From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:58667) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1c4RsC-00051G-9B for qemu-devel@nongnu.org; Wed, 09 Nov 2016 07:21:01 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1c4Rs9-0001GZ-2Z for qemu-devel@nongnu.org; Wed, 09 Nov 2016 07:21:00 -0500 Received: from mx1.redhat.com ([209.132.183.28]:36766) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1c4Rs8-0001GI-Qm for qemu-devel@nongnu.org; Wed, 09 Nov 2016 07:20:56 -0500 Date: Wed, 9 Nov 2016 13:20:51 +0100 From: Andrew Jones Message-ID: <20161109122051.ztllxmhwsalds2qw@kamzik.brq.redhat.com> References: <20161109030146.GA3802@dhcp-128-65.nay.redhat.com> <20161109104059.bvw5h4k4v77pw2rl@kamzik.brq.redhat.com> <9144d6b1-a1c9-e727-4673-9df10b227fdb@redhat.com> <20161109113735.GF22181@redhat.com> <20161109114809.cawi6tpsxwn5vfql@kamzik.brq.redhat.com> <20161109115819.GG22181@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161109115819.GG22181@redhat.com> Subject: Re: [Qemu-devel] virsh dump (qemu guest memory dump?): KASLR enabled linux guest support List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Daniel P. Berrange" Cc: Laszlo Ersek , Dave Young , qiaonuohan@cn.fujitsu.com, bhe@redhat.com, anderson@redhat.com, qemu-devel@nongnu.org On Wed, Nov 09, 2016 at 11:58:19AM +0000, Daniel P. Berrange wrote: > On Wed, Nov 09, 2016 at 12:48:09PM +0100, Andrew Jones wrote: > > 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. > > That doesn't help with trying to diagnose a crash during boot up, since > the guest agent isn't running till fairly late. I'm also concerned that > the QEMU guest agent is likely to be far from widely deployed in guests, > so reliance on the guest agent will mean the dump facility is no longer > reliably available. > It'd still be reliably available and useable during early boot, just like it is now, for kernels that don't use KASLR. This proposal is only attempting to *also* address KASLR kernels, for which there is currently no support whatsoever. Call it a best-effort. Of course we can get support for [probably] early boot and guest-agent-less guests using KASLR too if we introduce a paravirt solution, requiring guest kernel and KVM changes. Is it worth it? Thanks, drew