On Tue, Aug 12, 2014 at 11:05 AM, Stefan Bader <stefan.ba...@canonical.com> wrote: > On 12.08.2014 19:28, Kees Cook wrote: >> On Fri, Aug 8, 2014 at 7:35 AM, Stefan Bader <stefan.ba...@canonical.com> >> wrote: >>> On 08.08.2014 14:43, David Vrabel wrote: >>>> On 08/08/14 12:20, Stefan Bader wrote: >>>>> Unfortunately I have not yet figured out why this happens, but can >>>>> confirm by >>>>> compiling with or without CONFIG_RANDOMIZE_BASE being set that without >>>>> KASLR all >>>>> is ok, but with it enabled there are issues (actually a dom0 does not >>>>> even boot >>>>> as a follow up error). >>>>> >>>>> Details can be seen in [1] but basically this is always some portion of a >>>>> vmalloc allocation failing after hitting a freshly allocated PTE space >>>>> not being >>>>> PTE_NONE (usually from a module load triggered by systemd-udevd). In the >>>>> non-dom0 case this repeats many times but ends in a guest that allows >>>>> login. In >>>>> the dom0 case there is a more fatal error at some point causing a crash. >>>>> >>>>> I have not tried this for a normal PV guest but for dom0 it also does not >>>>> help >>>>> to add "nokaslr" to the kernel command-line. >>>> >>>> Maybe it's overlapping with regions of the virtual address space >>>> reserved for Xen? What the the VA that fails? >>>> >>>> David >>>> >>> Yeah, there is some code to avoid some regions of memory (like initrd). >>> Maybe >>> missing p2m tables? I probably need to add debugging to find the failing VA >>> (iow >>> not sure whether it might be somewhere in the stacktraces in the report). >>> >>> The kernel-command line does not seem to be looked at. It should put >>> something >>> into dmesg and that never shows up. Also today's random feature is other PV >>> guests crashing after a bit somewhere in the check_for_corruption area... >> >> Right now, the kaslr code just deals with initrd, cmdline, etc. If >> there are other reserved regions that aren't listed in the e820, it'll >> need to locate and skip them. >> >> -Kees >> > Making my little steps towards more understanding I figured out that it isn't > the code that does the relocation. Even with that completely disabled there > were > the vmalloc issues. What causes it seems to be the default of the upper limit > and that this changes the split between kernel and modules to 1G+1G instead of > 512M+1.5G. That is the reason why nokaslr has no effect.
Oh! That's very interesting. There must be some assumption in Xen about the kernel VM layout then? -Kees -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
