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kasan.txt

kasan.txt 2.4 KB
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  1. KASAN is supported on powerpc on 32-bit and Radix 64-bit only.
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  2. 

  3. 32 bit support
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  4. ==============
    5. 

  5. 

  6. KASAN is supported on both hash and nohash MMUs on 32-bit.
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  7. 

  8. The shadow area sits at the top of the kernel virtual memory space above the
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  9. fixmap area and occupies one eighth of the total kernel virtual memory space.
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  10. 

  11. Instrumentation of the vmalloc area is optional, unless built with modules,
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  12. in which case it is required.
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  13. 

  14. 64 bit support
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  15. ==============
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  16. 

  17. Currently, only the radix MMU is supported. There have been versions for hash
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  18. and Book3E processors floating around on the mailing list, but nothing has been
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  19. merged.
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  20. 

  21. KASAN support on Book3S is a bit tricky to get right:
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  22. 

  23.  - It would be good to support inline instrumentation so as to be able to catch
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  24.    stack issues that cannot be caught with outline mode.
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  25. 

  26.  - Inline instrumentation requires a fixed offset.
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  27. 

  28.  - Book3S runs code with translations off ("real mode") during boot, including a
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  29.    lot of generic device-tree parsing code which is used to determine MMU
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  30.    features.
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  31. 

  32.  - Some code - most notably a lot of KVM code - also runs with translations off
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  33.    after boot.
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  34. 

  35.  - Therefore any offset has to point to memory that is valid with
    36. 

  36.    translations on or off.
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  37. 

  38. One approach is just to give up on inline instrumentation. This way boot-time
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  39. checks can be delayed until after the MMU is set is up, and we can just not
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  40. instrument any code that runs with translations off after booting. This is the
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  41. current approach.
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  42. 

  43. To avoid this limitiation, the KASAN shadow would have to be placed inside the
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  44. linear mapping, using the same high-bits trick we use for the rest of the linear
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  45. mapping. This is tricky:
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  46. 

  47.  - We'd like to place it near the start of physical memory. In theory we can do
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  48.    this at run-time based on how much physical memory we have, but this requires
    49. 

  49.    being able to arbitrarily relocate the kernel, which is basically the tricky
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  50.    part of KASLR. Not being game to implement both tricky things at once, this
    51. 

  51.    is hopefully something we can revisit once we get KASLR for Book3S.
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  52. 

  53.  - Alternatively, we can place the shadow at the _end_ of memory, but this
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  54.    requires knowing how much contiguous physical memory a system has _at compile
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  55.    time_. This is a big hammer, and has some unfortunate consequences: inablity
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  56.    to handle discontiguous physical memory, total failure to boot on machines
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  57.    with less memory than specified, and that machines with more memory than
    58. 

  58.    specified can't use it. This was deemed unacceptable.
    59.