59 lines
2.4 KiB
Plaintext
59 lines
2.4 KiB
Plaintext
KASAN is supported on powerpc on 32-bit and Radix 64-bit only.
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32 bit support
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==============
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KASAN is supported on both hash and nohash MMUs on 32-bit.
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The shadow area sits at the top of the kernel virtual memory space above the
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fixmap area and occupies one eighth of the total kernel virtual memory space.
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Instrumentation of the vmalloc area is optional, unless built with modules,
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in which case it is required.
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64 bit support
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==============
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Currently, only the radix MMU is supported. There have been versions for hash
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and Book3E processors floating around on the mailing list, but nothing has been
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merged.
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KASAN support on Book3S is a bit tricky to get right:
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- It would be good to support inline instrumentation so as to be able to catch
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stack issues that cannot be caught with outline mode.
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- Inline instrumentation requires a fixed offset.
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- Book3S runs code with translations off ("real mode") during boot, including a
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lot of generic device-tree parsing code which is used to determine MMU
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features.
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- Some code - most notably a lot of KVM code - also runs with translations off
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after boot.
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- Therefore any offset has to point to memory that is valid with
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translations on or off.
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One approach is just to give up on inline instrumentation. This way boot-time
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checks can be delayed until after the MMU is set is up, and we can just not
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instrument any code that runs with translations off after booting. This is the
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current approach.
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To avoid this limitiation, the KASAN shadow would have to be placed inside the
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linear mapping, using the same high-bits trick we use for the rest of the linear
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mapping. This is tricky:
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- We'd like to place it near the start of physical memory. In theory we can do
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this at run-time based on how much physical memory we have, but this requires
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being able to arbitrarily relocate the kernel, which is basically the tricky
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part of KASLR. Not being game to implement both tricky things at once, this
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is hopefully something we can revisit once we get KASLR for Book3S.
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- Alternatively, we can place the shadow at the _end_ of memory, but this
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requires knowing how much contiguous physical memory a system has _at compile
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time_. This is a big hammer, and has some unfortunate consequences: inablity
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to handle discontiguous physical memory, total failure to boot on machines
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with less memory than specified, and that machines with more memory than
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specified can't use it. This was deemed unacceptable.
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