Re: Discussion on race between freed page_ext access and memory offline operation

[Charan Teja Kalla]

Thanks David for the inputs!!

On 6/27/2022 10:05 PM, David Hildenbrand wrote:
> On 27.06.22 18:09, Charan Teja Kalla wrote:
>> The below race between page_ext and online/offline of the respective
>> memory blocks will cause use-after-free on the access of page_ext structure.
>>
>> process1			    process2
>> ---------			    ---------
>> a)doing /proc/page_owner            doing memory offline	
>> 			            through offline_pages
>>
>> b)PageBuddy check is failed
>> thus proceed to get the
>> page_owner information
>> through page_ext access.
>> page_ext = lookup_page_ext(page);
>> 				
>> 				  migrate_pages();
>> 				 ................
>> 				Since all pages are successfully
>> 				migrated as part of the offline
>> 				operation,send MEM_OFFLINE notification
>> 				where for page_ext it calls:
>> 				offline_page_ext()-->
>> 				  __free_page_ext()-->
>> 				    free_page_ext()-->
>> 				      vfree(ms->page_ext)
>> 				mem_section->page_ext = NULL
>>
>> c) Check for the PAGE_EXT flags
>> in the page_ext->flags access
>> results into the use-after-free(leading
>> to the translation faults).
>>
>> As mentioned above, there is really no synchronization between page_ext
>> access and its freeing in the memory_offline.  The above is just one
>> example but the problem persists in the other paths too involving
>> page_ext->flags access(eg: page_is_idle()).
>>
>> The memory offline steps(roughly) on a memory block is as below:
>> 1) Isolate all the pages
>> 2) while(1)
>>   try free the pages to buddy.(->free_list[MIGRATE_ISOLATE])
>> 3) delete the pages from this buddy list.
>> 4) Then free page_ext.(Note: The struct page is still alive as it is
>> freed only during hot remove of the memory which frees the memmap, which
>> steps the user might not perform).
>>
>> This design leads to the state where struct page is alive but the struct
>> page_ext is freed, where the later is ideally part of the former which
>> just representing the page_flags. This seems to be a wrong design where
>> 'struct page' as a whole is not accessible(Thanks to Minchan for
>> pointing this out).
> Accessing the struct page -- including any extensions -- is invalid if
> the memory section is marked offline.
> 
> Usual PFN walkers use pfn_to_online_page() to make sure we have PFN with
> an actual meaning in it.

Is there such enforcement from the kernel side to use the
pfn_to_online_page() while doing the pfn walk?  Eg: In the same
read_page_owner()(Not sure of the other places), it is not used while
doing the pfn walk.

> 
> There is no real synchronization between pfn_to_online_page() and memory
> offline code. For now it wasn't required because it was never relevant
> in practice.
> 

Isn't the race here makes the code to still use the page despite it got
offlined parallel there by making the statement 'Accessing the struct
page -- including any extensions -- is invalid' applicable here.  Eg: In
the same read_page_owner(), it can go and try to dump the page_owner of
a page(agree that it dumps the proper page_owner) in print_page_owner(),
where it accesses the page->flags?

> After pfn_to_online_page() it takes quite a long time until memory is
> actually offlined and then, the memmap is removed. Maybe it's different
> for page_ext.
> 
As you already well aware, the memmap will not be removed as long as we
are playing just with the offline/online operation but page_ext is freed
even during the offline operation making **part of the struct page is
mapped and the other part is not**.

> 
> It smells like page_ext should use some mechanism during  MEM_OFFLINE to
> synchronize against any users of its metadata. Generic memory offlining
> code might be the wrong place for that.
> 
> page_ext needs a mechanism to synchronize against any users of the data
> it manages. Maybe RCU can help?

Let me give a thought about the feasibility of this. But this requires
making code at all the places where moving the page_ext users under
rcu_lock.