Re: [Qemu-devel] [RFC PATCH 2/6] virtio/console: Add a failback for unstealable pipe buffer

[Masami Hiramatsu]

(2012/08/09 18:03), Amit Shah wrote:
> On (Tue) 24 Jul 2012 [11:37:18], Yoshihiro YUNOMAE wrote:
>> From: Masami Hiramatsu <address@hidden>
>>
>> Add a failback memcpy path for unstealable pipe buffer.
>> If buf->ops->steal() fails, virtio-serial tries to
>> copy the page contents to an allocated page, instead
>> of just failing splice().
>>
>> Signed-off-by: Masami Hiramatsu <address@hidden>
>> Cc: Amit Shah <address@hidden>
>> Cc: Arnd Bergmann <address@hidden>
>> Cc: Greg Kroah-Hartman <address@hidden>
>> ---
>>
>>  drivers/char/virtio_console.c |   28 +++++++++++++++++++++++++---
>>  1 files changed, 25 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
>> index fe31b2f..911cb3e 100644
>> --- a/drivers/char/virtio_console.c
>> +++ b/drivers/char/virtio_console.c
>> @@ -794,7 +794,7 @@ static int pipe_to_sg(struct pipe_inode_info *pipe, 
>> struct pipe_buffer *buf,
>>                      struct splice_desc *sd)
>>  {
>>      struct sg_list *sgl = sd->u.data;
>> -    unsigned int len = 0;
>> +    unsigned int offset, len;
>>  
>>      if (sgl->n == MAX_SPLICE_PAGES)
>>              return 0;
>> @@ -807,9 +807,31 @@ static int pipe_to_sg(struct pipe_inode_info *pipe, 
>> struct pipe_buffer *buf,
>>  
>>              len = min(buf->len, sd->len);
>>              sg_set_page(&(sgl->sg[sgl->n]), buf->page, len, buf->offset);
>> -            sgl->n++;
>> -            sgl->len += len;
>> +    } else {
>> +            /* Failback to copying a page */
>> +            struct page *page = alloc_page(GFP_KERNEL);
> 
> I prefer zeroing out the page.  If there's not enough data to be
> filled in the page, the remaining data can be leaked to the host.

Yeah, it is really easy to fix that.
But out of curiosity, would that be really a problem?
I guess that host can access any guest page if need. If that
is right, is that really insecure to leak randomly allocated
unused page to the host?

Thank you,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: address@hidden