Messages in this thread | | | From | Thomas Garnier <> | Date | Thu, 5 Jan 2017 08:39:05 -0800 | Subject | Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location |
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On Thu, Jan 5, 2017 at 12:11 AM, Ingo Molnar <mingo@kernel.org> wrote: > > * Thomas Garnier <thgarnie@google.com> wrote: > >> Each processor holds a GDT in its per-cpu structure. The sgdt >> instruction gives the base address of the current GDT. This address can >> be used to bypass KASLR memory randomization. With another bug, an >> attacker could target other per-cpu structures or deduce the base of the >> main memory section (PAGE_OFFSET). >> >> In this change, a space is reserved at the end of the memory range >> available for KASLR memory randomization. The space is big enough to hold >> the maximum number of CPUs (as defined by setup_max_cpus). Each GDT is >> mapped at specific offset based on the target CPU. Note that if there is >> not enough space available, the GDTs are not remapped. >> >> The document was changed to mention GDT remapping for KASLR. This patch >> also include dump page tables support. >> >> This patch was tested on multiple hardware configurations and for >> hibernation support. > >> void kernel_randomize_memory(void); >> +void kernel_randomize_smp(void); >> +void* kaslr_get_gdt_remap(int cpu); > > Yeah, no fundamental objections from me to the principle, but I get some bad vibes > from the naming here: seeing that kernel_randomize_smp() actually makes things > less random. >
I agree, I went back and forth on the name. I will change it to something better.
> Also, don't we want to do this unconditionally and not allow remapping failures? > > The GDT is fairly small, plus making the SGDT instruction expose fewer kernel > internals would be (marginally) useful on non-randomized kernels as well. > > It also makes the code more common, more predictable, more debuggable and less > complex overall - which is pretty valuable in terms of long term security as well. >
Okay, I will add BUG_ON on failures to remap.
> Thanks, > > Ingo
Ingo: I saw the 5-level page table support being sent through. Do you want me to wait for it to be -next? (Given it will need to be changed too).
-- Thomas
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