Re: [PATCH] mm: Prevent derefencing NULL ptr in pfn_section_valid()

[Waiman Long <longman@redhat.com>]

On 7/1/24 09:50, Charan Teja Kalla wrote:
> Hi Waiman,
>
> On 6/26/2024 5:46 AM, Waiman Long wrote:
>> Commit 5ec8e8ea8b77 ("mm/sparsemem: fix race in accessing
>> memory_section->usage") changed pfn_section_valid() to add a READ_ONCE()
>> call around "ms->usage" to fix a race with section_deactivate() where
>> ms->usage can be cleared.  The READ_ONCE() call, by itself, is not enough
>> to prevent NULL pointer dereference.  We need to check its value before
>> dereferencing it.
> I am unable to see a scenario where ms->usage will be NULL when
> pfn_section_valid() is called:
>
> 1) In pfn_valid, valid_section() check ensures that pfn_section_valid()
> is not called as the section is marked as invalid.
>
> 2) In pfn_to_online_page, online_section() check ensures that
> pfn_section_valid() is not called.
>
> and in the update path, we do:
>       kfree_rcu(ms->usage, rcu);
>       WRITE_ONCE(ms->usage, NULL);
>
> Could you help me in understanding about what I am missing here, please?
>
With the below timing sequence:

      CPU 0                                      CPU 1
      -----                                      -----
                                         if (!valid_section(ms))
                                             return 0;
  ms->section_mem_map &=
     ~SECTION_HAS_MEM_MAP                <interrupt>

  WRITE_ONCE(ms->usage, NULL);
                                         READ_ONCE(ms->usage)->subsection_map


In the time gap between valid_section() check and accessing ms->usage,
it may have been cleared leading to dereferencing a NULL pointer. That is
why it will be prudent to do a NULL check first.

Regards,
Longman