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  • Andrey Konovalov's avatar
    kasan, mm: change hooks signatures · 0116523c
    Andrey Konovalov authored
    Patch series "kasan: add software tag-based mode for arm64", v13.
    This patchset adds a new software tag-based mode to KASAN [1].  (Initially
    this mode was called KHWASAN, but it got renamed, see the naming rationale
    at the end of this section).
    The plan is to implement HWASan [2] for the kernel with the incentive,
    that it's going to have comparable to KASAN performance, but in the same
    time consume much less memory, trading that off for somewhat imprecise bug
    detection and being supported only for arm64.
    The underlying ideas of the approach used by software tag-based KASAN are:
    1. By using the Top Byte Ignore (TBI) arm64 CPU feature, we can store
       pointer tags in the top byte of each kernel pointer.
    2. Using shadow memory, we can store memory tags for each chunk of kernel
    3. On each memory allocation, we can generate a random tag, embed it into
       the returned pointer and set the memory tags that correspond to this
       chunk of memory to the same value.
    4. By using compiler instrumentation, before each memory access we can add
       a check that the pointer tag matches the tag of the memory that is being
    5. On a tag mismatch we report an error.
    With this patchset the existing KASAN mode gets renamed to generic KASAN,
    with the word "generic" meaning that the implementation can be supported
    by any architecture as it is purely software.
    The new mode this patchset adds is called software tag-based KASAN.  The
    word "tag-based" refers to the fact that this mode uses tags embedded into
    the top byte of kernel pointers and the TBI arm64 CPU feature that allows
    to dereference such pointers.  The word "software" here means that shadow
    memory manipulation and tag checking on pointer dereference is done in
    software.  As it is the only tag-based implementation right now, "software
    tag-based" KASAN is sometimes referred to as simply "tag-based" in this
    A potential expansion of this mode is a hardware tag-based mode, which
    would use hardware memory tagging support (announced by Arm [3]) instead
    of compiler instrumentation and manual shadow memory manipulation.
    Same as generic KASAN, software tag-based KASAN is strictly a debugging
    ====== Rationale
    On mobile devices generic KASAN's memory usage is significant problem.
    One of the main reasons to have tag-based KASAN is to be able to perform a
    similar set of checks as the generic one does, but with lower memory
    Comment from Vishwath Mohan <>:
    I don't have data on-hand, but anecdotally both ASAN and KASAN have proven
    problematic to enable for environments that don't tolerate the increased
    memory pressure well.  This includes
    (a) Low-memory form factors - Wear, TV, Things, lower-tier phones like Go,
    (c) Connected components like Pixel's visual core [1].
    These are both places I'd love to have a low(er) memory footprint option at
    my disposal.
    Comment from Evgenii Stepanov <>:
    Looking at a live Android device under load, slab (according to
    /proc/meminfo) + kernel stack take 8-10% available RAM (~350MB).  KASAN's
    overhead of 2x - 3x on top of it is not insignificant.
    Not having this overhead enables near-production use - ex.  running
    KASAN/KHWASAN kernel on a personal, daily-use device to catch bugs that do
    not reproduce in test configuration.  These are the ones that often cost
    the most engineering time to track down.
    CPU overhead is bad, but generally tolerable.  RAM is critical, in our
    experience.  Once it gets low enough, OOM-killer makes your life
    ====== Technical details
    Software tag-based KASAN mode is implemented in a very similar way to the
    generic one. This patchset essentially does the following:
    1. TCR_TBI1 is set to enable Top Byte Ignore.
    2. Shadow memory is used (with a different scale, 1:16, so each shadow
       byte corresponds to 16 bytes of kernel memory) to store memory tags.
    3. All slab objects are aligned to shadow scale, which is 16 bytes.
    4. All pointers returned from the slab allocator are tagged with a random
       tag and the corresponding shadow memory is poisoned with the same value.
    5. Compiler instrumentation is used to insert tag checks. Either by
       calling callbacks or by inlining them (CONFIG_KASAN_OUTLINE and
       CONFIG_KASAN_INLINE flags are reused).
    6. When a tag mismatch is detected in callback instrumentation mode
       KASAN simply prints a bug report. In case of inline instrumentation,
       clang inserts a brk instruction, and KASAN has it's own brk handler,
       which reports the bug.
    7. The memory in between slab objects is marked with a reserved tag, and
       acts as a redzone.
    8. When a slab object is freed it's marked with a reserved tag.
    Bug detection is imprecise for two reasons:
    1. We won't catch some small out-of-bounds accesses, that fall into the
       same shadow cell, as the last byte of a slab object.
    2. We only have 1 byte to store tags, which means we have a 1/256
       probability of a tag match for an incorrect access (actually even
       slightly less due to reserved tag values).
    Despite that there's a particular type of bugs that tag-based KASAN can
    detect compared to generic KASAN: use-after-free after the object has been
    allocated by someone else.
