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,
