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

[John Stultz]

On Tue, Apr 14, 2020 at 9:11 AM Ãrjan Eide <orjan.eide@xxxxxxx> wrote:
>
> On Tue, Apr 14, 2020 at 04:28:10PM +0200, Greg Kroah-Hartman wrote:
> > On Tue, Apr 14, 2020 at 04:18:47PM +0200, ïrjan 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: ïrjan 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?
>
> It looks like the new dma-heaps have the same issue as ion. The
> heap-helpers also do dma_sync_sg_for_device() unconditionally on
> end_cpu_access which may happen before dma_map_sg(), leading to use of
> the 0 dma_address in the sg list of a, yet unmapped, attachment.

Yea, the dma-buf heaps code came from the ION logic, so it likely has
the same faults.

> It could be fixed in dma-heaps just like this patch does for ion. Is
> patch a valid way to fix this problem? Or, should this rather be handled
> in dma-buf core by tracking the mapped state of attachments there?

In the short-term, I'd definitely prefer to have a fix to dmabuf heaps
rather then ION, but I also agree that long term it probably shouldn't
just be up to the dma-buf exporter (as there are other dmabuf
exporters that may have it wrong too), and that we need to address
some DMA API expectations/limitations to better handle multiple device
pipelines. (I actually gave a talk last fall on some of the issues I
see around it: https://youtu.be/UsEVoWD_o0c )

thanks
-john