On Tue, Apr 14, 2020 at 7:28 AM Greg Kroah-Hartman gregkh@linuxfoundation.org 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@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?
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.
That sound ok?
thanks -john