[PATCH v2 0/2] media: videobuf2: publish buffers in bufs_bitmap under mmap_lock

[PATCH v2 0/2] media: videobuf2: publish buffers in bufs_bitmap under mmap_lock

[Ricardo Ribalda]

vb2_mmap() only takes q->mmap_lock, not the main queue serialization
lock (q->lock). Commit a286b0837e13 ("media: core: Add bitmap manage
bufs array entries") started setting bits in q->bufs_bitmap inside
__vb2_queue_alloc() before buffer memory is allocated and without
holding q->mmap_lock, allowing a concurrent mmap() to access a
half-initialized buffer and hit a use-after-free if allocation or
.buf_init() fails, and prematurely counting tentative buffers during
partial-allocation .queue_setup() re-checks.

Patch 1 fixes this by dropping set_bit() from vb2_queue_add_buffer() and
publishing the range in q->bufs_bitmap via vb2_queue_publish_buffers()
under q->mmap_lock once allocation and .queue_setup() succeed.

Patch 2 adds smp_mb__before_atomic() / test_bit_acquire() ordering on
q->bufs_bitmap for lockless vb2_get_buffer() callers (such as
vb2_find_buffer() called from stateless mem2mem .device_run() while
VIDIOC_CREATE_BUFS runs on weakly-ordered architectures).

This series was generated by an LLM, but supervised and reviewed by a
human (not the most handsome human, but a human).

---
Changes in v2:
- Keep vb2_queue_add_buffer() without set_bit() so __vb2_queue_alloc()
  stays unchanged, and add vb2_queue_publish_buffers() using atomic
  set_bit() (rather than non-atomic bitmap_set(), since vb2_get_buffer()
  has lockless readers).
- Split the smp_mb__before_atomic() / test_bit_acquire() memory barrier
  pair into a separate patch with an explanation of the arm64
  VIDIOC_CREATE_BUFS vs vb2_find_buffer() race.
- Drop lockdep_assert_held() from vb2_queue_remove_buffer().
- Simplify the !q->bufs guard in __vb2_queue_free() to 'if (!q->bufs)
  return;' (still needed as a NULL check now that __vb2_queue_free()
  indexes q->bufs[] directly instead of going through vb2_get_buffer(),
  since vb2_core_queue_release() and REQBUFS(0) call it when
  q->bufs == NULL).
- Clarify @bufs and @bufs_bitmap documentation in struct vb2_queue.
- Link to v1: https://lore.kernel.org/r/20261102-vb2-mmap_lock-v1-1-fe550fe03ed8@chromium.org

---
Ricardo Ribalda (2):
      media: videobuf2: publish buffers in bufs_bitmap under mmap_lock
      media: videobuf2: order buffer initialization before bufs_bitmap publication

 drivers/media/common/videobuf2/videobuf2-core.c | 74 +++++++++++++++++++++----
 include/media/videobuf2-core.h                  | 14 +++--
 2 files changed, 74 insertions(+), 14 deletions(-)
---
base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
change-id: 20261102-vb2-mmap_lock-959e0cc4edb3

Best regards,
-- 
Ricardo Ribalda <ribalda@chromium.org>

[PATCH v2 1/2] media: videobuf2: publish buffers in bufs_bitmap under mmap_lock

[Ricardo Ribalda]

vb2_mmap() only takes q->mmap_lock, not the main queue lock (q->lock).
To keep vb2_mmap() safe, newly allocated buffers must not become visible
in vb2_get_buffer() until they are fully initialized and q->mmap_lock is
held.

Commit a286b0837e13 ("media: core: Add bitmap manage bufs array entries")
started setting q->bufs_bitmap inside __vb2_queue_alloc() before buffer
memory is allocated and without holding q->mmap_lock. As a result:

- A concurrent mmap() can find a half-initialized buffer, and if buffer
  allocation or .buf_init() fails, __vb2_queue_alloc() frees that buffer
  without holding q->mmap_lock, causing a use-after-free in vb2_mmap().
- If fewer buffers are allocated than requested, they are already
  counted as active in q->bufs_bitmap before .queue_setup() checks
  whether the smaller count is acceptable.

Fix this by keeping new buffers only in q->bufs[] during allocation and
setting their bits in q->bufs_bitmap under q->mmap_lock once allocation
and .queue_setup() succeed, and let __vb2_queue_free() read q->bufs[]
directly when cleaning up unexposed buffers.

