This is a note to let you know that I've just added the patch titled
mm/page_alloc.c: fix freeing non-compound pages
to the 5.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
mm-page_alloc.c-fix-freeing-non-compound-pages.patch
and it can be found in the queue-5.9 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 4437ab2c19e2f1941ff53b1fc88d1e0e11667805
Author: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Date: Tue Oct 13 16:56:04 2020 -0700
mm/page_alloc.c: fix freeing non-compound pages
[ Upstream commit e320d3012d25b1fb5f3df4edb7bd44a1c362ec10 ]
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@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Acked-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
Cc: Nick Piggin <npiggin@xxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200926213919.26642-1-willy@xxxxxxxxxxxxx
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 0c781f912f9f0..491789a793ae5 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2367,6 +2367,15 @@ 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 a4a4c6864f518..071b687b7363f 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 0000000000000..074e76bd76b2b
--- /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@xxxxxxxxxxxxx>
+ */
+
+#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@xxxxxxxxxxxxx>");
+MODULE_LICENSE("GPL");
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c449d74f55842..d99c2db7f254f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4961,6 +4961,9 @@ 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);
