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

[Vitaly Kuznetsov]

Sean Christopherson <seanjc@xxxxxxxxxx> writes:

> On Tue, Aug 24, 2021, Maxim Levitsky wrote:
>> On Tue, 2021-08-24 at 16:42 +0200, Vitaly Kuznetsov wrote:
>> > Eduardo Habkost <ehabkost@xxxxxxxxxx> writes:
>> > 
>> > > On Tue, Aug 24, 2021 at 3:13 AM Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
>> > > > Eduardo Habkost <ehabkost@xxxxxxxxxx> 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);
>

Yes, thanks!

>> > > > > 
>> > > > > 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!  :-)
>

I'm not forgetting, I'm deliberately ignoring its existence :-)

Whoever tries to raise KVM_MAX_VCPUS from '288' may limit the change to
x86_64, I seriosly doubt 32bit users want to run guests with thouthands
of CPUs.

>> > 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.

Thanks for the CONFIG_FRAME_WARN pointer, I said '1024' out of top of my
head but it seems the number wasn't random after all)

>
>> 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.

+1

>
>> 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.
>

-- 
Vitaly