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    SubjectRe: [RFC/PATCH RESEND -next 01/21] Add kernel address sanitizer infrastructure.
    On 07/09/2014 07:29 AM, Andrey Ryabinin wrote:
    > Address sanitizer for kernel (kasan) is a dynamic memory error detector.
    >
    > The main features of kasan is:
    > - is based on compiler instrumentation (fast),
    > - detects out of bounds for both writes and reads,
    > - provides use after free detection,
    >
    > This patch only adds infrastructure for kernel address sanitizer. It's not
    > available for use yet. The idea and some code was borrowed from [1].
    >
    > This feature requires pretty fresh GCC (revision r211699 from 2014-06-16 or
    > latter).
    >
    > Implementation details:
    > The main idea of KASAN is to use shadow memory to record whether each byte of memory
    > is safe to access or not, and use compiler's instrumentation to check the shadow memory
    > on each memory access.
    >
    > Address sanitizer dedicates 1/8 of the low memory to the shadow memory and uses direct
    > mapping with a scale and offset to translate a memory address to its corresponding
    > shadow address.
    >
    > Here is function to translate address to corresponding shadow address:
    >
    > unsigned long kasan_mem_to_shadow(unsigned long addr)
    > {
    > return ((addr - PAGE_OFFSET) >> KASAN_SHADOW_SCALE_SHIFT)
    > + kasan_shadow_start;
    > }
    >
    > where KASAN_SHADOW_SCALE_SHIFT = 3.
    >
    > So for every 8 bytes of lowmemory there is one corresponding byte of shadow memory.
    > The following encoding used for each shadow byte: 0 means that all 8 bytes of the
    > corresponding memory region are valid for access; k (1 <= k <= 7) means that
    > the first k bytes are valid for access, and other (8 - k) bytes are not;
    > Any negative value indicates that the entire 8-bytes are unaccessible.
    > Different negative values used to distinguish between different kinds of
    > unaccessible memory (redzones, freed memory) (see mm/kasan/kasan.h).
    >
    > To be able to detect accesses to bad memory we need a special compiler.
    > Such compiler inserts a specific function calls (__asan_load*(addr), __asan_store*(addr))
    > before each memory access of size 1, 2, 4, 8 or 16.
    >
    > These functions check whether memory region is valid to access or not by checking
    > corresponding shadow memory. If access is not valid an error printed.
    >
    > [1] https://code.google.com/p/address-sanitizer/wiki/AddressSanitizerForKernel
    >
    > Signed-off-by: Andrey Ryabinin <a.ryabinin@samsung.com>

    I gave it a spin, and it seems that it fails for what you might call a "regular"
    memory size these days, in my case it was 18G:

    [ 0.000000] Kernel panic - not syncing: ERROR: Failed to allocate 0xe0c00000 bytes below 0x0.
    [ 0.000000]
    [ 0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 3.16.0-rc4-next-20140710-sasha-00044-gb7b0579-dirty #784
    [ 0.000000] ffffffffb9c2d3c8 cd9ce91adea4379a 0000000000000000 ffffffffb9c2d3c8
    [ 0.000000] ffffffffb9c2d330 ffffffffb7fe89b7 ffffffffb93c8c28 ffffffffb9c2d3b8
    [ 0.000000] ffffffffb7fcff1d 0000000000000018 ffffffffb9c2d3c8 ffffffffb9c2d360
    [ 0.000000] Call Trace:
    [ 0.000000] <UNK> dump_stack (lib/dump_stack.c:52)
    [ 0.000000] panic (kernel/panic.c:119)
    [ 0.000000] memblock_alloc_base (mm/memblock.c:1092)
    [ 0.000000] memblock_alloc (mm/memblock.c:1097)
    [ 0.000000] kasan_alloc_shadow (mm/kasan/kasan.c:151)
    [ 0.000000] zone_sizes_init (arch/x86/mm/init.c:684)
    [ 0.000000] paging_init (arch/x86/mm/init_64.c:677)
    [ 0.000000] setup_arch (arch/x86/kernel/setup.c:1168)
    [ 0.000000] ? printk (kernel/printk/printk.c:1839)
    [ 0.000000] start_kernel (include/linux/mm_types.h:462 init/main.c:533)
    [ 0.000000] ? early_idt_handlers (arch/x86/kernel/head_64.S:344)
    [ 0.000000] x86_64_start_reservations (arch/x86/kernel/head64.c:194)
    [ 0.000000] x86_64_start_kernel (arch/x86/kernel/head64.c:183)

    It got better when I reduced memory to 1GB, but then my system just failed to boot
    at all because that's not enough to bring everything up.


    Thanks,
    Sasha


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