Re: [PATCH] virtio-console: avoid DMA from vmalloc area

[Xianting Tian]

在 2021/7/28 下午3:25, Arnd Bergmann 写道:
> On Wed, Jul 28, 2021 at 4:59 AM Xianting Tian
> <xianting.tian@xxxxxxxxxxxxxxxxx> wrote:
> > Arnd, thanks for your quick reply,
> >
> > As we know put_chars() of virtio-console is registered to hvc framework.
> > I go throughed the code, actually there are totally three places that
> > put_chars() is called in hvc driver,  but only 1 has issue which is
> > fixed by commit c4baad5029.
> Ah, good. Knowing what the callers are definitely helps. ;-)
>
> > So I think the scenario that the buf is from "ioremap(), kmap_atomic() ,
> > fixmap, loadable module" doesn't exist for virtio-console.
> > If there is something wrong about above description, please correct me,
> > thanks.
> The description is good then.
>
> > Three places that put_chars() is called in hvc driver:
> > 1， it is on stack buf,  it is not ok for dma
> >       hvc_console_print():
> >           char c[N_OUTBUF] __ALIGNED__;
> >           cons_ops[index]->put_chars(vtermnos[index], c, i);
> >
> > 2， just one byte, no issue for dma
> >       static void hvc_poll_put_char(struct tty_driver *driver, int line,
> > char ch)
> >       {
> >           struct tty_struct *tty = driver->ttys[0];
> >           struct hvc_struct *hp = tty->driver_data;
> >           int n;
> >
> >           do {
> >               n = hp->ops->put_chars(hp->vtermno, &ch, 1);
> >           } while (n <= 0);
> >       }
> This is actually the same as the first, taking the address of a
> function argument forces it onto the stack.
>
> > 3,  hp->outbuf is allocated in hvc_alloc() via kzalloc(), no issue for dma
> >       static int hvc_push(struct hvc_struct *hp)
> >       {
> >           int n;
> >
> >           n = hp->ops->put_chars(hp->vtermno, hp->outbuf, hp->n_outbuf);
> >           …
> >       }
> ok.
>
> I have a new question then: are there any other hvc backends that do
> DMA, or is the virtio-console driver the only one? If there are any others,
> I think this should better be fixed in the hvc framework, by changing it
> to never pass stack data into the put_chars() function in the first place.
>
> It may be possible to just use the 'hp->n_outbuf' buffer in all three cases.

thanks,

I checked several hvc backends, like drivers/tty/hvc/hvc_riscv_sbi.c, 
drivers/tty/hvc/hvc_iucv.c, drivers/tty/hvc/hvc_rtas.c, they don't use dma.

I not finished all hvc backends check yet. But I think even if all hvc 
backends don't use dma currently, it is still possible that the hvc 
backend using dma will be added in the furture.

So I agree with you it should better be fixed in the hvc framework, 
solve the issue in the first place.

Looking forward to your reply.

>         Arnd