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@xxxxxxxxxxx>
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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