linux-hardening - Re: [PATCH 32/82] vringh: Refactor intentional wrap-around calculation

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Date: Fri, 26 Jan 2024 11:42:46 -0800
From: Kees Cook <keescook@...omium.org>
To: Eugenio Perez Martin <eperezma@...hat.com>
Cc: linux-hardening@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org,
	virtualization@...ts.linux.dev, netdev@...r.kernel.org,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 32/82] vringh: Refactor intentional wrap-around
 calculation

On Fri, Jan 26, 2024 at 08:31:04PM +0100, Eugenio Perez Martin wrote:
> On Tue, Jan 23, 2024 at 2:42 AM Kees Cook <keescook@...omium.org> wrote:
> >
> > In an effort to separate intentional arithmetic wrap-around from
> > unexpected wrap-around, we need to refactor places that depend on this
> > kind of math. One of the most common code patterns of this is:
> >
> >         VAR + value < VAR
> >
> > Notably, this is considered "undefined behavior" for signed and pointer
> > types, which the kernel works around by using the -fno-strict-overflow
> > option in the build[1] (which used to just be -fwrapv). Regardless, we
> > want to get the kernel source to the position where we can meaningfully
> > instrument arithmetic wrap-around conditions and catch them when they
> > are unexpected, regardless of whether they are signed[2], unsigned[3],
> > or pointer[4] types.
> >
> > Refactor open-coded unsigned wrap-around addition test to use
> > check_add_overflow(), retaining the result for later usage (which removes
> > the redundant open-coded addition). This paves the way to enabling the
> > unsigned wrap-around sanitizer[2] in the future.
> >
> > Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
> > Link: https://github.com/KSPP/linux/issues/26 [2]
> > Link: https://github.com/KSPP/linux/issues/27 [3]
> > Link: https://github.com/KSPP/linux/issues/344 [4]
> > Cc: "Michael S. Tsirkin" <mst@...hat.com>
> > Cc: Jason Wang <jasowang@...hat.com>
> > Cc: kvm@...r.kernel.org
> > Cc: virtualization@...ts.linux.dev
> > Cc: netdev@...r.kernel.org
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> >  drivers/vhost/vringh.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> > index 7b8fd977f71c..07442f0a52bd 100644
> > --- a/drivers/vhost/vringh.c
> > +++ b/drivers/vhost/vringh.c
> > @@ -145,6 +145,8 @@ static inline bool range_check(struct vringh *vrh, u64 addr, size_t *len,
> >                                bool (*getrange)(struct vringh *,
> >                                                 u64, struct vringh_range *))
> >  {
> > +       u64 sum;
> 
> I understand this is part of a bulk change so little time to think
> about names :). But what about "end" or similar?
> 
> Either way,
> Acked-by: Eugenio Pérez <eperezma@...hat.com>

Thanks! Yeah, you are not alone in suggesting "end" in a several of
these patches. :)

-Kees

-- 
Kees Cook