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Date: Wed, 27 Jul 2022 19:49:44 +0530
From: Charan Teja Kalla <quic_charante@...cinc.com>
To: <akpm@...ux-foundation.org>, <david@...hat.com>,
<quic_pkondeti@...cinc.com>, <pasha.tatashin@...een.com>,
<sjpark@...zon.de>, <sieberf@...zon.com>, <shakeelb@...gle.com>,
<dhowells@...hat.com>, <willy@...radead.org>,
<liuting.0x7c00@...edance.com>, <minchan@...nel.org>,
Michal Hocko <mhocko@...e.com>
CC: <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH V2] mm: fix use-after free of page_ext after race with
memory-offline
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@quicinc.com/
>
> Suggested-by: David Hildenbrand <david@...hat.com>
> Suggested-by: Michal Hocko <mhocko@...e.com>
> Signed-off-by: Charan Teja Kalla <quic_charante@...cinc.com>
> ---
> 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@quicinc.com/
>
> 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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