linux-kernel - Re: [PATCH] arm64: KVM: VHE: reset PSTATE.UAO when switch to host

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Date:   Thu, 7 Sep 2017 15:03:49 +0000
From:   gengdongjiu <gengdongjiu@...wei.com>
To:     Marc Zyngier <marc.zyngier@....com>,
        James Morse <james.morse@....com>
CC:     "christoffer.dall@...aro.org" <christoffer.dall@...aro.org>,
        "vladimir.murzin@....com" <vladimir.murzin@....com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "shankerd@...eaurora.org" <shankerd@...eaurora.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Zhanghaibin (Euler)" <zhanghaibin7@...wei.com>,
        Huangshaoyu <huangshaoyu@...wei.com>
Subject: Re: [PATCH] arm64: KVM: VHE: reset PSTATE.UAO when switch to host

> On 07/09/17 12:49, gengdongjiu wrote:
> >
> >
> > On 2017/9/7 18:13, Marc Zyngier wrote:
> >> On 07/09/17 11:05, gengdongjiu wrote:
> >>> Hi James,
> >>>
> >>> On 2017/9/7 17:20, James Morse wrote:
> >>>> Hi Dongjiu Geng,
> >>>>
> >>>> On 07/09/17 06:54, Dongjiu Geng wrote:
> >>>>> In VHE mode, host kernel runs in the EL2 and can enable 'User
> >>>>> Access Override' when fs==KERNEL_DS so that it can access kernel
> >>>>> memory. However, PSTATE.UAO is set to 0 on an exception taken from
> >>>>> EL1 to EL2. Thus when VHE is used and exception taken from a guest
> >>>>> UAO will be disabled and host will use the incorrect PSTATE.UAO.
> >>>>> So check and reset the PSTATE.UAO when switching to host.
> >>>>
> >>>> This would only be a problem if KVM were calling into world-switch
> >>>> with fs==KERNEL_DS. I can't see where this happens.
> >>>  Not only KVM, may also kernel sets the fs == KERNEL_DS before
> >>> calling into world-switch
> >>
> >> How? Please describe the exact sequence of event that lead to this
> >> situation with the current code base.
> >
> > Hi Marc,
> >
> >    Different tasks have different fs, such as USER_DS or KERNEL_DS. In
> > the context switch, it will restore the task's fs. Thus, that depends
> > on task itself, as shown below code. UAO is different with PAN, PAN will be always enabled if hardware CPU supports PAN feature, but
> UAO is dynamical change.
> 
> You haven't answered my question: There is exactly one point where we enter the world-switch. Show me that, at this point, PSTATE.UAO
> *before* the call is different from PSTATE.UAO after the call. Give me the exact sequence of event that leads to this situation. Show me a
> stack trace.

Hi Marc,

  If using current mainline KVM code + Qemu and modify nothing, may not exist broken issue,  because the Qemu progress FS should be USER_DS. But if I make a modification for user space( Qemu or KVM tools) to change its FS property to KERNEL_DS or use third party application with KERNEL_DS FS to run the guest, it will have problem(the KERNEL_DS is cleared). If you think my case is reasonable and should support, I can show you the calling stack trace. If you think, my case is not reasonable and KVM should not support the application with KERNEL_DS fs to run guest. You can ignore this patch, thanks.


> 
> Until you do this, I will ignore any further comment coming from you on this subject.
> 
> Thanks,
> 
> 	M.
> --
> Jazz is not dead. It just smells funny...