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Subject: Re: [PATCH] KVM: LAPIC: Fix pv ipis out-of-bounds access
From:   Liran Alon <liran.alon@oracle.com>
In-Reply-To: <20180829102910.rkyato47chayt22s@mwanda>
Date:   Wed, 29 Aug 2018 13:43:21 +0300
Cc:     Wanpeng Li <kernellwp@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim Krcmar <rkrcmar@redhat.com>
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 <20180829102910.rkyato47chayt22s@mwanda>
To:     Dan Carpenter <dan.carpenter@oracle.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On 29 Aug 2018, at 13:29, Dan Carpenter <dan.carpenter@oracle.com> =
wrote:
>=20
> 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:
>>>=20
>>> 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(-)
>>>>>>=20
>>>>>> 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 =3D rcu_dereference(kvm->arch.apic_map);
>>>>>>=20
>>>>>> + if (unlikely((s32)(map->max_apic_id - __fls(ipi_bitmap_low)) < =
min))
>>>>>> +         goto out;
>>>>>=20
>>>>> I personally think =E2=80=9Cif ((min + __fls(ipi_bitmap_low)) > =
map->max_apic_id)=E2=80=9D is more readable.
>>>>> But that=E2=80=99s just a matter of taste :)
>>>>=20
>>>> That's an integer overflow.
>>>>=20
>>>> 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.
>>>>=20
>>>>      if (min > (s32)(map->max_apic_id - __fls(ipi_bitmap_low))
>>>=20
>>> Wait, the (s32) cast doesn't make sense.  We want negative min =
values to
>>> be treated as invalid.
>>=20
>> In v2, how about:
>>=20
>> if (unlikely(min > map->max_apic_id || (min + __fls(ipi_bitmap_low)) =
>
>> map->max_apic_id))
>>    goto out;
>=20
> That works, too.  It still has the off by one and we should set
> "count =3D -KVM_EINVAL;".
>=20
> Is the unlikely() really required?  I don't know what the fast paths =
are
> in KVM, so I don't know.
>=20
> regards,
> dan carpenter

Why is =E2=80=9Cmin=E2=80=9D defined as =E2=80=9Cint=E2=80=9D instead of =
=E2=80=9Cunsigned int=E2=80=9D?
It represents the lowest APIC ID in bitmap so it can=E2=80=99t be =
negative=E2=80=A6

"if (unlikely(min > map->max_apic_id || (min + __fls(ipi_bitmap_low)) > =
map->max_apic_id))=E2=80=9D
should indeed be ok.

-Liran