Address Collision using i386 4G/4G Memory Split

Alexander Lochmann alexander.lochmann at
Tue Dec 18 10:22:56 UTC 2018

>> Some context: We are doing VM-based tracing in the FreeBSD kernel. For
>> that, we observe parts of the kernel memory (allocations, accesses,...).
>> Before 12.0 we simply knew that kernel addresses that we logged were
>> unique. Moreover, when a memory access to a region of interest happened
>> we knew that could only be kernel memory.
>> We know have to ensure that we only record memory accesses that happen
>> within the kernel.
>> Our approach is to record the kernels value for the CR3 register, and
>> record memory accesses if the CR3 registers holds the aforementioned value.
> You must use CPL to see if the current operation mode is user or kernel.
> If user, nothing should be done (this would avoid vm86). If kernel, you
> need to compare current %cr3 with IdlePTD (IdlePTDP for PAE case).
Thanks for the advice!  We'll include that in our toolchain.
Do you use PLs other than 0(=kernel) and 3(=user)?

- Alex

Technische Universität Dortmund
Alexander Lochmann                PGP key: 0xBC3EF6FD
Otto-Hahn-Str. 16                 phone:  +49.231.7556141
D-44227 Dortmund                  fax:    +49.231.7556116

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