Re: [PATCH] staging: android: ion: Skip sync if not mapped

[Greg Kroah-Hartman]

On Tue, Apr 14, 2020 at 09:41:31PM -0700, John Stultz wrote:
> On Tue, Apr 14, 2020 at 7:28 AM Greg Kroah-Hartman
> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Tue, Apr 14, 2020 at 04:18:47PM +0200, Ørjan Eide wrote:
> > > Only sync the sg-list of an Ion dma-buf attachment when the attachment
> > > is actually mapped on the device.
> > >
> > > dma-bufs may be synced at any time. It can be reached from user space
> > > via DMA_BUF_IOCTL_SYNC, so there are no guarantees from callers on when
> > > syncs may be attempted, and dma_buf_end_cpu_access() and
> > > dma_buf_begin_cpu_access() may not be paired.
> > >
> > > Since the sg_list's dma_address isn't set up until the buffer is used
> > > on the device, and dma_map_sg() is called on it, the dma_address will be
> > > NULL if sync is attempted on the dma-buf before it's mapped on a device.
> > >
> > > Before v5.0 (commit 55897af63091 ("dma-direct: merge swiotlb_dma_ops
> > > into the dma_direct code")) this was a problem as the dma-api (at least
> > > the swiotlb_dma_ops on arm64) would use the potentially invalid
> > > dma_address. How that failed depended on how the device handled physical
> > > address 0. If 0 was a valid address to physical ram, that page would get
> > > flushed a lot, while the actual pages in the buffer would not get synced
> > > correctly. While if 0 is an invalid physical address it may cause a
> > > fault and trigger a crash.
> > >
> > > In v5.0 this was incidentally fixed by commit 55897af63091 ("dma-direct:
> > > merge swiotlb_dma_ops into the dma_direct code"), as this moved the
> > > dma-api to use the page pointer in the sg_list, and (for Ion buffers at
> > > least) this will always be valid if the sg_list exists at all.
> > >
> > > But, this issue is re-introduced in v5.3 with
> > > commit 449fa54d6815 ("dma-direct: correct the physical addr in
> > > dma_direct_sync_sg_for_cpu/device") moves the dma-api back to the old
> > > behaviour and picks the dma_address that may be invalid.
> > >
> > > dma-buf core doesn't ensure that the buffer is mapped on the device, and
> > > thus have a valid sg_list, before calling the exporter's
> > > begin_cpu_access.
> > >
> > > Signed-off-by: Ørjan Eide <orjan.eide@xxxxxxx>
> > > ---
> > >  drivers/staging/android/ion/ion.c | 12 ++++++++++++
> > >  1 file changed, 12 insertions(+)
> > >
> > > Resubmit without disclaimer, sorry about that.
> > >
> > > This seems to be part of a bigger issue where dma-buf exporters assume
> > > that their dma-buf begin_cpu_access and end_cpu_access callbacks have a
> > > certain guaranteed behavior, which isn't ensured by dma-buf core.
> > >
> > > This patch fixes this in ion only, but it also needs to be fixed for
> > > other exporters, either handled like this in each exporter, or in
> > > dma-buf core before calling into the exporters.
> > >
> > > diff --git a/drivers/staging/android/ion/ion.c b/drivers/staging/android/ion/ion.c
> > > index 38b51eace4f9..7b752ba0cb6d 100644
> > > --- a/drivers/staging/android/ion/ion.c
> > > +++ b/drivers/staging/android/ion/ion.c
> >
> > Now that we have the dma-buff stuff in the tree, do we even need the
> > ion code in the kernel anymore?  Can't we delete it now?
> >
> 
> I agree that we shouldn't be taking further (non-security/cleanup)
> patches to the ION code.
> 
> I'd like to give developers a little bit of a transition period (I was
> thinking a year, but really just one LTS release that has both would
> do) where they can move their ION heaps over to dmabuf heaps and test
> both against the same tree.
> 
> But I do think we can mark it as deprecated and let folks know that
> around the end of the year it will be deleted.

No one ever notices "depreciated" things, they only notice if the code
is no longer there :)

So I'm all for just deleting it and seeing who even notices...

thanks,

greg k-h