Re: [PATCH V2] mm: fix use-after free of page_ext after race with memory-offline

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Adding Michal Hocko. Sorry for the spam.

On 7/27/2022 7:45 PM, 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.
> 
> 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 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.
> 
> Thanks to David Hildenbrand for his views/suggestions on the initial
> discussion[1] and Pavan kondeti for various inputs on this patch.
> 
> FAQ's:
> Q) Should page_ext_[get|put]() needs to be used for every page_ext
> access?
> A) NO, the synchronization is really not needed in all the paths of
> accessing page_ext. One case is where extra refcount is taken on a
> page for which memory block, this pages falls into, offline operation is
> being performed. This extra refcount makes the offline operation not to
> succeed hence the freeing of page_ext.  Another case is where the page
> is already being freed and we do reset its page_owner.
> 
> Some examples where the rcu_lock is not taken while accessing the
> page_ext are:
> 1) In migration (where we also migrate the page_owner information), we
> take the extra refcount on the source and destination pages and then
> start the migration. This extra refcount makes the test_pages_isolated()
> function to fail thus retry the offline operation.
> 
> 2) In free_pages_prepare(), we do reset the page_owner(through page_ext)
> which again doesn't need the protection to access because the page is
> already freeing (through only one path).
> 
> So, users need not to use page_ext_[get|put]() when they are sure that
> extra refcount is taken on a page preventing the offline operation.
> 
> Q) Why can't the page_ext is freed in the hot_remove path, where memmap
> is also freed ?
> 
> A) As per David's answers, there are many reasons and a few are:
> 1) Discussions had happened in the past to eventually also use rcu
> protection for handling pfn_to_online_page(). So doing it cleanly here
> is certainly an improvement.
> 
> 2) It's not good having to scatter section online checks all over the
> place in page ext code. Once there is a difference between active vs.
> stale page ext data things get a bit messy and error prone. This is
> already ugly enough in our generic memmap handling code.
> 
> 3) Having on-demand allocations, such as KASAN or page ext from the
> memory online notifier is at least currently cleaner, because we don't
> have to handle each and every subsystem that hooks into that during the
> core memory hotadd/remove phase, which primarily only setups the
> vmemmap, direct map and memory block devices.
> 
> [1] https://lore.kernel.org/linux-mm/59edde13-4167-8550-86f0-11fc67882107@xxxxxxxxxxx/
> 
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Suggested-by: Michal Hocko <mhocko@xxxxxxxx>
> Signed-off-by: Charan Teja Kalla <quic_charante@xxxxxxxxxxx>
> ---
> Changes in V2:
>    o Use only get/put_page_ext() to get the page_ext in the 
>      required paths. Add proper comments for them.
>    o Use synchronize_rcu() only once instead of calling it for
>      every mem_section::page_ext of a memory block.
>    o Free'd page_ext in 3 steps of invalidate, wait till all the
>      users are finished using and then finally free page_ext.
> 
> Changes in V1:
>    o Used the RCU lock while accessing the page_ext in the paths that
>      can race with the memory offline operation.
>    o Introduced (get|put)_page_ext() function to get the page_ext of page.
