linux-kernel - Re: [PATCH] vfio: hisi_acc_vfio_pci: fix integer overflow check in hisi_acc_vf_resume

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Date:   Wed, 6 Jul 2022 13:18:12 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Longfang Liu <liulongfang@...wei.com>,
        Yishai Hadas <yishaih@...dia.com>,
        Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] vfio: hisi_acc_vfio_pci: fix integer overflow check in
 hisi_acc_vf_resume_write()

On Wed, Jul 06, 2022 at 08:51:24AM +0300, Dan Carpenter wrote:
> On Tue, Jul 05, 2022 at 03:06:49PM -0300, Jason Gunthorpe wrote:
> > On Tue, Jul 05, 2022 at 12:05:28PM +0300, Dan Carpenter wrote:
> > > The casting on this makes the integer overflow check slightly wrong.
> > > "len" is an unsigned long. "*pos" and "requested_length" are signed
> > > long longs.  Imagine "len" is ULONG_MAX and "*pos" is 2.
> > > "ULONG_MAX + 2 = 1". 
> > 
> > I wonder if this can happen, len is a kernel controlled value bounded
> > by a memory allocation..
> > 
> 
> Oh.  Smatch uses a model which says that all read/writes come from
> vfs_write().  The problem with tracking kernel read/writes is that
> recursion is tricky.  So Smatch just deletes those from the DB.

Oh, maybe I got it wrong, len is the user input, so yes that does look
bad

> > This code was copy and pasted from drivers/vfio/pci/mlx5/main.c, so it
> > should be fixed too
> 
> Sure.
> 
> I created a static checker warning for this type of thing but it didn't
> catch the issue in drivers/vfio/pci/mlx5/main.c because Smatch says that
> the bug is impossible.  Which is true.

How come it is different?

Jason