Re: Potential bug in TDP MMU

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On Tue, Nov 30, 2021, Ignat Korchagin wrote:
> I have managed to reliably reproduce the issue on a QEMU VM (on a host
> with nested virtualisation enabled). Here are the steps:
> 
> 1. Install gvisor as per
> https://gvisor.dev/docs/user_guide/install/#install-latest
> 2. Run
> $ for i in $(seq 1 100); do sudo runsc --platform=kvm --network=none
> do echo ok; done
> 
> I've tried to recompile the kernel with the above patch, but
> unfortunately it does fix the issue. I'm happy to try other
> patches/fixes queued for 5.16-rc4

My best guest would be https://lore.kernel.org/all/20211120045046.3940942-5-seanjc@xxxxxxxxxx/,
that bug results in KVM installing SPTEs into an invalid root.  I think that could
lead to a use-after-free and/or double-free, which is usually what leads to the
"Bad page state" errors.

In the meantime, I'll try to repro.

> > > arch/x86/kvm/../../../virt/kvm/kvm_main.c:171

...

> > > After this the machine starts spitting some traces starting with:
> > >
> > > [177247.871683][T2343516] BUG: Bad page state in process <comm>  pfn:fe680a



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