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

[David Hildenbrand]

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.

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.

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.


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.

> 
> Some solutions we think are:
> ----------------------------
> 1) Take the mem_hotplug_lock read_lock every time page_ext access.

That would be the big hammer. But it feels wrong, because page_ext is
another subsystem that's synchronized from generic memory offlining code
via the notifier.

> 
> 2) Take the extra refcount on the page every time page_ext access is
> made, so that parallel offline operation can't free the page to buddy.

No, that's no good. Just racy.

> 
> 3) Change the design where the page_ext is valid as long as the struct
> page is alive.

:/ Doesn't spark joy.

> 
> Any other inputs here?


page_ext needs a mechanism to synchronize against any users of the data
it manages. Maybe RCU can help?


-- 
Thanks,

David / dhildenb