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    Subject[PATCH v4 06/11] kfence, kasan: make KFENCE compatible with KASAN
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    From: Alexander Potapenko <glider@google.com>

    We make KFENCE compatible with KASAN for testing KFENCE itself. In
    particular, KASAN helps to catch any potential corruptions to KFENCE
    state, or other corruptions that may be a result of freepointer
    corruptions in the main allocators.

    To indicate that the combination of the two is generally discouraged,
    CONFIG_EXPERT=y should be set. It also gives us the nice property that
    KFENCE will be build-tested by allyesconfig builds.

    Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
    Co-developed-by: Marco Elver <elver@google.com>
    Signed-off-by: Marco Elver <elver@google.com>
    Signed-off-by: Alexander Potapenko <glider@google.com>
    ---
    lib/Kconfig.kfence | 2 +-
    mm/kasan/common.c | 7 +++++++
    2 files changed, 8 insertions(+), 1 deletion(-)

    diff --git a/lib/Kconfig.kfence b/lib/Kconfig.kfence
    index 4c2ea1c722de..6825c1c07a10 100644
    --- a/lib/Kconfig.kfence
    +++ b/lib/Kconfig.kfence
    @@ -10,7 +10,7 @@ config HAVE_ARCH_KFENCE_STATIC_POOL

    menuconfig KFENCE
    bool "KFENCE: low-overhead sampling-based memory safety error detector"
    - depends on HAVE_ARCH_KFENCE && !KASAN && (SLAB || SLUB)
    + depends on HAVE_ARCH_KFENCE && (!KASAN || EXPERT) && (SLAB || SLUB)
    depends on JUMP_LABEL # To ensure performance, require jump labels
    select STACKTRACE
    help
    diff --git a/mm/kasan/common.c b/mm/kasan/common.c
    index 950fd372a07e..f5c49f0fdeff 100644
    --- a/mm/kasan/common.c
    +++ b/mm/kasan/common.c
    @@ -18,6 +18,7 @@
    #include <linux/init.h>
    #include <linux/kasan.h>
    #include <linux/kernel.h>
    +#include <linux/kfence.h>
    #include <linux/kmemleak.h>
    #include <linux/linkage.h>
    #include <linux/memblock.h>
    @@ -396,6 +397,9 @@ static bool __kasan_slab_free(struct kmem_cache *cache, void *object,
    tagged_object = object;
    object = reset_tag(object);

    + if (is_kfence_address(object))
    + return false;
    +
    if (unlikely(nearest_obj(cache, virt_to_head_page(object), object) !=
    object)) {
    kasan_report_invalid_free(tagged_object, ip);
    @@ -444,6 +448,9 @@ static void *__kasan_kmalloc(struct kmem_cache *cache, const void *object,
    if (unlikely(object == NULL))
    return NULL;

    + if (is_kfence_address(object))
    + return (void *)object;
    +
    redzone_start = round_up((unsigned long)(object + size),
    KASAN_SHADOW_SCALE_SIZE);
    redzone_end = round_up((unsigned long)object + cache->object_size,
    --
    2.28.0.709.gb0816b6eb0-goog
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