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

[Hans Verkuil <hverkuil+cisco@kernel.org>]

Hi Ricardo,

On 11/3/26 11:56, Ricardo Ribalda wrote:
> 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.
> 

Thank you for making the change, it much easier to follow now.

Marek, Tomasz, can one (or both) of you review this patch as well?

I think this can go to the fixes branch. Patch 2/2 needs more discussion and
is something for the next branch.

Regards,

	Hans

> 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.
>