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

[Pavan Kondeti]

Hi Charan,

On Thu, Jul 14, 2022 at 08:17:43PM +0530, 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. This seems to be a wrong design where
> 'struct page' as a whole is not accessible(Thanks to Minchan for
> pointing this out).
> 
> 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())__. Since offline waits till the last reference on the
> page goes down i.e. any path that took the refcount on the page can make
> the memory offline operation to wait. Eg: In the migrate_pages()
> operation, we do take the extra refcount on the pages that are under
> migration and then we do copy page_owner by accessing page_ext. For
> 
> Fix those paths where offline races with page_ext access by maintaining
> synchronization with rcu lock.
> 
> Thanks to David Hildenbrand for his views/suggestions on the initial
> discussion[1].
> 
> [1] https://lore.kernel.org/linux-mm/59edde13-4167-8550-86f0-11fc67882107@xxxxxxxxxxx/
> 
> Signed-off-by: Charan Teja Kalla <quic_charante@xxxxxxxxxxx>
> ---
>  include/linux/page_ext.h  | 19 +++++++++++++++++++
>  include/linux/page_idle.h | 40 ++++++++++++++++++++++++++++++----------
>  mm/page_ext.c             |  3 ++-
>  mm/page_owner.c           | 18 +++++++++++++++---
>  mm/page_table_check.c     | 10 +++++++---
>  5 files changed, 73 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h
> index fabb2e1..df5d353 100644
> --- a/include/linux/page_ext.h
> +++ b/include/linux/page_ext.h
> @@ -64,6 +64,25 @@ static inline struct page_ext *page_ext_next(struct page_ext *curr)
>  	return next;
>  }
>  
> +static inline struct page_ext *get_page_ext(struct page *page)
> +{
> +	struct page_ext *page_ext;
> +
> +	rcu_read_lock();
> +	page_ext = lookup_page_ext(page);
> +	if (!page_ext) {
> +		rcu_read_unlock();
> +		return NULL;
> +	}
> +
> +	return page_ext;
> +}
> +
> +static inline void put_page_ext(void)
> +{
> +	rcu_read_unlock();
> +}
> +
Would it be a harm if we make lookup_page_ext() completely a private function?
Or is there any codepath that have the benefit of calling lookup_page_ext()
without going through get_page_ext()? If that is the case, we should add
RCU lockdep check inside lookup_page_ext() to make sure that this function is
called with RCUs.

Thanks,
Pavan