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

Dan Carpenter
2018-08-29T10:29:10Z

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@oracle.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;".

Is the unlikely() really required? I don't know what the fast paths are
in KVM, so I don't know.

regards,
dan carpenter