ISTM some kind of mutual exclusion with proper acquire/release
ordering is indeed needed.  So the userfaultfd code seems bogus.

I think userfaultfd either needs to take a real lock (probably doesn't
matter which) or the core rules about PTEs need to be rewritten.