[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v11 6/9] xen: Add ring 3 vmware_port support
On 06/03/2015 05:36 PM, Don Slutz wrote: > On 06/03/15 11:58, Andrew Cooper wrote: >> On 03/06/15 16:26, George Dunlap wrote: >>> On 05/22/2015 04:50 PM, Don Slutz wrote: >>>> Summary is that VMware treats "in (%dx),%eax" (or "out %eax,(%dx)") >>>> to port 0x5658 specially. Note: since many operations return data >>>> in EAX, "in (%dx),%eax" is the one to use. The other lengths like >>>> "in (%dx),%al" will still do things, only AL part of EAX will be >>>> changed. For "out %eax,(%dx)" of all lengths, EAX will remain >>>> unchanged. >>>> >>>> This instruction is allowed to be used from ring 3. To >>>> support this the vmexit for GP needs to be enabled. I have not >>>> fully tested that nested HVM is doing the right thing for this. >>>> >>>> Enable no-fault of pio in x86_emulate for VMware port >>>> >>>> Also adjust the emulation registers after doing a VMware >>>> backdoor operation. >>>> >>>> Add new routine hvm_emulate_one_gp() to be used by the #GP fault >>>> handler. >>>> >>>> Some of the best info is at: >>>> >>>> https://sites.google.com/site/chitchatvmback/backdoor >>>> >>>> Signed-off-by: Don Slutz <dslutz@xxxxxxxxxxx> >>> So let me get this straight. >>> >>> VMWare allows ring3 to access the magic port regardless of whether the >>> guest OS has enabled access to that IO port or not. >>> >>> In order to emulate this, we need to: >>> * Trap to Xen on #GPs rather than just letting the hardware handle it >>> * Emulate all instructions which cause a #GP, just to see if they might >>> be an IO instruction accessing the magic port. >>> * If it is an IO instruction, and it's accessing the magic port, then we >>> skip the ioport access checks (which will cause the instruction to >>> execute as though it had been given access). >>> * Under all other circumstances (we hope) the emulator in Xen will do >>> exactly what the hardware just did, and deliver a #GP to the guest. >>> >>> In an attempt to make this more safe, emulation ops that write (such as >>> write and cmpxchg) are replaced with stubs which always return an error. >>> >>> Is that about right? > > Yes, however it is missing that Jan Beulich wanted the emulator in Xen > to be used. I had started with code that did not use the emulator. I agree with him that the emulator should be used to emulate the instructions we *want* to emulate. I'm just not happy with using the emulator to emulate all the instructions we *don't* want to emulate (i.e., all the ones that really do need to #GP). >>> That sounds completely insane. It opens up an almost infinite surface >>> of attack onto the Xen emulator. >>> >>> I understand that having the "VMWare compatible" is a nice tick-box to >>> have, but seriously, I cannot imagine that having unprivileged >>> user-space tools know the real clock frequency without having to involve >>> the OS is anywhere close to worth the risk involved. > > Not sure how you moved from attack surface to "real clock frequency" > (which I am not sure which of the many "clock frequency" you are > referring to. The only new one that leaps to mind is the emulated lapic > bus frequency (which Linux attempts to determine from other clocks). I'm talking about cost-benefits analysis. What's the benefit of accepting this patch, and is it worth the cost? My argument here is that the cost of this change is opening up a massive attack surface on the Xen emulation code. The benefit of this change: Allowing guest processes access to the VMWare backdoor without guest OS cooperation. (Guest OSes can access the backdoor without this patch.) I hadn't gotten to the part of the series where Qemu was roped in to do mouse and clipboard stuff; so at the time I wrote that, the only functionality that it looked like was being made available to the guest was reading the clock and a couple of other random bits. But I see I have another e-mail from Andy with information of material importance to this discussion. -George _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
|
Lists.xenproject.org is hosted with RackSpace, monitoring our |