Messages in this thread | | | From | Thomas Garnier <> | Date | Thu, 5 Jan 2017 13:58:17 -0800 | Subject | Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location |
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On Thu, Jan 5, 2017 at 1:19 PM, Andy Lutomirski <luto@amacapital.net> wrote: > On Thu, Jan 5, 2017 at 1:08 PM, Thomas Garnier <thgarnie@google.com> wrote: >> On Thu, Jan 5, 2017 at 12:18 PM, Andy Lutomirski <luto@kernel.org> wrote: >>> On Thu, Jan 5, 2017 at 11:03 AM, Thomas Garnier <thgarnie@google.com> wrote: >>>> On Thu, Jan 5, 2017 at 10:58 AM, Arjan van de Ven <arjan@linux.intel.com> wrote: >>>>> On 1/5/2017 9:54 AM, Thomas Garnier wrote: >>>>> >>>>>> >>>>>> That's my goal too. I started by doing a RO remap and got couple >>>>>> problems with hibernation. I can try again for the next iteration or >>>>>> delay it for another patch. I also need to look at KVM GDT usage, I am >>>>>> not familiar with it yet. >>>>> >>>>> >>>>> don't we write to the GDT as part of the TLS segment stuff for glibc ? >>>>> >>>> >>>> Not sure which glibc feature it is. >>>> >>>> In this design, you can write to the GDT per-cpu variable that will >>>> remain read-write. You just need to make the remapping writeable when >>>> we load task registers (ltr) then the processor use the current GDT >>>> address. At least that the case I know, I might find more through >>>> testing. >>> >>> Hmm. I bet that if we preset the accessed bits in all the segments >>> then we don't need it to be writable in general. But your point about >>> set_thread_area (TLS) is well taken. However, I strongly suspect that >>> we could make set_thread_area unconditionally set the accessed bit and >>> no one would ever notice. >> >> Not sure I fully understood and I don't want to miss an important >> point. Do you mean making GDT (remapping and per-cpu) read-only and >> switch the writeable flag only when we write to the per-cpu entry? >> > > What I mean is: write to the GDT through normal percpu access (or > whatever the normal mapping is) but load a read-only alias into the > GDT register. As long as nothing ever tries to write through the GDTR > alias, no page faults will be generated. So we just need to make sure > that nothing ever writes to it through GDTR. AFAIK the only reason > the CPU ever writes to the address in GDTR is to set an accessed bit. >
A write is made when we use load_TR_desc (ltr). I didn't see any other yet.
> --Andy
-- Thomas
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