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    SubjectRe: [PATCH 1/4] staging: android: ion: Support cpu access during dma_buf_detach
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    On 1/18/19 12:37 PM, Liam Mark wrote:
    > Often userspace doesn't know when the kernel will be calling dma_buf_detach
    > on the buffer.
    > If userpace starts its CPU access at the same time as the sg list is being
    > freed it could end up accessing the sg list after it has been freed.
    >
    > Thread A Thread B
    > - DMA_BUF_IOCTL_SYNC IOCT
    > - ion_dma_buf_begin_cpu_access
    > - list_for_each_entry
    > - ion_dma_buf_detatch
    > - free_duped_table
    > - dma_sync_sg_for_cpu
    >

    The window for this seems really small, but it does seem technically
    possible, good find. for what it's worth:

    Acked-by: Andrew F. Davis <afd@ti.com>

    > Fix this by getting the ion_buffer lock before freeing the sg table memory.
    >
    > Fixes: 2a55e7b5e544 ("staging: android: ion: Call dma_map_sg for syncing and mapping")
    > Signed-off-by: Liam Mark <lmark@codeaurora.org>
    > ---
    > drivers/staging/android/ion/ion.c | 2 +-
    > 1 file changed, 1 insertion(+), 1 deletion(-)
    >
    > diff --git a/drivers/staging/android/ion/ion.c b/drivers/staging/android/ion/ion.c
    > index a0802de8c3a1..6f5afab7c1a1 100644
    > --- a/drivers/staging/android/ion/ion.c
    > +++ b/drivers/staging/android/ion/ion.c
    > @@ -248,10 +248,10 @@ static void ion_dma_buf_detatch(struct dma_buf *dmabuf,
    > struct ion_dma_buf_attachment *a = attachment->priv;
    > struct ion_buffer *buffer = dmabuf->priv;
    >
    > - free_duped_table(a->table);
    > mutex_lock(&buffer->lock);
    > list_del(&a->list);
    > mutex_unlock(&buffer->lock);
    > + free_duped_table(a->table);
    >
    > kfree(a);
    > }
    >

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