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

Dan Carpenter
2018-08-29T10:18:22Z

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.

regards,
dan carpenter