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    Subject[PATCH 5.9 393/757] mm/page_alloc.c: fix freeing non-compound pages
    Date
    From: Matthew Wilcox (Oracle) <willy@infradead.org>

    [ 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@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: Sasha Levin <sashal@kernel.org>
    ---
    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 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@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 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);

    --
    2.25.1


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