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Date: Tue, 11 Feb 2020 22:02:55 +0800 From: Xiaoyao Li <xiaoyao.li@...el.com> To: Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Sean Christopherson <sean.j.christopherson@...el.com>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...capital.net> Cc: x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, David Laight <David.Laight@...lab.com> Subject: Re: [PATCH v2 3/6] kvm: x86: Emulate split-lock access as a write On 2/11/2020 9:34 PM, Paolo Bonzini wrote: > On 11/02/20 14:22, Thomas Gleixner wrote: >> Paolo Bonzini <pbonzini@...hat.com> 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. So you think there is no need to emulate this feature and return #AC to guest? Anyway, I'm fine with killing the guest. BTW, Can it really be used for DoS purpose by malicious guest? Since it's in kvm emulator so it needs vm-exit first and won't the die() in kernel handler kill KVM? (Actually I'm not clear about KVM after die()) >>> 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. Alright, I don't know the history of TEST_CTRL, there is a bit 31 in it which means "Disable LOCK# assertion for split locked access" when set. Bit 31 exists for a long period, but linux seems not use it so I guess it may be a testing purpose bit. However, when it comes to bit 29, split lock #AC, the main purpose is to prevent any split lock more than debugging. BTW, I guess the reason putting it in MSR_TEST_CTRL is that it's related with split lock as bit 31. > Paolo >
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