On 18.12.18 11:32, Konstantin Belousov wrote: > On Tue, Dec 18, 2018 at 11:22:53AM +0100, Alexander Lochmann wrote: >> >>>> 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)? > No, only 0 and 3. But be careful with vm86 (I am not sure how your VM > reports it to your instrumentation). > Ok. Thx!
- Alex -- Technische Universität Dortmund Alexander Lochmann PGP key: 0xBC3EF6FD Otto-Hahn-Str. 16 phone: +49.231.7556141 D-44227 Dortmund fax: +49.231.7556116 http://ess.cs.tu-dortmund.de/Staff/al
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