Re: [PATCH RT] Revert "ARM: Initialize split page table locks for vector page"

[Matthew Wilcox]

On Mon, Oct 14, 2019 at 06:02:38PM +0200, Sebastian Andrzej Siewior wrote:
> I'm dropping this patch, with its original description:
> 
> |ARM: Initialize split page table locks for vector page
> |
> |Without this patch, ARM can not use SPLIT_PTLOCK_CPUS if
> |PREEMPT_RT_FULL=y because vectors_user_mapping() creates a
> |VM_ALWAYSDUMP mapping of the vector page (address 0xffff0000), but no
> |ptl->lock has been allocated for the page.  An attempt to coredump
> |that page will result in a kernel NULL pointer dereference when
> |follow_page() attempts to lock the page.
> |
> |The call tree to the NULL pointer dereference is:
> |
> |   do_notify_resume()
> |      get_signal_to_deliver()
> |         do_coredump()
> |            elf_core_dump()
> |               get_dump_page()
> |                  __get_user_pages()
> |                     follow_page()
> |                        pte_offset_map_lock() <----- a #define
> |                           ...
> |                              rt_spin_lock()
> |
> |The underlying problem is exposed by mm-shrink-the-page-frame-to-rt-size.patch.
> 
> The patch named mm-shrink-the-page-frame-to-rt-size.patch was dropped
> from the RT queue once the SPLIT_PTLOCK_CPUS feature (in a slightly
> different shape) went upstream (somewhere between v3.12 and v3.14).
> 
> I can see that the patch still allocates a lock which wasn't there
> before. However I can't trigger a kernel oops like described in the
> patch by triggering a coredump.

Did your test build have ALLOC_SPLIT_PTLOCKS defined?