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

[Daniel P. Berrange]

On Wed, Nov 09, 2016 at 12:26:17PM +0100, Laszlo Ersek wrote:
> On 11/09/16 11:40, Andrew Jones wrote:
> > On Wed, Nov 09, 2016 at 11:01:46AM +0800, Dave Young wrote:
> >> Hi,
> >>
> >> Latest linux kernel enabled kaslr to randomiz phys/virt memory
> >> addresses, we had some effort to support kexec/kdump so that crash
> >> utility can still works in case crashed kernel has kaslr enabled.
> >>
> >> But according to Dave Anderson virsh dump does not work, quoted messages
> >> from Dave below:
> >>
> >> """
> >> with virsh dump, there's no way of even knowing that KASLR
> >> has randomized the kernel __START_KERNEL_map region, because there is no
> >> virtual address information -- e.g., like "SYMBOL(_stext)" in the kdump
> >> vmcoreinfo data to compare against the vmlinux file symbol value.
> >> Unless virsh dump can export some basic virtual memory data, which
> >> they say it can't, I don't see how KASLR can ever be supported.
> >> """
> >>
> >> I assume virsh dump is using qemu guest memory dump facility so it
> >> should be first addressed in qemu. Thus post this query to qemu devel
> >> list. If this is not correct please let me know.
> >>
> >> Could you qemu dump people make it work? Or we can not support virt dump
> >> as long as KASLR being enabled. Latest Fedora kernel has enabled it in 
> >> x86_64.
> >>
> > 
> > When the -kernel command line option is used, then it may be possible
> > to extract some information that could be used to supplement the memory
> > dump that dump-guest-memory provides. However, that would be a specific
> > use. In general, QEMU knows nothing about the guest kernel. It doesn't
> > know where it is in the disk image, and it doesn't even know if it's
> > Linux.
> > 
> > Is there anything a guest userspace application could probe from e.g.
> > /proc that would work? If so, then the guest agent could gain a new
> > feature providing that.
> 
> I fully agree. This is exactly what I suggested too, independently, in
> the downstream thread, before arriving at this upstream thread. Let me
> quote that email:
> 
> On 11/09/16 12:09, Laszlo Ersek wrote:
> > [...] the dump-guest-memory QEMU command supports an option called
> > "paging". Here's its documentation, from the "qapi-schema.json" source
> > file:
> >
> >> # @paging: if true, do paging to get guest's memory mapping. This allows
> >> #          using gdb to process the core file.
> >> #
> >> #          IMPORTANT: this option can make QEMU allocate several gigabytes
> >> #                     of RAM. This can happen for a large guest, or a
> >> #                     malicious guest pretending to be large.
> >> #
> >> #          Also, paging=true has the following limitations:
> >> #
> >> #             1. The guest may be in a catastrophic state or can have 
> >> corrupted
> >> #                memory, which cannot be trusted
> >> #             2. The guest can be in real-mode even if paging is enabled. 
> >> For
> >> #                example, the guest uses ACPI to sleep, and ACPI sleep 
> >> state
> >> #                goes in real-mode
> >> #             3. Currently only supported on i386 and x86_64.
> >> #
> >
> > "virsh dump --memory-only" sets paging=false, for obvious reasons.
> >
> > [...] the dump-guest-memory command provides a raw snapshot of the
> > virtual machine's memory (and of the registers of the VCPUs); it is
> > not enlightened about the guest.
> >
> > If the additional information you are looking for can be retrieved
> > within the running Linux guest, using an appropriately privieleged
> > userspace process, then I would recommend considering an extension to
> > the qemu guest agent. The management layer (libvirt, [...]) could
> > first invoke the guest agent (a process with root privileges running
> > in the guest) from the host side, through virtio-serial. The new guest
> > agent command would return the information necessary to deal with
> > KASLR. Then the management layer would initiate the dump like always.
> > Finally, the extra information would be combined with (or placed
> > beside) the dump file in some way.
> >
> > So, this proposal would affect the guest agent and the management
> > layer (= libvirt).
> 
> Given that we already dislike "paging=true", enlightening
> dump-guest-memory with even more guest-specific insight is the wrong
> approach, IMO. That kind of knowledge belongs to the guest agent.

If you're trying to debug a hung/panicked guest, then using a guest
agent to fetch info is a complete non-starter as it'll be dead.

Regards,
Daniel
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