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
