Re: [PATCH v2 4/4] KVM: x86: Fix stack-out-of-bounds memory access from ioapic_write_indirect()

[Sean Christopherson <seanjc@google.com>]

On Tue, Aug 24, 2021, Maxim Levitsky wrote:
> On Tue, 2021-08-24 at 16:42 +0200, Vitaly Kuznetsov wrote:
> > Eduardo Habkost <ehabkost@redhat.com> writes:
> > 
> > > On Tue, Aug 24, 2021 at 3:13 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> > > > Eduardo Habkost <ehabkost@redhat.com> writes:
> > > > 
> > > > > On Mon, Aug 23, 2021 at 04:30:28PM +0200, Vitaly Kuznetsov wrote:
> > > > > > diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c
> > > > > > index ff005fe738a4..92cd4b02e9ba 100644
> > > > > > --- a/arch/x86/kvm/ioapic.c
> > > > > > +++ b/arch/x86/kvm/ioapic.c
> > > > > > @@ -319,7 +319,7 @@ static void ioapic_write_indirect(struct kvm_ioapic *ioapic, u32 val)
> > > > > >      unsigned index;
> > > > > >      bool mask_before, mask_after;
> > > > > >      union kvm_ioapic_redirect_entry *e;
> > > > > > -    unsigned long vcpu_bitmap;
> > > > > > +    unsigned long vcpu_bitmap[BITS_TO_LONGS(KVM_MAX_VCPUS)];

The preferred pattern is:

	DECLARE_BITMAP(vcpu_bitmap, KVM_MAX_VCPUS);

> > > > > 
> > > > > Is there a way to avoid this KVM_MAX_VCPUS-sized variable on the
> > > > > stack?  This might hit us back when we increase KVM_MAX_VCPUS to
> > > > > a few thousand VCPUs (I was planning to submit a patch for that
> > > > > soon).
> > > > 
> > > > What's the short- or mid-term target?
> > > 
> > > Short term target is 2048 (which was already tested). Mid-term target
> > > (not tested yet) is 4096, maybe 8192.
> > > 
> > > > Note, we're allocating KVM_MAX_VCPUS bits (not bytes!) here, this means
> > > > that for e.g. 2048 vCPUs we need 256 bytes of the stack only. In case
> > > > the target much higher than that, we will need to either switch to
> > > > dynamic allocation or e.g. use pre-allocated per-CPU variables and make
> > > > this a preempt-disabled region. I, however, would like to understand if
> > > > the problem with allocating this from stack is real or not first.
> > > 
> > > Is 256 bytes too much here, or would that be OK?
> > > 
> > 
> > AFAIR, on x86_64 stack size (both reqular and irq) is 16k, eating 256

Don't forget i386!  :-)

> > bytes of it is probably OK. I'd start worrying when we go to 1024 (8k
> > vCPUs) and above (but this is subjective of course).

256 is fine, 1024 would indeed be problematic, e.g. CONFIG_FRAME_WARN defaults to
1024 on 32-bit kernels.  That's not a hard limit per se, but ideally KVM will stay
warn-free on all flavors of x86.

> On the topic of enlarging these bitmaps to cover all vCPUs.
> 
> I also share the worry of having the whole bitmap on kernel stack for very
> large number of vcpus.
> Maybe we need to abstract and use a bitmap for a sane number of vcpus, 
> and use otherwise a 'kmalloc'ed buffer?

That's a future problem.  More specifically, it's the problem of whoever wants to
push KVM_MAX_VCPUS > ~2048.  There are a lot of ways to solve the problem, e.g.
this I/O APIC code runs under a spinlock so a dedicated bitmap in struct kvm_ioapic
could be used to avoid a large stack allocation.

> Also in theory large bitmaps might affect performance a bit.

Maybe.  The only possible degredation for small VMs, i.e. VMs that don't need the
full bitmap, is if the compiler puts other variables below the bitmap and causes
sub-optimal cache line usage.  But I suspect/hope the compiler is smart enough to
use GPRs and/or organize the local variables on the stack so that doesn't happen.