Re: usbdev_mmap causes type confusion in page_table_check

[Pasha Tatashin <pasha.tatashin@soleen.com>]

On Mon, May 8, 2023 at 4:37 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 09.05.23 01:21, Pasha Tatashin wrote:
> >> For normal Kernel-MM operations, vm_normal_page() should be used to
> >> get "struct page" based on vma+addr+pte combination, but
> >> page_table_check does not use vma for its operation in order to
> >> strengthen the verification of no invalid page sharing. But, even
>
> I'm not sure if that's the right approach for this case here, though.
>
> >> vm_normal_page() can cause access to the "struct page" for VM_PFNMAP
> >> if pfn_valid(pfn) is true. So, vm_normal_page() can return a struct
> >> page for a user mapped slab page.
> >
> > Only for !ARCH_HAS_PTE_SPECIAL case, otherwise NULL is returned.
>
> That would violate VM_PFNMAP semantics, though. I remember that there
> was a trick to it.
>
> Assuming we map /dev/mem, what stops a page we mapped and determined to
> be !anon to be freed and reused, such that we suddenly have an anon page
> mappped?
>
> In that case, we really don't want to look at the "struct page" ever, no?

Good point. page_table_check just does not work well /dev/mem. I am
thinking of adding BUG_ON(PageSlab(page); and also "depends on
!DEVMEM" for the PAGE_TABLE_CHECK config option.

Pasha