Re: [PATCH] mm/sparse: Fix race on mem_section->usage in pfn walkers

[Muchun Song <muchun.song@linux.dev>]

> On Apr 15, 2026, at 13:44, Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> On Wed, 15 Apr 2026 10:23:26 +0800 Muchun Song <songmuchun@bytedance.com> wrote:
> 
>> When memory is hot-removed, section_deactivate() can tear down
>> mem_section->usage while concurrent pfn walkers still inspect the
>> subsection map via pfn_section_valid() or pfn_section_first_valid().
>> 
>> After commit 5ec8e8ea8b77 ("mm/sparsemem: fix race in accessing
>> memory_section->usage") converted the teardown to an RCU-based
>> scheme, the code still relies on SECTION_HAS_MEM_MAP becoming visible
>> to readers before ms->usage is cleared and queued for freeing.
>> 
>> That ordering is not guaranteed. section_deactivate() can clear
>> ms->usage and queue kfree_rcu() before another CPU observes the
>> SECTION_HAS_MEM_MAP clear. A concurrent pfn walker can therefore see
>> valid_section() return true, enter its sched-RCU read-side critical
>> section after kfree_rcu() has already been queued, and then dereference
>> a stale ms->usage pointer.
> 
> Then what happens?  Can it oops?

Probably not, because struct mem_section_usage has no pointer members,
so there will be no dereference of a pointer. The UAF here may lead to
incorrect logic judgments later on.

> 
>> And pfn_to_online_page() can call pfn_section_valid() without its
>> own sched-RCU read-side critical section, which has similar problem.
>> 
>> The race looks like this:
>> 
>>  compact_zone()                    memunmap_pages
>>  ==============                    ==============
>>                                    __remove_pages()->
>>                                      sparse_remove_section()->
>>                                        section_deactivate():
>>                                          a) [ Clear SECTION_HAS_MEM_MAP
>>                                               is reordered to b) ]
>>                                          kfree_rcu(ms->usage)
>>      __pageblock_pfn_to_page
>>         ......
>>          pfn_valid():
>>            rcu_read_lock_sched()
>>            valid_section() // return true
>>            pfn_section_valid()
>>              [Access ms->usage which is UAF]
>>                                          WRITE_ONCE(ms->usage, NULL)
>>            rcu_read_unlock_sched()       b) Clear SECTION_HAS_MEM_MAP
>> 
>> Fix this by using rcu_replace_pointer() when clearing ms->usage in
>> section_deactivate(), then it does not rely on the order of clearing
>> of SECTION_HAS_MEM_MAP.
>> 
>> Fixes: 5ec8e8ea8b77 ("mm/sparsemem: fix race in accessing memory_section->usage")
> 
> December 2023.

The probability of reordering is relatively low, and as mentioned above,
serious issues are unlikely to occur, so it will be hard to be discovered.

Thanks,
Muchun.

> 
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>> This patch is focused on the ms->usage lifetime race only.
>> 
>> ...
>> 
>> I am not fully sure whether that reasoning is correct, or whether current
>> callers are expected to rely on additional hotplug serialization instead.
>> Comments on whether this is a real issue, and how the vmemmap lifetime is
>> expected to be handled here, would be very helpful.
> 
> Thanks.  Quite a bit for consideration.
> 
>> --- a/mm/sparse-vmemmap.c
>> +++ b/mm/sparse-vmemmap.c
>> @@ -601,8 +601,10 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>>  * was allocated during boot.
>>  */
>> 	if (!PageReserved(virt_to_page(ms->usage))) {
>> - 		kfree_rcu(ms->usage, rcu);
>> - 		WRITE_ONCE(ms->usage, NULL);
>> + 		struct mem_section_usage *usage;
>> +
>> + 		usage = rcu_replace_pointer(ms->usage, NULL, true);
>> + 		kfree_rcu(usage, rcu);
>> 	}
>> 	memmap = pfn_to_page(SECTION_ALIGN_DOWN(pfn));
>> }
> 
> This part isn't applicable to 7.0 - it depends on material I've sent to
> Linus for 7.1-rc1.
> 
> So for now I'll drop this into mm-unstable to get it some runtime
> testing.  If people like this patch and we decide to proceed with it
> then I can make it a hotfix for 7.1-rcX.  But the -stable people will
> be wanting a backportable version of it, if we decide to backport,