>    o https://lore.kernel.org/all/1657810063-28938-1-git-send-email-quic_charante@xxxxxxxxxxx/
> 
>  include/linux/page_ext.h  | 53 +++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/page_idle.h | 40 ++++++++++++++++++++++++++---------
>  mm/page_ext.c             | 41 ++++++++++++++++++++++++++++++++----
>  mm/page_owner.c           | 35 +++++++++++++++++++++++--------
>  mm/page_table_check.c     | 10 ++++++---
>  5 files changed, 153 insertions(+), 26 deletions(-)
> 
> diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h
> index fabb2e1..3a35c95 100644
> --- a/include/linux/page_ext.h
> +++ b/include/linux/page_ext.h
> @@ -5,6 +5,7 @@
>  #include <linux/types.h>
>  #include <linux/stacktrace.h>
>  #include <linux/stackdepot.h>
> +#include <linux/rcupdate.h>
>  
>  struct pglist_data;
>  struct page_ext_operations {
> @@ -36,6 +37,8 @@ struct page_ext {
>  	unsigned long flags;
>  };
>  
> +#define PAGE_EXT_INVALID       (0x1)
> +
>  extern unsigned long page_ext_size;
>  extern void pgdat_page_ext_init(struct pglist_data *pgdat);
>  
> @@ -57,6 +60,11 @@ static inline void page_ext_init(void)
>  
>  struct page_ext *lookup_page_ext(const struct page *page);
>  
> +static inline bool page_ext_invalid(struct page_ext *page_ext)
> +{
> +	return !page_ext || (((unsigned long)page_ext & PAGE_EXT_INVALID) == 1);
> +}
> +
>  static inline struct page_ext *page_ext_next(struct page_ext *curr)
>  {
>  	void *next = curr;
> @@ -64,6 +72,37 @@ static inline struct page_ext *page_ext_next(struct page_ext *curr)
>  	return next;
>  }
>  
> +/*
> + * This function gives proper page_ext of a memory section
> + * during race with the offline operation on a memory block
> + * this section falls into. Not using this function to get
> + * page_ext of a page, in code paths where extra refcount
> + * is not taken on that page eg: pfn walking, can lead to
> + * use-after-free access of page_ext.
> + */
> +static inline struct page_ext *page_ext_get(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;
> +}
> +
> +/*
> + * Must be called after work is done with the page_ext received
> + * with page_ext_get().
> + */
> +static inline void page_ext_put(void)
> +{
> +	rcu_read_unlock();
> +}
> +
>  #else /* !CONFIG_PAGE_EXTENSION */
>  struct page_ext;
>  
> @@ -87,5 +126,19 @@ static inline void page_ext_init_flatmem_late(void)
>  static inline void page_ext_init_flatmem(void)
>  {
>  }
> +
> +static inline struct page_ext *page_ext_get(struct page *page)
> +{
> +	return NULL;
> +}
> +
> +static inline bool page_ext_invalid(struct page_ext *page_ext)
> +{
> +	return true;
> +}
> +
> +static inline void page_ext_put(void)
> +{
> +}
>  #endif /* CONFIG_PAGE_EXTENSION */
>  #endif /* __LINUX_PAGE_EXT_H */
> diff --git a/include/linux/page_idle.h b/include/linux/page_idle.h
> index 4663dfe..3dd3718 100644
> --- a/include/linux/page_idle.h
> +++ b/include/linux/page_idle.h
> @@ -13,65 +13,85 @@
>   * If there is not enough space to store Idle and Young bits in page flags, use
>   * page ext flags instead.
>   */
> -
>  static inline bool folio_test_young(struct folio *folio)
>  {
> -	struct page_ext *page_ext = lookup_page_ext(&folio->page);
> +	struct page_ext *page_ext;
> +	bool page_young;
>  
> +	page_ext = page_ext_get(&folio->page);
>  	if (unlikely(!page_ext))
>  		return false;
>  
> -	return test_bit(PAGE_EXT_YOUNG, &page_ext->flags);
> +	page_young = test_bit(PAGE_EXT_YOUNG, &page_ext->flags);
> +	page_ext_put();
> +
> +	return page_young;
>  }
>  
>  static inline void folio_set_young(struct folio *folio)
>  {
> -	struct page_ext *page_ext = lookup_page_ext(&folio->page);
> +	struct page_ext *page_ext;
>  
> +	page_ext = page_ext_get(&folio->page);
