[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 04:58 PM, 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? >> >> 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. > > The attack surface sadly is not enlarged in the slightest by this change. > > We already trap and emulate all #UD exceptions in an attempt to support > migration of VMs between Intel and AMD hardware. See XSA-105. (There > is a good argument to be made for not trapping #UD, but that doesn't > completely close the hole) So at the moment, an attacker on Intel can force the emulation of any AMD-only instruction (and vice versa), is that right? This would allow an attacker to force the emulation of every #GP condition of every instruction we emulate. Those two sets may be within an order of magnitude of each other, but they will only overlap a little bit. So my guess is that enabling this would double the surface of attack (give or take). I'd be a lot happier with this patch if we could make it so that on a #GP the only instruction that could get emulated would be an IO instruction. -George _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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