linux-kernel - Re: [PATCH] KVM: LAPIC: Fix pv ipis out-of-bounds access

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Date:   Thu, 30 Aug 2018 10:15:39 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Radim Krcmar <rkrcmar@...hat.com>
Cc:     Dan Carpenter <dan.carpenter@...cle.com>,
        Liran Alon <liran.alon@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] KVM: LAPIC: Fix pv ipis out-of-bounds access

On Wed, 29 Aug 2018 at 23:42, Radim Krcmar <rkrcmar@...hat.com> wrote:
>
> 2018-08-29 13:29+0300, Dan Carpenter:
> > On Wed, Aug 29, 2018 at 06:23:08PM +0800, Wanpeng Li wrote:
> > > On Wed, 29 Aug 2018 at 18:18, Dan Carpenter <dan.carpenter@...cle.com> wrote:
> > > >
> > > > On Wed, Aug 29, 2018 at 01:12:05PM +0300, Dan Carpenter wrote:
> > > > > On Wed, Aug 29, 2018 at 12:05:06PM +0300, Liran Alon wrote:
> > > > > > > arch/x86/kvm/lapic.c | 17 +++++++++++++----
> > > > > > > 1 file changed, 13 insertions(+), 4 deletions(-)
> > > > > > >
> > > > > > > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > > > > > > index 0cefba2..86e933c 100644
> > > > > > > --- a/arch/x86/kvm/lapic.c
> > > > > > > +++ b/arch/x86/kvm/lapic.c
> > > > > > > @@ -571,18 +571,27 @@ int kvm_pv_send_ipi(struct kvm *kvm, unsigned long ipi_bitmap_low,
> > > > > > >   rcu_read_lock();
> > > > > > >   map = rcu_dereference(kvm->arch.apic_map);
> > > > > > >
> > > > > > > + if (unlikely((s32)(map->max_apic_id - __fls(ipi_bitmap_low)) < min))
> > > > > > > +         goto out;
> > > > > >
> > > > > > I personally think “if ((min + __fls(ipi_bitmap_low)) > map->max_apic_id)” is more readable.
> > > > > > But that’s just a matter of taste :)
> > > > >
> > > > > That's an integer overflow.
> > > > >
> > > > > But I do prefer to put the variable on the left.  The truth is that some
> > > > > Smatch checks just ignore code which is backwards written because
> > > > > otherwise you have to write duplicate code and the most code is written
> > > > > with the variable on the left.
> > > > >
> > > > >       if (min > (s32)(map->max_apic_id - __fls(ipi_bitmap_low))
> > > >
> > > > Wait, the (s32) cast doesn't make sense.  We want negative min values to
> > > > be treated as invalid.
> > >
> > > In v2, how about:
> > >
> > > if (unlikely(min > map->max_apic_id || (min + __fls(ipi_bitmap_low)) >
> > > map->max_apic_id))
> > >     goto out;
> >
> > That works, too.  It still has the off by one and we should set
> > "count = -KVM_EINVAL;".
>
> I'd prefer to ignore destinations that are not present and deliver the
> rest, possibly nothing, instead of returning an error.
> (It's closer to how the real hardware behaves and we already return the
>  number of notified VCPUs, so the caller can tell whether something went
>  wrong.)
>
> Either in the form that I have posted earlier, or as:
>
>         if (min > map->max_apic_id)
>                 goto out;
>
>         for_each_set_bit(i, &ipi_bitmap_low, MIN(BITS_PER_LONG, map->max_apic_id - min + 1))

Do it in v2.

Regards,
Wanpeng Li