Re: [RFC PATCH] cw1200: use kmalloc() allocation instead of stack

[Ulf Hansson]

On Wed, 30 Jun 2021 at 13:30, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Wed, Jun 30, 2021 at 11:56 AM Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote:
> >
> > On Tue, 22 Jun 2021 at 22:33, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > >
> > > On Tue, Jun 22, 2021 at 10:24 PM Jernej Skrabec
> > > <jernej.skrabec@xxxxxxxxx> wrote:
> > > >
> > > > It turns out that if CONFIG_VMAP_STACK is enabled and src or dst is
> > > > memory allocated on stack, SDIO operations fail due to invalid memory
> > > > address conversion:
> > >
> > > Thank you for sending this!
> > >
> > > It's worth pointing out that even without CONFIG_VMAP_STACK, using
> > > dma_map_sg() on a stack variable is broken, though it will appear to
> > > work most of the time but rarely cause a stack data corruption when
> > > the cache management goes wrong.
> > >
> > > This clearly needs to be fixed somewhere, if not with your patch, then
> > > a similar one.
> > >
> > > > diff --git a/drivers/net/wireless/st/cw1200/hwio.c b/drivers/net/wireless/st/cw1200/hwio.c
> > > > index 3ba462de8e91..5521cb7f2233 100644
> > > > --- a/drivers/net/wireless/st/cw1200/hwio.c
> > > > +++ b/drivers/net/wireless/st/cw1200/hwio.c
> > > > @@ -66,33 +66,65 @@ static int __cw1200_reg_write(struct cw1200_common *priv, u16 addr,
> > > >  static inline int __cw1200_reg_read_32(struct cw1200_common *priv,
> > > >                                         u16 addr, u32 *val)
> > > >  {
> > > > -       __le32 tmp;
> > > > -       int i = __cw1200_reg_read(priv, addr, &tmp, sizeof(tmp), 0);
> > > > -       *val = le32_to_cpu(tmp);
> > > > +       __le32 *tmp;
> > > > +       int i;
> > > > +
> > > > +       tmp = kmalloc(sizeof(*tmp), GFP_KERNEL);
> > > > +       if (!tmp)
> > > > +               return -ENOMEM;
> > > > +
> > > > +       i = __cw1200_reg_read(priv, addr, tmp, sizeof(*tmp), 0);
> > > > +       *val = le32_to_cpu(*tmp);
> > > > +       kfree(tmp);
> > > >         return i;
> > > >  }
> > >
> > > There is a possible problem here when the function gets called from
> > > atomic context, so it might need to use GFP_ATOMIC instead of
> > > GFP_KERNEL. If it's never called from atomic context, then this patch
> > > looks correct to me.
> >
> > I would be surprised if this is called from atomic context (when IRQs
> > are turned off), because in most cases, to complete the read/write
> > request the mmc controller driver relies on IRQs being delivered.
>
> I thought I had seen a spinlock in the forked driver, but I don't see
> it now, so I probably misremembered that bit.
>
> > > The alternative would be to add a bounce buffer check based on
> > > is_vmalloc_or_module_addr() in sdio_io_rw_ext_helper(), which would
> > > add a small bit of complexity there but solve the problem for
> > > all drivers at once. In this case, it would probably have to use
> > > GFP_ATOMIC regardless of whether __cw1200_reg_read_32()
> > > is allowed to sleep, since other callers might not.
> >
> > I like the idea, but...
> >
> > I don't think we should see this as an alternative, but rather as a
> > complement which would have performance issues. A warning should be
> > printed, if the buffer isn't properly allocated.
>
> Fair enough. I found the function call I was looking for: object_is_on_stack(),
> the patch below should print a warning once when a driver passes
> a bad buffer, but I did not test that.
>
> There are some possible variations on that: an on-stack buffer by
> itself can work as long as the DMA is cache-coherent and stacks
> are not vmapped. For the is_vmalloc_or_module_addr() case,
> we may decide to just return an error, rather than running into
> a kernel oops.
>
> > Additionally, I don't think GFT_ATOMIC should be needed.
>
> Ok, I now see the mmc_wait_for_req() in mmc_io_rw_extended()
> that probably means it can not be called in atomic context at all,
> and that GFP_KERNEL is safe, and that any driver calling it with
> a spinlock held is already broken.
>
>        Arnd
>
> 8<---
> diff --git a/drivers/mmc/core/sdio_ops.c b/drivers/mmc/core/sdio_ops.c
> index 4c229dd2b6e5..845f9ca3b200 100644
> --- a/drivers/mmc/core/sdio_ops.c
> +++ b/drivers/mmc/core/sdio_ops.c
> @@ -124,6 +124,7 @@ int mmc_io_rw_extended(struct mmc_card *card, int
> write, unsigned fn,
>         int err;
>
>         WARN_ON(blksz == 0);
> +       WARN_ON_ONCE(is_vmalloc_or_module_addr(buf) || object_is_on_stack(buf));

Looks reasonable to me, at least we should start giving a warning.
Would you like to send a formal patch that we can test?

Kind regards
Uffe