Re: [PATCH v2 for-4.14 1/2] x86/mem_sharing: block interrupt injection for forks

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Mon, May 25, 2020 at 6:18 AM Tamas K Lengyel <tamas@xxxxxxxxxxxxx> wrote:
>
> On Mon, May 25, 2020 at 12:06 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
> >
> > On 22.05.2020 18:33, Tamas K Lengyel wrote:
> > > When running shallow forks without device models it may be undesirable 
> > > for Xen
> > > to inject interrupts. With Windows forks we have observed the kernel 
> > > going into
> > > infinite loops when trying to process such interrupts, likely because it 
> > > attempts
> > > to interact with devices that are not responding without QEMU running. By
> > > disabling interrupt injection the fuzzer can exercise the target code 
> > > without
> > > interference.
> > >
> > > Forks & memory sharing are only available on Intel CPUs so this only 
> > > applies
> > > to vmx.
> >
> > Looking at e.g. mem_sharing_control() I can't seem to be able to confirm
> > this. Would you mind pointing me at where this restriction is coming from?
>
> Both mem_access and mem_sharing are only implemented for EPT:
> http://xenbits.xen.org/hg/xen-unstable.hg/file/5eadf9363c25/xen/arch/x86/mm/p2m-ept.c#l126.
>
> >
> > > --- a/xen/arch/x86/hvm/vmx/intr.c
> > > +++ b/xen/arch/x86/hvm/vmx/intr.c
> > > @@ -256,6 +256,12 @@ void vmx_intr_assist(void)
> > >      if ( unlikely(v->arch.vm_event) && v->arch.vm_event->sync_event )
> > >          return;
> > >
> > > +#ifdef CONFIG_MEM_SHARING
> > > +    /* Block event injection for VM fork if requested */
> > > +    if ( unlikely(v->domain->arch.hvm.mem_sharing.block_interrupts) )
> > > +        return;
> > > +#endif
> >
> > The two earlier returns are temporary as far as the guest is concerned,
> > i.e. eventually the interrupt(s) will get delivered. The one you add
> > looks as if it is a permanent thing, i.e. interrupt requests will pile
> > up and potentially confuse a guest down the road. This _may_ be okay
> > for your short-lived-shallow-fork scenario, but then wants at least
> > calling out in the public header by a comment (and I think the same
> > goes for XENMEM_FORK_WITH_IOMMU_ALLOWED that's already there).
>
> This is indeed only for the short-lived forks, that's why this is an
> optional flag that can be enabled when creating forks and it's not on
> by default. In that use-case the VM executes for fractions of a second
> and we want to only executes very specific code-segments with
> absolutely no interference. Interrupts in that case are just a
> nuisance that in the best case slow the fuzzing process down but as we
> observed in the worst case complete stall it.
>
> >
> > > --- a/xen/include/asm-x86/hvm/domain.h
> > > +++ b/xen/include/asm-x86/hvm/domain.h
> > > @@ -74,6 +74,8 @@ struct mem_sharing_domain
> > >       * to resume the search.
> > >       */
> > >      unsigned long next_shared_gfn_to_relinquish;
> > > +
> > > +    bool block_interrupts;
> > >  };
> >
> > Please can you avoid unnecessary growth of the structure by inserting
> > next to the pre-existing bool rather than at the end?
>
> Sure. Do you want me to resend the patch for that?

I'll just resend it anyway with the requested comments in the public header.

Tamas