From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx> Subject: mm/page_alloc.c: fix freeing non-compound pages 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. Link: https://lkml.kernel.org/r/20200926213919.26642-1-willy@xxxxxxxxxxxxx Fixes: e286781d5f2e ("mm: speculative page references") Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> Acked-by: Mike Rapoport <rppt@xxxxxxxxxxxxx> Cc: Nick Piggin <npiggin@xxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- lib/Kconfig.debug | 9 ++++++++ lib/Makefile | 1 lib/test_free_pages.c | 42 ++++++++++++++++++++++++++++++++++++++++ mm/page_alloc.c | 3 ++ 4 files changed, 55 insertions(+) --- a/lib/Kconfig.debug~page_alloc-fix-freeing-non-compound-pages +++ a/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 --- a/lib/Makefile~page_alloc-fix-freeing-non-compound-pages +++ a/lib/Makefile @@ -101,6 +101,7 @@ obj-$(CONFIG_TEST_BLACKHOLE_DEV) += test obj-$(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 --- /dev/null +++ a/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"); --- a/mm/page_alloc.c~page_alloc-fix-freeing-non-compound-pages +++ a/mm/page_alloc.c @@ -4952,6 +4952,9 @@ void __free_pages(struct page *page, uns { 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); _
