[PATCH v2 2/2] media: videobuf2: order buffer initialization before bufs_bitmap publication

[Ricardo Ribalda <ribalda@chromium.org>]

Stateless mem2mem decoders call vb2_find_buffer() -> vb2_get_buffer()
from .device_run() without holding q->lock or q->mmap_lock, which can
run concurrently with VIDIOC_CREATE_BUFS on a streaming queue.

Both set_bit() and test_bit() are unordered, and mutex_lock(&q->mmap_lock)
is only an acquire barrier. On weakly-ordered architectures such as
arm64, a lockless vb2_get_buffer() caller can observe a newly set bit in
q->bufs_bitmap before the kzalloc() zeroing, buffer initialization, and
q->bufs[] pointer store performed by __vb2_queue_alloc() become visible.

Pair smp_mb__before_atomic() in vb2_queue_publish_buffers() with
test_bit_acquire() in vb2_get_buffer() so that any reader observing a
set bit in q->bufs_bitmap also sees a fully initialized vb2_buffer.

Fixes: a286b0837e13 ("media: core: Add bitmap manage bufs array entries")
Cc: stable@vger.kernel.org
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
---
 drivers/media/common/videobuf2/videobuf2-core.c | 8 ++++++++
 include/media/videobuf2-core.h                  | 6 +++++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
index 70ef38c3d16a..d0dd4b22b646 100644
--- a/drivers/media/common/videobuf2/videobuf2-core.c
+++ b/drivers/media/common/videobuf2/videobuf2-core.c
@@ -461,6 +461,14 @@ static void vb2_queue_publish_buffers(struct vb2_queue *q, unsigned int start,
 
 	lockdep_assert_held(&q->mmap_lock);
 
+	/*
+	 * Order the buffer initialization and the q->bufs[] stores done by
+	 * __vb2_queue_alloc() before the bits that publish them. Pairs with
+	 * test_bit_acquire() in vb2_get_buffer(), which is also called
+	 * without q->lock or q->mmap_lock held, e.g. by vb2_find_buffer()
+	 * from a mem2mem .device_run().
+	 */
+	smp_mb__before_atomic();
 	for (i = start; i < start + count; i++) {
 		if (WARN_ON(i >= q->max_num_buffers || !q->bufs[i] ||
 			    test_bit(i, q->bufs_bitmap)))
diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
index 2e93e6985d5f..20b961da21e0 100644
--- a/include/media/videobuf2-core.h
+++ b/include/media/videobuf2-core.h
@@ -1288,7 +1288,11 @@ static inline struct vb2_buffer *vb2_get_buffer(struct vb2_queue *q,
 	if (index >= q->max_num_buffers)
 		return NULL;
 
-	if (test_bit(index, q->bufs_bitmap))
+	/*
+	 * A set bit guarantees that the buffer is fully initialized, pairs
+	 * with smp_mb__before_atomic() in vb2_queue_publish_buffers().
+	 */
+	if (test_bit_acquire(index, q->bufs_bitmap))
 		return q->bufs[index];
 	return NULL;
 }

-- 
2.56.0.385.gac671c819a-goog