On 07/10/2014 09:01 AM, Andrey Ryabinin wrote:
> On 07/10/14 15:55, Sasha Levin wrote:
>> > 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.
> I think memory size is not a problem here. I tested on my desktop with 16G.
> Seems it's a problem with memory holes cited by Dave.
> kasan tries to allocate ~3.5G. It means that lowmemsize is 28G in your case.
That's correct (I've mistyped and got 18 instead of 28 above).
However, I'm a bit confused here, I thought highmem/lowmem split was a 32bit
thing, so I'm not sure how it applies here.
Anyways, the machine won't boot with more than 1GB of RAM, is there a solution to
get KASAN running on my machine?
Thanks,
Sasha
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