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

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Date:   Wed, 29 Aug 2018 13:18:22 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Liran Alon <liran.alon@...cle.com>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH] KVM: LAPIC: Fix pv ipis out-of-bounds access

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