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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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@arm.com>
> ---
>  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?

thanks,

greg k-h