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

[Ãrjan Eide]

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: Ãrjan Eide <orjan.eide@xxxxxxx>
---
 drivers/staging/android/ion/ion.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

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
@@ -173,6 +173,7 @@ struct ion_dma_buf_attachment {
        struct device *dev;
        struct sg_table *table;
        struct list_head list;
+       bool mapped:1;
 };

 static int ion_dma_buf_attach(struct dma_buf *dmabuf,
@@ -195,6 +196,7 @@ static int ion_dma_buf_attach(struct dma_buf *dmabuf,
        a->table = table;
        a->dev = attachment->dev;
        INIT_LIST_HEAD(&a->list);
+       a->mapped = false;

        attachment->priv = a;

@@ -231,6 +233,8 @@ static struct sg_table *ion_map_dma_buf(struct dma_buf_attachment *attachment,
                        direction))
                return ERR_PTR(-ENOMEM);

+       a->mapped = true;
+
        return table;
 }

@@ -238,6 +242,10 @@ static void ion_unmap_dma_buf(struct dma_buf_attachment *attachment,
                              struct sg_table *table,
                              enum dma_data_direction direction)
 {
+       struct ion_dma_buf_attachment *a = attachment->priv;
+
+       a->mapped = false;
+
        dma_unmap_sg(attachment->dev, table->sgl, table->nents, direction);
 }

@@ -297,6 +305,8 @@ static int ion_dma_buf_begin_cpu_access(struct dma_buf *dmabuf,

        mutex_lock(&buffer->lock);
        list_for_each_entry(a, &buffer->attachments, list) {
+               if (!a->mapped)
+                       continue;
                dma_sync_sg_for_cpu(a->dev, a->table->sgl, a->table->nents,
                                    direction);
        }
@@ -320,6 +330,8 @@ static int ion_dma_buf_end_cpu_access(struct dma_buf *dmabuf,

        mutex_lock(&buffer->lock);
        list_for_each_entry(a, &buffer->attachments, list) {
+               if (!a->mapped)
+                       continue;
                dma_sync_sg_for_device(a->dev, a->table->sgl, a->table->nents,
                                       direction);
        }
--
2.17.1

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