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From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Sean Christopherson <seanjc@google.com>,
	Maxim Levitsky <mlevitsk@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>,
	kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Nitesh Narayan Lal <nitesh@redhat.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 4/4] KVM: x86: Fix stack-out-of-bounds memory access from ioapic_write_indirect()
Date: Wed, 25 Aug 2021 10:26:44 +0200	[thread overview]
Message-ID: <87eeaij5ff.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <YSUvESHcms6B3+DA@google.com>

Sean Christopherson <seanjc@google.com> writes:

> 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);
>

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


  reply	other threads:[~2021-08-25  8:26 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-23 14:30 [PATCH v2 0/4] KVM: Various fixes and improvements around kicking vCPUs Vitaly Kuznetsov
2021-08-23 14:30 ` [PATCH v2 1/4] KVM: Clean up benign vcpu->cpu data races when " Vitaly Kuznetsov
2021-08-23 14:30 ` [PATCH v2 2/4] KVM: Guard cpusmask NULL check with CONFIG_CPUMASK_OFFSTACK Vitaly Kuznetsov
2021-08-23 14:30 ` [PATCH v2 3/4] KVM: Optimize kvm_make_vcpus_request_mask() a bit Vitaly Kuznetsov
2021-08-23 14:30 ` [PATCH v2 4/4] KVM: x86: Fix stack-out-of-bounds memory access from ioapic_write_indirect() Vitaly Kuznetsov
2021-08-23 18:58   ` Eduardo Habkost
2021-08-24  7:13     ` Vitaly Kuznetsov
2021-08-24 14:23       ` Eduardo Habkost
2021-08-24 14:42         ` Vitaly Kuznetsov
2021-08-24 16:07           ` Maxim Levitsky
2021-08-24 17:40             ` Sean Christopherson
2021-08-25  8:26               ` Vitaly Kuznetsov [this message]
2021-08-25  8:21             ` Vitaly Kuznetsov
2021-08-25  9:11               ` Maxim Levitsky
2021-08-25  9:43                 ` Vitaly Kuznetsov
2021-08-25 10:41                   ` Maxim Levitsky
2021-08-25 13:19                   ` Eduardo Habkost
2021-08-26 12:40                     ` Vitaly Kuznetsov
2021-08-26 14:52                       ` Eduardo Habkost
2021-08-26 18:01                         ` Sean Christopherson
2021-08-26 18:13                           ` Eduardo Habkost
2021-08-26 19:27             ` Eduardo Habkost
2021-08-30 19:47             ` Nitesh Lal

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