Re: [Qemu-devel] virsh dump (qemu guest memory dump?): KASLR enabled linux guest support

[Dave Anderson <anderson@redhat.com>]

----- Original Message -----
> 
> 
> On 09/11/2016 16:28, Dave Anderson wrote:
> > I'm not sure whether this "guest userspace agent" is still in play here,
> > but if there were such a thing, it could theoretically do the same
> > thing that crash currently does when running on a live system.
> > 
> > Both of those are available or calculatable from the contents of
> > a kdump header.  However, on a live system, it's done like this:
> > 
> > - /proc/kallsyms is queried for the symbol value of "_text", which would
> >   be relocated if KASLR is in play.  That value is compared against the
> >   "_text" symbol value compiled into the vmlinux file to determine the
> >   relocation value generated by CONFIG_RANDOMIZE_BASE.
> > 
> > [...] in order to read kernel symbols from the
> > statically-mapped kernel region based at __START_KERNEL_map, it
> > translates a (possibly relocated) kernel virtual address into a
> > physical address like this:
> > 
> >   physical-address = virtual-address - __START_KERNEL_map + phys_base
> > 
> > But it's a chicken-and-egg deal, because the contents of the "phys_base"
> > symbol are needed to calculate the physical address, but it can't
> > read the "phys_base" symbol contents without first knowing its contents.
> > 
> > So on a live system, the "phys_base" is calculated by reading
> > the "Kernel Code:" value from /proc/iomem, and then doing this:
> > 
> >   phys_base = [Kernel Code: value] - ["_text" symbol value] - __START_KERNEL_map
>                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
> Should there be parentheses around this?  

Yes, sorry, that's correct -- that's what the code does, and what I meant to express...

Dave


> The physical-address formula above is equivalent to
> 
>     phys_base = physical-address - (virtual-address - __START_KERNEL_map)
> 
> > 
> > So theoretically, the guest agent could read /proc/iomem and /proc/kallsyms
> > for the information required.  (I think...)
> 
> Then yes, the guest-agent could add a command get-kernel-text-start with an output like:
> 
> { 'virtual': 0xffffffff86000000, 'physical': 0xb6000000 }
> 
> and libvirt can expose it to crash.  In this case, phys_base would be 0xb0000000
> if I did the math right, and the relocation value is obtained by comparing the
> "virtual" address with the vmlinux "_text".
> 
> IIRC the guest agent runs as root, so reading /proc/iomem is not a problem.
> 
> Paolo
>