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

在 2021/7/28 下午3:25, Arnd Bergmann 写道:
> On Wed, Jul 28, 2021 at 4:59 AM Xianting Tian
> <> 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.


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

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