Re: [PATCH] KVM: PPC: Book3S PR: close a race window when SVCPU pointer is hold before kvmppc_copy_from_svcpu()

[Simon Guo <wei.guo.simon@gmail.com>]

Hi Alex,
On Wed, Jan 31, 2018 at 10:28:05AM +0100, Alexander Graf wrote:
> 
> 
> On 31.01.18 05:23, wei.guo.simon@gmail.com wrote:
> > From: Simon Guo <wei.guo.simon@gmail.com>
> > 
> > commit 40fdd8c88c4a ("KVM: PPC: Book3S: PR: Make svcpu -> vcpu store
> > preempt savvy") and commit 3d3319b45eea ("KVM: PPC: Book3S: PR: Enable
> > interrupts earlier") is trying to turns on preemption early when
> > return into highmem guest exit handler.
> > 
> > However there is a race window in following example at
> > arch/powerpc/kvm/book3s_interrupts.S:
> > 
> > highmem guest exit handler:
> > ...
> > 195         GET_SHADOW_VCPU(r4)
> > 196         bl      FUNC(kvmppc_copy_from_svcpu)
> > ...
> > 239         bl      FUNC(kvmppc_handle_exit_pr)
> > 
> > If there comes a preemption between line 195 and 196, line 196
> > may hold an invalid SVCPU reference with following sequence:
> > 1) Qemu task T1 runs at GET_SHADOW_VCPU(r4) at line 195, on CPU A.
> > 2) T1 is preempted and switch out CPU A. As a result, it checks
> > CPU A's svcpu->in_use (=1 at present) and flush cpu A's svcpu to
> > T1's vcpu.
> > 3) Another task T2 switches into CPU A and it may update CPU A's
> > svcpu->in_use into 1.
> > 4) T1 is scheduled into CPU B. But it still holds CPU A's svcpu
> > reference as R4. Then it executes kvmppc_copy_from_svcpu() with
> > R4 and it will corrupt T1's VCPU with T2's content. T2's VCPU
> > will also be impacted.
> > 
> > This patch moves the svcpu->in_use into VCPU so that the vcpus
> > sharing the same svcpu can work properly and fix the above case.
> > 
> > Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
> 
> Sorry, the previous version would only compile on 32bit PPC ;). Please
> find the fixed one which just uses svcpu_get() and _put() here:
> 
> 
> https://github.com/agraf/linux-2.6/commit/f9e3ca44c9a9d4930d6dccaacb518734746059c3
> 
> 
> Alex
Your solution looks better than mine :)
Unfortunately somehow I cannot reproduce my issue without the fix. So 
I cannot test it currently.

Reviewed-by: Simon Guo <wei.guo.simon@gmail.com>

Thanks,
- Simon