On Tue, Feb 11, 2020 at 02:34:18PM +0100, Paolo Bonzini wrote: > On 11/02/20 14:22, Thomas Gleixner wrote: > > Paolo Bonzini <pbonzini@xxxxxxxxxx> writes: > >> On 03/02/20 16:16, Xiaoyao Li wrote: > >>> A sane guest should never tigger emulation on a split-lock access, but > >>> it cannot prevent malicous guest from doing this. So just emulating the > >>> access as a write if it's a split-lock access to avoid malicous guest > >>> polluting the kernel log. > >> > >> Saying that anything doing a split lock access is malicious makes little > >> sense. > > > > Correct, but we also have to accept, that split lock access can be used > > in a malicious way, aka. DoS. > > Indeed, a more accurate emulation such as temporarily disabling > split-lock detection in the emulator would allow the guest to use split > lock access as a vehicle for DoS, but that's not what the commit message > says. If it were only about polluting the kernel log, there's > printk_ratelimited for that. (In fact, if we went for incorrect > emulation as in this patch, a rate-limited pr_warn would be a good idea). > > It is much more convincing to say that since this is pretty much a > theoretical case, we can assume that it is only done with the purpose of > DoS-ing the host or something like that, and therefore we kill the guest. The problem with "kill the guest", and the reason I'd prefer to emulate the split-lock as a write, is that killing the guest in this case is annoyingly difficult. Returning X86EMUL_UNHANDLEABLE / EMULATION_FAILED gets KVM to handle_emulation_failure(), but handle_emulation_failure() will only "kill" the guest if emulation failed in L1 CPL==0. For all other modes, it will inject a #UD and resume the guest. KVM also injects a #UD for L1 CPL==0, but that's the least annoying thing. Adding a new emulation type isn't an option because this code can be triggered through normal emulation. A new return type could be added for split-lock, but that's code I'd really not add, both from an Intel perspective and a KVM maintenance perspective. And, we'd still have the conundrum of what to do if/when split-lock #AC is exposed to L1, e.g. in that case, KVM should inject an #AC into L1, not kill the guest. Again, totally doable, but ugly and IMO an unnecessary maintenance burden. I completely agree that poorly emulating the instruction from the (likely) malicious guest is a hack, but it's a simple and easy to maintain hack. > >> Split lock detection is essentially a debugging feature, there's a > >> reason why the MSR is called "TEST_CTL". So you don't want to make the > > > > The fact that it ended up in MSR_TEST_CTL does not say anything. That's > > where they it ended up to be as it was hastily cobbled together for > > whatever reason. > > Or perhaps it was there all the time in test silicon or something like > that... That would be a very plausible reason for all the quirks behind it. > > Paolo >