    ====== Testing
    Some kernel developers voiced a concern that changing the top byte of
    kernel pointers may lead to subtle bugs that are difficult to discover.
    To address this concern deliberate testing has been performed.
    It doesn't seem feasible to do some kind of static checking to find
    potential issues with pointer tagging, so a dynamic approach was taken.
    All pointer comparisons/subtractions have been instrumented in an LLVM
    compiler pass and a kernel module that would print a bug report whenever
    two pointers with different tags are being compared/subtracted (ignoring
    comparisons with NULL pointers and with pointers obtained by casting an
    error code to a pointer type) has been used.  Then the kernel has been
    booted in QEMU and on an Odroid C2 board and syzkaller has been run.
    This yielded the following results.
    The two places that look interesting are:
    is_vmalloc_addr in include/linux/mm.h
    is_kernel_rodata in mm/util.c
    Here we compare a pointer with some fixed untagged values to make sure
    that the pointer lies in a particular part of the kernel address space.
    Since tag-based KASAN doesn't add tags to pointers that belong to rodata
    or vmalloc regions, this should work as is.  To make sure debug checks to
    those two functions that check that the result doesn't change whether we
    operate on pointers with or without untagging has been added.
    A few other cases that don't look that interesting:
    Comparing pointers to achieve unique sorting order of pointee objects
    (e.g. sorting locks addresses before performing a double lock):
    tty_ldisc_lock_pair_timeout in drivers/tty/tty_ldisc.c
    pipe_double_lock in fs/pipe.c
    unix_state_double_lock in net/unix/af_unix.c
    lock_two_nondirectories in fs/inode.c
    mutex_lock_double in kernel/events/core.c
    ep_cmp_ffd in fs/eventpoll.c
    fsnotify_compare_groups fs/notify/mark.c
    Nothing needs to be done here, since the tags embedded into pointers
    don't change, so the sorting order would still be unique.
    Checks that a pointer belongs to some particular allocation:
    is_sibling_entry in lib/radix-tree.c
    object_is_on_stack in include/linux/sched/task_stack.h
    Nothing needs to be done here either, since two pointers can only belong
    to the same allocation if they have the same tag.
    Overall, since the kernel boots and works, there are no critical bugs.
    As for the rest, the traditional kernel testing way (use until fails) is
    the only one that looks feasible.
    Another point here is that tag-based KASAN is available under a separate
    config option that needs to be deliberately enabled. Even though it might
    be used in a "near-production" environment to find bugs that are not found
    during fuzzing or running tests, it is still a debug tool.
    ====== Benchmarks
    The following numbers were collected on Odroid C2 board. Both generic and
    tag-based KASAN were used in inline instrumentation mode.
    Boot time [1]:
    * ~1.7 sec for clean kernel
    * ~5.0 sec for generic KASAN
    * ~5.0 sec for tag-based KASAN
    Network performance [2]:
    * 8.33 Gbits/sec for clean kernel
    * 3.17 Gbits/sec for generic KASAN
    * 2.85 Gbits/sec for tag-based KASAN
    Slab memory usage after boot [3]:
    * ~40 kb for clean kernel
    * ~105 kb (~260% overhead) for generic KASAN
    * ~47 kb (~20% overhead) for tag-based KASAN
    KASAN memory overhead consists of three main parts:
    1. Increased slab memory usage due to redzones.
    2. Shadow memory (the whole reserved once during boot).
    3. Quaratine (grows gradually until some preset limit; the more the limit,
       the more the chance to detect a use-after-free).
    Comparing tag-based vs generic KASAN for each of these points:
    1. 20% vs 260% overhead.
    2. 1/16th vs 1/8th of physical memory.
    3. Tag-based KASAN doesn't require quarantine.
    [1] Time before the ext4 driver is initialized.
    [2] Measured as `iperf -s & iperf -c -t 30`.
    [3] Measured as `cat /proc/meminfo | grep Slab`.
    ====== Some notes
    A few notes:
    1. The patchset can be found here:
    2. Building requires a recent Clang version (7.0.0 or later).
    3. Stack instrumentation is not supported yet and will be added later.
    This patch (of 25):
    Tag-based KASAN changes the value of the top byte of pointers returned
    from the kernel allocation functions (such as kmalloc).  This patch
    updates KASAN hooks signatures and their usage in SLAB and SLUB code to
    reflect that.
    Signed-off-by: default avatarAndrey Konovalov <>
    Reviewed-by: default avatarAndrey Ryabinin <>
    Reviewed-by: default avatarDmitry Vyukov <>
    Cc: Christoph Lameter <>
    Cc: Mark Rutland <>
    Cc: Will Deacon <>
    Signed-off-by: default avatarAndrew Morton <>
    Signed-off-by: default avatarLinus Torvalds <>
kasan.h 4.97 KB