Re: [PATCH v2] can: mcba_usb: Fix memory leak when cancelling urb

[Minh Bùi Quang]

On Mon, Jan 11, 2021 at 9:31 PM Bui Quang Minh <minhquangbui99@xxxxxxxxx> wrote:
>
> On Mon, Jan 11, 2021 at 01:00:31PM +0100, Oliver Neukum wrote:
> > Am Montag, den 11.01.2021, 10:49 +0000 schrieb Bui Quang Minh:
> > > In mcba_usb_read_bulk_callback(), when we don't resubmit or fails to
> > > resubmit the urb, we need to deallocate the transfer buffer that is
> > > allocated in mcba_usb_start().
> > >
> > > Reported-by: syzbot+57281c762a3922e14dfe@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > Signed-off-by: Bui Quang Minh <minhquangbui99@xxxxxxxxx>
> > > ---
> > > v1: add memory leak fix when not resubmitting urb
> > > v2: add memory leak fix when failing to resubmit urb
> > >
> > >  drivers/net/can/usb/mcba_usb.c | 11 ++++++++---
> > >  1 file changed, 8 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/net/can/usb/mcba_usb.c b/drivers/net/can/usb/mcba_usb.c
> > > index df54eb7d4b36..30236e640116 100644
> > > --- a/drivers/net/can/usb/mcba_usb.c
> > > +++ b/drivers/net/can/usb/mcba_usb.c
> > > @@ -584,6 +584,8 @@ static void mcba_usb_read_bulk_callback(struct urb *urb)
> > >     case -EPIPE:
> > >     case -EPROTO:
> > >     case -ESHUTDOWN:
> > > +           usb_free_coherent(urb->dev, urb->transfer_buffer_length,
> > > +                             urb->transfer_buffer, urb->transfer_dma);
> > >             return;
> > >
> >
> > Can you call usb_free_coherent() in what can be hard IRQ context?
>
> You are right, I digged in the code and saw some comments that on some
> architectures, usb_free_coherent() cannot be called in hard IRQ context.
> I see the usb_free_coherent() is called in write_bulk_callback too. I will
> send a patch that uses usb_anchor to keep track of these urbs and cleanup
> the transfer buffer later in disconnect().

Hi, I have sent a version 3 patch. However, I found out that usb_free_coherent()
is ok in this situation. In usb_free_coherent(), if the buffer is allocated via
dma_alloc_coherent() in usb_alloc_coherent(), dma_free_coherent() is called.
In dma_free_coherent(), ops->free() may be called in some cases which may
contains calls to vunmap() that is not permitted in interrupt context. However,
in usb_alloc_coherent(), buffer can be allocated from dma pool if the
size is less
than 2048 and the buffer size in mcba_usb is obviously less than 2048.
As a result,
usb_free_coherent() will at most fall in the path that calls
dma_pool_free(), which is
safe. Am I right?

Thanks,
Quang Minh.