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

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Message-ID: <CANRm+Cy9n=2Ssy_cEWRtnoQJczjdqB+m-dA6zesz75r0h532iA@mail.gmail.com>
Date:   Wed, 29 Aug 2018 18:23:08 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Liran Alon <liran.alon@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>
Subject: Re: [PATCH] KVM: LAPIC: Fix pv ipis out-of-bounds access

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;

0xFFFFFFFF is converted to int min = -1;

Regards,
Wanpeng Li