From: "Matthew Wilcox (Oracle)" willy@infradead.org
mainline inclusion from mainline-v5.10-rc1 commit e320d3012d25b1fb5f3df4edb7bd44a1c362ec10 category: bugfix bugzilla: 43588 CVE: NA
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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 Signed-off-by: Andrew Morton akpm@linux-foundation.org Acked-by: Mike Rapoport rppt@linux.ibm.com Cc: Nick Piggin npiggin@gmail.com Cc: Hugh Dickins hughd@google.com Cc: Peter Zijlstra peterz@infradead.org Link: https://lkml.kernel.org/r/20200926213919.26642-1-willy@infradead.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Liu Shixin liushixin2@huawei.com Reviewed-by: Kefeng Wang wangkefeng.wang@huawei.com Signed-off-by: Yang Yingliang yangyingliang@huawei.com --- 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 2d2c51cbf4bb..a6cd1e8059e0 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -1981,6 +1981,15 @@ config TEST_DEBUG_VIRTUAL
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. + endif # RUNTIME_TESTING_MENU
config MEMTEST diff --git a/lib/Makefile b/lib/Makefile index f92a84934ab2..71443e03deb3 100644 --- a/lib/Makefile +++ b/lib/Makefile @@ -81,6 +81,7 @@ obj-$(CONFIG_TEST_UUID) += test_uuid.o obj-$(CONFIG_TEST_PARMAN) += test_parman.o obj-$(CONFIG_TEST_KMOD) += test_kmod.o obj-$(CONFIG_TEST_DEBUG_VIRTUAL) += test_debug_virtual.o +obj-$(CONFIG_TEST_FREE_PAGES) += test_free_pages.o
ifeq ($(CONFIG_DEBUG_KOBJECT),y) CFLAGS_kobject.o += -DDEBUG 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 45fda5c01141..67768c56d412 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -4585,6 +4585,9 @@ 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);