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

[Andrew Jones]

On Wed, Nov 09, 2016 at 11:37:35AM +0000, Daniel P. Berrange wrote:
> 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.

So don't wait. Management software can make this query immediately
after the guest agent goes live. The information needed won't change.

drew