>  	if (unlikely(!page_ext))
>  		return;
>  
>  	set_bit(PAGE_EXT_YOUNG, &page_ext->flags);
> +	page_ext_put();
>  }
>  
>  static inline bool folio_test_clear_young(struct folio *folio)
>  {
> -	struct page_ext *page_ext = lookup_page_ext(&folio->page);
> +	struct page_ext *page_ext;
> +	bool page_young;
>  
> +	page_ext = page_ext_get(&folio->page);
>  	if (unlikely(!page_ext))
>  		return false;
>  
> -	return test_and_clear_bit(PAGE_EXT_YOUNG, &page_ext->flags);
> +	page_young = test_and_clear_bit(PAGE_EXT_YOUNG, &page_ext->flags);
> +	page_ext_put();
> +
> +	return page_young;
>  }
>  
>  static inline bool folio_test_idle(struct folio *folio)
>  {
> -	struct page_ext *page_ext = lookup_page_ext(&folio->page);
> +	struct page_ext *page_ext;
> +	bool page_idle;
>  
> +	page_ext = page_ext_get(&folio->page);
>  	if (unlikely(!page_ext))
>  		return false;
>  
> -	return test_bit(PAGE_EXT_IDLE, &page_ext->flags);
> +	page_idle =  test_bit(PAGE_EXT_IDLE, &page_ext->flags);
> +	page_ext_put();
> +
> +	return page_idle;
>  }
>  
>  static inline void folio_set_idle(struct folio *folio)
>  {
> -	struct page_ext *page_ext = lookup_page_ext(&folio->page);
> +	struct page_ext *page_ext;
>  
> +	page_ext = page_ext_get(&folio->page);
>  	if (unlikely(!page_ext))
>  		return;
>  
>  	set_bit(PAGE_EXT_IDLE, &page_ext->flags);
> +	page_ext_put();
>  }
>  
>  static inline void folio_clear_idle(struct folio *folio)
>  {
> -	struct page_ext *page_ext = lookup_page_ext(&folio->page);
> +	struct page_ext *page_ext;
>  
> +	page_ext = page_ext_get(&folio->page);
>  	if (unlikely(!page_ext))
>  		return;
>  
>  	clear_bit(PAGE_EXT_IDLE, &page_ext->flags);
> +	page_ext_put();
>  }
>  #endif /* !CONFIG_64BIT */
>  
> diff --git a/mm/page_ext.c b/mm/page_ext.c
> index 3dc715d..404a2eb 100644
> --- a/mm/page_ext.c
> +++ b/mm/page_ext.c
> @@ -211,15 +211,17 @@ struct page_ext *lookup_page_ext(const struct page *page)
>  {
>  	unsigned long pfn = page_to_pfn(page);
>  	struct mem_section *section = __pfn_to_section(pfn);
> +	struct page_ext *page_ext = READ_ONCE(section->page_ext);
> +
>  	/*
>  	 * The sanity checks the page allocator does upon freeing a
>  	 * page can reach here before the page_ext arrays are
>  	 * allocated when feeding a range of pages to the allocator
>  	 * for the first time during bootup or memory hotplug.
>  	 */
> -	if (!section->page_ext)
> +	if (page_ext_invalid(page_ext))
>  		return NULL;
> -	return get_entry(section->page_ext, pfn);
> +	return get_entry(page_ext, pfn);
>  }
>  
>  static void *__meminit alloc_page_ext(size_t size, int nid)
> @@ -298,9 +300,26 @@ static void __free_page_ext(unsigned long pfn)
>  	ms = __pfn_to_section(pfn);
>  	if (!ms || !ms->page_ext)
>  		return;
> -	base = get_entry(ms->page_ext, pfn);
> +
> +	base = READ_ONCE(ms->page_ext);
> +	if (page_ext_invalid(base))
> +		base = (void *)base - PAGE_EXT_INVALID;
> +	WRITE_ONCE(ms->page_ext, NULL);
> +
> +	base = get_entry(base, pfn);
>  	free_page_ext(base);
> -	ms->page_ext = NULL;
> +}
> +
> +static void __invalidate_page_ext(unsigned long pfn)
> +{
> +	struct mem_section *ms;
> +	void *val;
> +
> +	ms = __pfn_to_section(pfn);
> +	if (!ms || !ms->page_ext)
> +		return;
> +	val = (void *)ms->page_ext + PAGE_EXT_INVALID;
> +	WRITE_ONCE(ms->page_ext, val);
>  }
>  
>  static int __meminit online_page_ext(unsigned long start_pfn,
> @@ -343,6 +362,20 @@ static int __meminit offline_page_ext(unsigned long start_pfn,
>  	start = SECTION_ALIGN_DOWN(start_pfn);
>  	end = SECTION_ALIGN_UP(start_pfn + nr_pages);
>  
> +	/*
> +	 * Freeing of page_ext is done in 3 steps to avoid
> +	 * use-after-free of it:
> +	 * 1) Traverse all the sections and mark their page_ext
> +	 *    as invalid.