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 | 66 +++++++++++++++++++++----
 include/media/videobuf2-core.h                  |  8 +--
 2 files changed, 61 insertions(+), 13 deletions(-)

diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
index b0a6084f1757..70ef38c3d16a 100644
--- a/drivers/media/common/videobuf2/videobuf2-core.c
+++ b/drivers/media/common/videobuf2/videobuf2-core.c
@@ -431,6 +431,9 @@ static void init_buffer_cache_hints(struct vb2_queue *q, struct vb2_buffer *vb)
  * @q:	pointer to &struct vb2_queue with videobuf2 queue.
  * @vb:	pointer to &struct vb2_buffer to be added to the queue.
  * @index: index where add vb2_buffer in the queue
+ *
+ * The buffer is stored in @q->bufs[] but remains invisible to vb2_get_buffer()
+ * until published with vb2_queue_publish_buffers().
  */
 static void vb2_queue_add_buffer(struct vb2_queue *q, struct vb2_buffer *vb, unsigned int index)
 {
@@ -439,7 +442,31 @@ static void vb2_queue_add_buffer(struct vb2_queue *q, struct vb2_buffer *vb, uns
 	q->bufs[index] = vb;
 	vb->index = index;
 	vb->vb2_queue = q;
-	set_bit(index, q->bufs_bitmap);
+}
+
+/**
+ * vb2_queue_publish_buffers() - publish buffers staged by __vb2_queue_alloc()
+ * @q:		pointer to &struct vb2_queue with videobuf2 queue.
+ * @start:	index of the first buffer to publish.
+ * @count:	number of buffers to publish.
+ *
+ * Set the @q->bufs_bitmap bits of the @count fully initialized buffers that
+ * __vb2_queue_alloc() stored in @q->bufs[] starting at @start, which makes
+ * them visible to vb2_get_buffer().
+ */
+static void vb2_queue_publish_buffers(struct vb2_queue *q, unsigned int start,
+				      unsigned int count)
+{
+	unsigned int i;
+
+	lockdep_assert_held(&q->mmap_lock);
+
+	for (i = start; i < start + count; i++) {
+		if (WARN_ON(i >= q->max_num_buffers || !q->bufs[i] ||
+			    test_bit(i, q->bufs_bitmap)))
+			continue;
+		set_bit(i, q->bufs_bitmap);
+	}
 }
 
 /**
@@ -460,6 +487,11 @@ static void vb2_queue_remove_buffer(struct vb2_buffer *vb)
  * @first_index: index of the first created buffer, all newly allocated buffers
  *		 have indices in the range [first_index..first_index+count-1]
  *
+ * The allocated buffers are stored in q->bufs[] but not published in
+ * q->bufs_bitmap: the caller must either publish them with
+ * vb2_queue_publish_buffers() or free them with __vb2_queue_free(), both under
+ * q->mmap_lock.
+ *
  * Returns the number of buffers successfully allocated.
  */
 static int __vb2_queue_alloc(struct vb2_queue *q, enum vb2_memory memory,
@@ -562,7 +594,8 @@ static void __vb2_free_mem(struct vb2_queue *q, unsigned int start, unsigned int
 	struct vb2_buffer *vb;
 
 	for (i = start; i < start + count; i++) {
-		vb = vb2_get_buffer(q, i);
+		/* See __vb2_queue_free() for why vb2_get_buffer() isn't used */
+		vb = q->bufs[i];
 		if (!vb)
 			continue;
 
@@ -580,6 +613,11 @@ static void __vb2_free_mem(struct vb2_queue *q, unsigned int start, unsigned int
  * __vb2_queue_free() - free @count buffers from @start index of the queue - video memory and
  * related information, if no buffers are left return the queue to an
  * uninitialized state. Might be called even if the queue has already been freed.
+ *
+ * Buffers are looked up directly in q->bufs[] instead of through
+ * vb2_get_buffer() so that buffers staged by __vb2_queue_alloc() that were
+ * never published in q->bufs_bitmap (because a later .queue_setup() call
+ * rejected the allocated number of buffers) are freed as well.
  */
 static void __vb2_queue_free(struct vb2_queue *q, unsigned int start, unsigned int count)
 {
@@ -587,9 +625,13 @@ static void __vb2_queue_free(struct vb2_queue *q, unsigned int start, unsigned i
 
 	lockdep_assert_held(&q->mmap_lock);
 
