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

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Message-ID: <20180829102910.rkyato47chayt22s@mwanda>
Date:   Wed, 29 Aug 2018 13:29:10 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Wanpeng Li <kernellwp@...il.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, 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;".

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

regards,
dan carpenter