Re: [PATCH] misc: tifm: Remove VLA

[Laura Abbott]

On 04/10/2018 12:57 AM, Arnd Bergmann wrote:
> On Mon, Apr 9, 2018 at 11:07 PM, Laura Abbott <labbott@xxxxxxxxxx> wrote:
> > There's an ongoing effort to remove VLAs[1] from the kernel to eventually
> > turn on -Wvla. The single VLA can either take a value of 2 or 4 so switch
> > to the upper bound.
> >
> > [1] https://lkml.org/lkml/2018/3/7/621
> >
> > Signed-off-by: Laura Abbott <labbott@xxxxxxxxxx>
> > ---
> >   drivers/misc/tifm_7xx1.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/misc/tifm_7xx1.c b/drivers/misc/tifm_7xx1.c
> > index e5f108713dd8..690eaaea5ce4 100644
> > --- a/drivers/misc/tifm_7xx1.c
> > +++ b/drivers/misc/tifm_7xx1.c
> > @@ -239,7 +239,8 @@ static int tifm_7xx1_resume(struct pci_dev *dev)
> >          unsigned long timeout;
> >          unsigned int good_sockets = 0, bad_sockets = 0;
> >          unsigned long flags;
> > -       unsigned char new_ids[fm->num_sockets];
> > +       /* Maximum number of entries is 4 */
> > +       unsigned char new_ids[4];
> >          DECLARE_COMPLETION_ONSTACK(finish_resume);
>
> I like the idea of removing all the VLAs, but this one appears to make
> the code less robust rather than more: In case of an unexpected
> fm->num_sockets value, we now not only overflow the kernel stack
> area but also the local variable into the neighboring stack slots.
>
> Maybe add an extra overflow check?

Sure.

>         Arnd
>
> diff --git a/drivers/misc/tifm_7xx1.c b/drivers/misc/tifm_7xx1.c
> index e5f108713dd8..c702cd92d396 100644
> --- a/drivers/misc/tifm_7xx1.c
> +++ b/drivers/misc/tifm_7xx1.c
> @@ -239,9 +239,12 @@ static int tifm_7xx1_resume(struct pci_dev *dev)
>          unsigned long timeout;
>          unsigned int good_sockets = 0, bad_sockets = 0;
>          unsigned long flags;
> -       unsigned char new_ids[fm->num_sockets];
> +      /* Maximum number of entries is 4 */
> +       unsigned char new_ids[4];
>          DECLARE_COMPLETION_ONSTACK(finish_resume);
>
> +       if (WARN_ON(fm->num_sockets > ARRAY_SIZE(new_ids)))
> +               return -ENXIO;
> +
>          pci_set_power_state(dev, PCI_D0);
>          pci_restore_state(dev);
>          rc = pci_enable_device(dev);