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

[Charan Teja Kalla <quic_charante@quicinc.com>]

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?

> 
> Fixes: 5ec8e8ea8b77 ("mm/sparsemem: fix race in accessing memory_section->usage")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  include/linux/mmzone.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 8f9c9590a42c..b1dcf6ddb406 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1980,8 +1980,9 @@ static inline int subsection_map_index(unsigned long pfn)
>  static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
>  {
>  	int idx = subsection_map_index(pfn);
> +	struct mem_section_usage *usage = READ_ONCE(ms->usage);
>  
> -	return test_bit(idx, READ_ONCE(ms->usage)->subsection_map);
> +	return usage ? test_bit(idx, usage->subsection_map) : 0;
>  }
>  #else
>  static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)