On Thu, Jan 5, 2017 at 9:54 AM, Thomas Garnier <[email protected]> wrote: > On Thu, Jan 5, 2017 at 9:51 AM, Andy Lutomirski <[email protected]> wrote: >> On Wed, Jan 4, 2017 at 2:16 PM, Thomas Garnier <[email protected]> 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. >> >> Can we remap it read-only? I.e. use PAGE_KERNEL_RO instead of >> PAGE_KERNEL. After all, the ability to modify the GDT is instant >> root. > > 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.
If you want a small adventure, I think a significant KVM-related performance improvement is available. Specifically, on VMX exits, the GDT limit is hardwired to 0xffff (IIRC -- I could be remembering the actual vaue wrong). KVM does LGDT to fix it. If we actually made the GDT have limit 0xffff (presumably by mapping the zero page a few times to pad it out without wasting memory), then we would avoid the LGDT. LGDT is incredibly slow, so this would be a big win. Want to see if you can make this work with your patch set?
