mm: fix use-after free of page_ext after race with memory-offline

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 abovementioned 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 until 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@quicinc.com/
[2] https://lore.kernel.org/all/a26ce299-aed1-b8ad-711e-a49e82bdd180@quicinc.com/T/#u
[3] https://lore.kernel.org/all/6fa6b7aa-731e-891c-3efb-a03d6a700efa@redhat.com/

[quic_charante@quicinc.com: rename label `loop' to `ext_put_continue' per David]
  Link: https://lkml.kernel.org/r/1661496993-11473-1-git-send-email-quic_charante@quicinc.com
Link: https://lkml.kernel.org/r/1660830600-9068-1-git-send-email-quic_charante@quicinc.com
Signed-off-by: Charan Teja Kalla <quic_charante@quicinc.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Suggested-by: Michal Hocko <mhocko@suse.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Fernand Sieber <sieberf@amazon.com>
Cc: Minchan Kim <minchan@google.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Pavan Kondeti <quic_pkondeti@quicinc.com>
Cc: SeongJae Park <sjpark@amazon.de>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: William Kucharski <william.kucharski@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 97 insertions, 6 deletions

diff --git a/mm/page_ext.c b/mm/page_ext.c
index e22a928dd66a..b236bdd59fa8 100644
--- a/mm/page_ext.c
+++ b/mm/page_ext.c
@@ -9,6 +9,7 @@
 #include <linux/page_owner.h>
 #include <linux/page_idle.h>
 #include <linux/page_table_check.h>
+#include <linux/rcupdate.h>
 
 /*
  * struct page extension
@@ -59,6 +60,10 @@
  * can utilize this callback to initialize the state of it correctly.
  */
 
+#ifdef CONFIG_SPARSEMEM
+#define PAGE_EXT_INVALID       (0x1)
+#endif
+
 #if defined(CONFIG_PAGE_IDLE_FLAG) && !defined(CONFIG_64BIT)
 static bool need_page_idle(void)
 {
@@ -84,6 +89,7 @@ static struct page_ext_operations *page_ext_ops[] __initdata = {
 unsigned long page_ext_size = sizeof(struct page_ext);
 
 static unsigned long total_usage;
+static struct page_ext *lookup_page_ext(const struct page *page);
 
 static bool __init invoke_need_callbacks(void)
 {
@@ -125,6 +131,48 @@ static inline struct page_ext *get_entry(void *base, unsigned long index)
 	return base + page_ext_size * index;
 }
 
+/**
+ * page_ext_get() - Get the extended information for a page.
+ * @page: The page we're interested in.
+ *
+ * Ensures that the page_ext will remain valid until page_ext_put()
+ * is called.
+ *
+ * Return: NULL if no page_ext exists for this page.
+ * Context: Any context.  Caller may not sleep until they have called
+ * page_ext_put().
+ */
+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;
+}
+
+/**
+ * page_ext_put() - Working with page extended information is done.
+ * @page_ext - Page extended information received from page_ext_get().
+ *
+ * The page extended information of the page may not be valid after this
+ * function is called.
+ *
+ * Return: None.
+ * Context: Any context with corresponding page_ext_get() is called.
+ */
+void page_ext_put(struct page_ext *page_ext)
+{
+	if (unlikely(!page_ext))
+		return;
+
+	rcu_read_unlock();
+}
 #ifndef CONFIG_SPARSEMEM
 
 
@@ -133,12 +181,13 @@ void __meminit pgdat_page_ext_init(struct pglist_data *pgdat)
 	pgdat->node_page_ext = NULL;
 }
 
-struct page_ext *lookup_page_ext(const struct page *page)
+static struct page_ext *lookup_page_ext(const struct page *page)
 {
 	unsigned long pfn = page_to_pfn(page);
 	unsigned long index;
 	struct page_ext *base;
 
+	WARN_ON_ONCE(!rcu_read_lock_held());
 	base = NODE_DATA(page_to_nid(page))->node_page_ext;
 	/*
 	 * The sanity checks the page allocator does upon freeing a
@@ -206,20 +255,27 @@ fail:
 }
 
 #else /* CONFIG_SPARSEMEM */
+static bool page_ext_invalid(struct page_ext *page_ext)
+{
+	return !page_ext || (((unsigned long)page_ext & PAGE_EXT_INVALID) == PAGE_EXT_INVALID);
+}
 
-struct page_ext *lookup_page_ext(const struct page *page)
+static 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);
+
+	WARN_ON_ONCE(!rcu_read_lock_held());
 	/*
 	 * 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 +354,30 @@ 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);
+	/*
+	 * page_ext here can be valid while doing the roll back
+	 * operation in online_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 +420,20 @@ 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;