+	/* q->bufs is NULL until vb2_core_allocated_buffers_storage() runs */
+	if (!q->bufs)
+		return;
+
 	/* Call driver-provided cleanup function for each buffer, if provided */
 	for (i = start; i < start + count; i++) {
-		struct vb2_buffer *vb = vb2_get_buffer(q, i);
+		struct vb2_buffer *vb = q->bufs[i];
 
 		if (vb && vb->planes[0].mem_priv)
 			call_void_vb_qop(vb, buf_cleanup, vb);
@@ -624,7 +666,7 @@ static void __vb2_queue_free(struct vb2_queue *q, unsigned int start, unsigned i
 		q->cnt_unprepare_streaming = 0;
 	}
 	for (i = start; i < start + count; i++) {
-		struct vb2_buffer *vb = vb2_get_buffer(q, i);
+		struct vb2_buffer *vb = q->bufs[i];
 		bool unbalanced;
 
 		if (!vb)
@@ -676,7 +718,7 @@ static void __vb2_queue_free(struct vb2_queue *q, unsigned int start, unsigned i
 
 	/* Free vb2 buffers */
 	for (i = start; i < start + count; i++) {
-		struct vb2_buffer *vb = vb2_get_buffer(q, i);
+		struct vb2_buffer *vb = q->bufs[i];
 
 		if (!vb)
 			continue;
@@ -1000,14 +1042,16 @@ int vb2_core_reqbufs(struct vb2_queue *q, enum vb2_memory memory,
 
 	if (ret < 0) {
 		/*
-		 * Note: __vb2_queue_free() will subtract 'allocated_buffers'
-		 * from already queued buffers and it will reset q->memory to
+		 * Note: the buffers allocated by __vb2_queue_alloc() were never
+		 * published in q->bufs_bitmap, __vb2_queue_free() frees them
+		 * straight from q->bufs[] and resets q->memory to
 		 * VB2_MEMORY_UNKNOWN.
 		 */
 		__vb2_queue_free(q, first_index, allocated_buffers);
 		mutex_unlock(&q->mmap_lock);
 		return ret;
 	}
+	vb2_queue_publish_buffers(q, first_index, allocated_buffers);
 	mutex_unlock(&q->mmap_lock);
 
 	/*
@@ -1125,14 +1169,16 @@ int vb2_core_create_bufs(struct vb2_queue *q, enum vb2_memory memory,
 
 	if (ret < 0) {
 		/*
-		 * Note: __vb2_queue_free() will subtract 'allocated_buffers'
-		 * from already queued buffers and it will reset q->memory to
-		 * VB2_MEMORY_UNKNOWN.
+		 * Note: the buffers allocated by __vb2_queue_alloc() were never
+		 * published in q->bufs_bitmap, __vb2_queue_free() frees them
+		 * straight from q->bufs[] and resets q->memory to
+		 * VB2_MEMORY_UNKNOWN if no buffers are left.
 		 */
 		__vb2_queue_free(q, *first_index, allocated_buffers);
 		mutex_unlock(&q->mmap_lock);
 		return -ENOMEM;
 	}
+	vb2_queue_publish_buffers(q, *first_index, allocated_buffers);
 	mutex_unlock(&q->mmap_lock);
 
 	/*
diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
index 4b4f4c15c53a..2e93e6985d5f 100644
--- a/include/media/videobuf2-core.h
+++ b/include/media/videobuf2-core.h
@@ -564,9 +564,11 @@ struct vb2_buf_ops {
  * @mmap_lock:	private mutex used when buffers are allocated/freed/mmapped
  * @memory:	current memory type used
  * @dma_dir:	DMA mapping direction.
- * @bufs:	videobuf2 buffer structures. If it is non-NULL then
- *		bufs_bitmap is also non-NULL.
- * @bufs_bitmap: bitmap tracking whether each bufs[] entry is used
+ * @bufs:	videobuf2 buffer structures. Must be accessed with
+ *		vb2_get_buffer(), as an allocated buffer cannot be used
+ *		until it is published.
+ * @bufs_bitmap: bitmap tracking published buffers. Bits are set and cleared
+ *		under @mmap_lock.
  * @max_num_buffers: upper limit of number of allocated/used buffers.
  *		     If set to 0 v4l2 core will change it VB2_MAX_FRAME
  *		     for backward compatibility.

-- 
2.56.0.385.gac671c819a-goog

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

[Ricardo Ribalda]

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