[PATCH v2] page_alloc: Fix freeing non-compound pages

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

Here is a very rare race which leaks memory:

Page P0 is allocated to the page cache.  Page P1 is free.

Thread A                Thread B                Thread C
find_get_entry():
xas_load() returns P0
						Removes P0 from page cache
						P0 finds its buddy P1
			alloc_pages(GFP_KERNEL, 1) returns P0
			P0 has refcount 1
page_cache_get_speculative(P0)
P0 has refcount 2
			__free_pages(P0)
			P0 has refcount 1
put_page(P0)
P1 is not freed

Fix this by freeing all the pages in __free_pages() that won't be freed
by the call to put_page().  It's usually not a good idea to split a page,
but this is a very unlikely scenario.

Fixes: e286781d5f2e ("mm: speculative page references")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
v2: Add a test module.  Verified it works by:
 (1) loading the test module on an unmodified kernel and watching it OOM
 (2) modifying __free_pages() with my v1 patch that neglected to add the
     PageHead test and hitting VM_BUG_ON_PAGE(PageTail(page))
 (3) applying all of this patch and seeing it survive

 lib/Kconfig.debug     |  9 +++++++++
 lib/Makefile          |  1 +
 lib/test_free_pages.c | 42 ++++++++++++++++++++++++++++++++++++++++++
 mm/page_alloc.c       |  3 +++
 4 files changed, 55 insertions(+)
 create mode 100644 lib/test_free_pages.c

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index e068c3c7189a..eed59af0e907 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2369,6 +2369,15 @@ config TEST_HMM
 
 	  If unsure, say N.
 
+config TEST_FREE_PAGES
+	tristate "Test freeing pages"
+	help
+	  Test that a memory leak does not occur due to a race between
+	  freeing a block of pages and a speculative page reference.
+	  Loading this module is safe if your kernel has the bug fixed.
+	  If the bug is not fixed, it will leak gigabytes of memory and
+	  probably OOM your system.
+
 config TEST_FPU
 	tristate "Test floating point operations in kernel space"
 	depends on X86 && !KCOV_INSTRUMENT_ALL
diff --git a/lib/Makefile b/lib/Makefile
index a4a4c6864f51..071b687b7363 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -99,6 +99,7 @@ obj-$(CONFIG_TEST_BLACKHOLE_DEV) += test_blackhole_dev.o
 obj-$(CONFIG_TEST_MEMINIT) += test_meminit.o
 obj-$(CONFIG_TEST_LOCKUP) += test_lockup.o
 obj-$(CONFIG_TEST_HMM) += test_hmm.o
+obj-$(CONFIG_TEST_FREE_PAGES) += test_free_pages.o
 
 #
 # CFLAGS for compiling floating point code inside the kernel. x86/Makefile turns
diff --git a/lib/test_free_pages.c b/lib/test_free_pages.c
new file mode 100644
index 000000000000..074e76bd76b2
--- /dev/null
+++ b/lib/test_free_pages.c
@@ -0,0 +1,42 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * test_free_pages.c: Check that free_pages() doesn't leak memory
+ * Copyright (c) 2020 Oracle
+ * Author: Matthew Wilcox <willy@infradead.org>
+ */
+
+#include <linux/gfp.h>
+#include <linux/mm.h>
+#include <linux/module.h>
+
+static void test_free_pages(gfp_t gfp)
+{
+	unsigned int i;
+
+	for (i = 0; i < 1000 * 1000; i++) {
+		unsigned long addr = __get_free_pages(gfp, 3);
+		struct page *page = virt_to_page(addr);
+
+		/* Simulate page cache getting a speculative reference */
+		get_page(page);
+		free_pages(addr, 3);
+		put_page(page);
+	}
+}
+
+static int m_in(void)
+{
+	test_free_pages(GFP_KERNEL);
+	test_free_pages(GFP_KERNEL | __GFP_COMP);
+
+	return 0;
+}
+
+static void m_ex(void)
+{
+}
+
+module_init(m_in);
+module_exit(m_ex);
+MODULE_AUTHOR("Matthew Wilcox <willy@infradead.org>");
+MODULE_LICENSE("GPL");
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fab5e97dc9ca..9b259c76e285 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4947,6 +4947,9 @@ void __free_pages(struct page *page, unsigned int order)
 {
 	if (put_page_testzero(page))
 		free_the_page(page, order);
+	else if (!PageHead(page))
+		while (order-- > 0)
+			free_the_page(page + (1 << order), order);
 }
 EXPORT_SYMBOL(__free_pages);
 
-- 
2.28.0