linux-kernel - Re: [PATCH v7 4/4] powerpc: Book3S 64-bit "heavyweight" KASAN support

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Date:   Thu, 13 Feb 2020 07:08:27 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Daniel Axtens <dja@...ens.net>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        kasan-dev@...glegroups.com, aneesh.kumar@...ux.ibm.com,
        bsingharora@...il.com
Cc:     Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH v7 4/4] powerpc: Book3S 64-bit "heavyweight" KASAN support



Le 13/02/2020 à 01:47, Daniel Axtens a écrit :
> KASAN support on Book3S is a bit tricky to get right:
> 
>   - It would be good to support inline instrumentation so as to be able to
>     catch stack issues that cannot be caught with outline mode.
> 
>   - Inline instrumentation requires a fixed offset.
> 
>   - Book3S runs code in real mode after booting. Most notably a lot of KVM
>     runs in real mode, and it would be good to be able to instrument it.
> 
>   - Because code runs in real mode after boot, the offset has to point to
>     valid memory both in and out of real mode.
> 
>      [ppc64 mm note: The kernel installs a linear mapping at effective
>      address c000... onward. This is a one-to-one mapping with physical
>      memory from 0000... onward. Because of how memory accesses work on
>      powerpc 64-bit Book3S, a kernel pointer in the linear map accesses the
>      same memory both with translations on (accessing as an 'effective
>      address'), and with translations off (accessing as a 'real
>      address'). This works in both guests and the hypervisor. For more
>      details, see s5.7 of Book III of version 3 of the ISA, in particular
>      the Storage Control Overview, s5.7.3, and s5.7.5 - noting that this
>      KASAN implementation currently only supports Radix.]
> 
> One approach is just to give up on inline instrumentation. This way all
> checks can be delayed until after everything set is up correctly, and the
> address-to-shadow calculations can be overridden. However, the features and
> speed boost provided by inline instrumentation are worth trying to do
> better.
> 
> If _at compile time_ it is known how much contiguous physical memory a
> system has, the top 1/8th of the first block of physical memory can be set
> aside for the shadow. This is a big hammer and comes with 3 big
> consequences:
> 
>   - there's no nice way to handle physically discontiguous memory, so only
>     the first physical memory block can be used.
> 
>   - kernels will simply fail to boot on machines with less memory than
>     specified when compiling.
> 
>   - kernels running on machines with more memory than specified when
>     compiling will simply ignore the extra memory.
> 
> Implement and document KASAN this way. The current implementation is Radix
> only.
> 
> Despite the limitations, it can still find bugs,
> e.g. http://patchwork.ozlabs.org/patch/1103775/
> 
> At the moment, this physical memory limit must be set _even for outline
> mode_. This may be changed in a later series - a different implementation
> could be added for outline mode that dynamically allocates shadow at a
> fixed offset. For example, see https://patchwork.ozlabs.org/patch/795211/
> 
> Suggested-by: Michael Ellerman <mpe@...erman.id.au>
> Cc: Balbir Singh <bsingharora@...il.com> # ppc64 out-of-line radix version
> Cc: Christophe Leroy <christophe.leroy@....fr> # ppc32 version
> Signed-off-by: Daniel Axtens <dja@...ens.net>

Reviewed-by: <christophe.leroy@....fr> # focussed mainly on 
Documentation and things impacting PPC32