> +	 * 2) Wait for all the existing users of page_ext who
> +	 *    started before invalidation to finish.
> +	 * 3) Free the page_ext.
> +	 */
> +	for (pfn = start; pfn < end; pfn += PAGES_PER_SECTION)
> +		__invalidate_page_ext(pfn);
> +
> +	synchronize_rcu();
> +
>  	for (pfn = start; pfn < end; pfn += PAGES_PER_SECTION)
>  		__free_page_ext(pfn);
>  	return 0;
> diff --git a/mm/page_owner.c b/mm/page_owner.c
> index e4c6f3f..0520dda 100644
> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
> @@ -195,14 +195,16 @@ noinline void __set_page_owner(struct page *page, unsigned short order,
>  
>  void __set_page_owner_migrate_reason(struct page *page, int reason)
>  {
> -	struct page_ext *page_ext = lookup_page_ext(page);
> +	struct page_ext *page_ext;
>  	struct page_owner *page_owner;
>  
> +	page_ext = page_ext_get(page);
>  	if (unlikely(!page_ext))
>  		return;
>  
>  	page_owner = get_page_owner(page_ext);
>  	page_owner->last_migrate_reason = reason;
> +	page_ext_put();
>  }
>  
>  void __split_page_owner(struct page *page, unsigned int nr)
> @@ -307,12 +309,12 @@ void pagetypeinfo_showmixedcount_print(struct seq_file *m,
>  			if (PageReserved(page))
>  				continue;
>  
> -			page_ext = lookup_page_ext(page);
> +			page_ext = page_ext_get(page);
>  			if (unlikely(!page_ext))
>  				continue;
>  
>  			if (!test_bit(PAGE_EXT_OWNER_ALLOCATED, &page_ext->flags))
> -				continue;
> +				goto loop;
>  
>  			page_owner = get_page_owner(page_ext);
>  			page_mt = gfp_migratetype(page_owner->gfp_mask);
> @@ -323,9 +325,12 @@ void pagetypeinfo_showmixedcount_print(struct seq_file *m,
>  					count[pageblock_mt]++;
>  
>  				pfn = block_end_pfn;
> +				page_ext_put();
>  				break;
>  			}
>  			pfn += (1UL << page_owner->order) - 1;
> +loop:
> +			page_ext_put();
>  		}
>  	}
>  
> @@ -508,6 +513,14 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
>  	/* Find an allocated page */
>  	for (; pfn < max_pfn; pfn++) {
>  		/*
> +		 * This temporary page_owner is required so
> +		 * that we can avoid the context switches while holding
> +		 * the rcu lock and copying the page owner information to
> +		 * user through copy_to_user() or GFP_KERNEL allocations.
> +		 */
> +		struct page_owner page_owner_tmp = {0};
> +
> +		/*
>  		 * If the new page is in a new MAX_ORDER_NR_PAGES area,
>  		 * validate the area as existing, skip it if not
>  		 */
> @@ -525,7 +538,7 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
>  			continue;
>  		}
>  
> -		page_ext = lookup_page_ext(page);
> +		page_ext = page_ext_get(page);
>  		if (unlikely(!page_ext))
>  			continue;
>  
> @@ -534,14 +547,14 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
>  		 * because we don't hold the zone lock.
