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