Re: [PATCH V5] mm: fix use-after free of page_ext after race with memory-offline

[Michal Hocko]

On Fri 26-08-22 12:26:33, Charan Teja Kalla wrote:
> The below is one path where race between page_ext and  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 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 (check [3] for why this design is
> chosen).
> 
> The above mentioned race is just one example __but the problem persists
> in the other paths too involving page_ext->flags access(eg:
> page_is_idle())__.
> 
> Fix all the paths where offline races with page_ext access by
> maintaining synchronization with rcu lock and is achieved in 3 steps:
> 1) Invalidate all the page_ext's of the sections of a memory block by
> storing a flag in the LSB of mem_section->page_ext.
> 
> 2) Wait till all the existing readers to finish working with the
> ->page_ext's with synchronize_rcu(). Any parallel process that starts
> after this call will not get page_ext, through lookup_page_ext(), for
> the block parallel offline operation is being performed.
> 
> 3) Now safely free all sections ->page_ext's of the block on which
> offline operation is being performed.
> 
> Note: If synchronize_rcu() takes time then optimizations can be done in
> this path through call_rcu()[2].
> 
> Thanks to David Hildenbrand for his views/suggestions on the initial
> discussion[1] and Pavan kondeti for various inputs on this patch.
> 
> [1] https://lore.kernel.org/linux-mm/59edde13-4167-8550-86f0-11fc67882107@xxxxxxxxxxx/
> [2] https://lore.kernel.org/all/a26ce299-aed1-b8ad-711e-a49e82bdd180@xxxxxxxxxxx/T/#u
> [3] https://lore.kernel.org/all/6fa6b7aa-731e-891c-3efb-a03d6a700efa@xxxxxxxxxx/
> 
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Suggested-by: Michal Hocko <mhocko@xxxxxxxx>
> Signed-off-by: Charan Teja Kalla <quic_charante@xxxxxxxxxxx>
> ---
> Changes in V5:
>    o Used the loop variable name as ext_put_continue -- David

If this is the only change since the last version (I didn't have much
time to read through the whole patch) then my ack still applies.
-- 
Michal Hocko
SUSE Labs