>  		 */
>  		if (!test_bit(PAGE_EXT_OWNER, &page_ext->flags))
> -			continue;
> +			goto loop;
>  
>  		/*
>  		 * Although we do have the info about past allocation of free
>  		 * pages, it's not relevant for current memory usage.
>  		 */
>  		if (!test_bit(PAGE_EXT_OWNER_ALLOCATED, &page_ext->flags))
> -			continue;
> +			goto loop;
>  
>  		page_owner = get_page_owner(page_ext);
>  
> @@ -550,7 +563,7 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
>  		 * would inflate the stats.
>  		 */
>  		if (!IS_ALIGNED(pfn, 1 << page_owner->order))
> -			continue;
> +			goto loop;
>  
>  		/*
>  		 * Access to page_ext->handle isn't synchronous so we should
> @@ -558,13 +571,17 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
>  		 */
>  		handle = READ_ONCE(page_owner->handle);
>  		if (!handle)
> -			continue;
> +			goto loop;
>  
>  		/* Record the next PFN to read in the file offset */
>  		*ppos = (pfn - min_low_pfn) + 1;
>  
> +		memcpy(&page_owner_tmp, page_owner, sizeof(struct page_owner));
> +		page_ext_put();
>  		return print_page_owner(buf, count, pfn, page,
> -				page_owner, handle);
> +				&page_owner_tmp, handle);
> +loop:
> +		page_ext_put();
>  	}
>  
>  	return 0;
> diff --git a/mm/page_table_check.c b/mm/page_table_check.c
> index e206274..ec371b9 100644
> --- a/mm/page_table_check.c
> +++ b/mm/page_table_check.c
> @@ -68,7 +68,7 @@ static void page_table_check_clear(struct mm_struct *mm, unsigned long addr,
>  		return;
>  
>  	page = pfn_to_page(pfn);
> -	page_ext = lookup_page_ext(page);
> +	page_ext = page_ext_get(page);
>  	anon = PageAnon(page);
>  
>  	for (i = 0; i < pgcnt; i++) {
> @@ -83,6 +83,7 @@ static void page_table_check_clear(struct mm_struct *mm, unsigned long addr,
>  		}
>  		page_ext = page_ext_next(page_ext);
>  	}
> +	page_ext_put();
>  }
>  
>  /*
> @@ -103,7 +104,7 @@ static void page_table_check_set(struct mm_struct *mm, unsigned long addr,
>  		return;
>  
>  	page = pfn_to_page(pfn);
> -	page_ext = lookup_page_ext(page);
> +	page_ext = page_ext_get(page);
>  	anon = PageAnon(page);
>  
>  	for (i = 0; i < pgcnt; i++) {
> @@ -118,6 +119,7 @@ static void page_table_check_set(struct mm_struct *mm, unsigned long addr,
>  		}
>  		page_ext = page_ext_next(page_ext);
>  	}
> +	page_ext_put();
>  }
>  
>  /*
> @@ -126,9 +128,10 @@ static void page_table_check_set(struct mm_struct *mm, unsigned long addr,
>   */
>  void __page_table_check_zero(struct page *page, unsigned int order)
>  {
> -	struct page_ext *page_ext = lookup_page_ext(page);
> +	struct page_ext *page_ext;
>  	unsigned long i;
>  
> +	page_ext = page_ext_get(page);
>  	BUG_ON(!page_ext);
>  	for (i = 0; i < (1ul << order); i++) {
>  		struct page_table_check *ptc = get_page_table_check(page_ext);
> @@ -137,6 +140,7 @@ void __page_table_check_zero(struct page *page, unsigned int order)
>  		BUG_ON(atomic_read(&ptc->file_map_count));
>  		page_ext = page_ext_next(page_ext);
>  	}
> +	page_ext_put();
>  }
>  
>  void __page_table_check_pte_clear(struct mm_struct *mm, unsigned long addr,




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