Re: [PATCH v4] i2c: virtio: retain xfer with kref to fix UAF on interrupted wait

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jun 10, 2026 at 11:02:32AM -0400, Gavin Li wrote:
> commit a663b3c47ab1 ("i2c: virtio: Avoid hang by using interruptible
> completion wait") switched virtio_i2c_complete_reqs() to
> wait_for_completion_interruptible() so a stuck device cannot hang a
> task forever. That left a use-after-free: if the wait returns early on
> a signal, virtio_i2c_xfer() frees reqs and DMA bounce buffers while the
> device may still hold virtqueue tokens pointing at &reqs[i] and DMA
> into read buffers. When those requests complete later,
> virtio_i2c_msg_done() calls complete() on freed memory.
> 
> Waiting uninterruptibly for every completion before freeing avoids the
> UAF but can hang the caller indefinitely if the virtio side never
> completes the request. Performing a queue reset / device reset is
> possible in this scenario, but adds complexity.
> 
> This commit manages the freeing of the xfer allocations via kref, and
> ensures that each in-flight request holds a reference. This fixes the
> use-after-free by ensuring that the virtio device has a valid location
> to write to until the request completes. This will cause a memory leak
> in cases where the device hangs, but that is much preferable to memory
> corruption.
> 
> Additionally, force usage of a bounce buffer even if the i2c_msg buf is
> DMA-safe, since the buffer passed to the virtqueue must remain valid
> even if the transfer is interrupted. Remove usage of
> i2c_get_dma_safe_msg_buf() since it may pass through msg->buf directly.
> All bounce buffers are part of the single xfer allocation, so there is
> no additional allocation overhead.
> 
> Signed-off-by: Gavin Li <gavin.li@samsara.com>
> 
> ---
> 
> Changes in v4:
> - Pack bounce buffers into a single allocation after reqs[]
> - Remove per-request buf pointer and xfer->num
> - Remove req.msg, track message direction with req->read
> - Simplify xfer release to a single kfree()
> - Restore using j to track successful transfers in _complete_xfer()
> ---
>  drivers/i2c/busses/i2c-virtio.c | 121 +++++++++++++++++++++++++-------
>  1 file changed, 94 insertions(+), 27 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-virtio.c b/drivers/i2c/busses/i2c-virtio.c
> index 5da6fef92bec3..33b469ca39d4b 100644
> --- a/drivers/i2c/busses/i2c-virtio.c
> +++ b/drivers/i2c/busses/i2c-virtio.c
> @@ -13,7 +13,9 @@
>  #include <linux/err.h>
>  #include <linux/i2c.h>
>  #include <linux/kernel.h>
> +#include <linux/kref.h>
>  #include <linux/module.h>
> +#include <linux/overflow.h>
>  #include <linux/virtio.h>
>  #include <linux/virtio_ids.h>
>  #include <linux/virtio_config.h>
> @@ -31,39 +33,74 @@ struct virtio_i2c {
>  	struct virtqueue *vq;
>  };
>  
> +struct virtio_i2c_xfer;
> +
>  /**
>   * struct virtio_i2c_req - the virtio I2C request structure
> + * @xfer: owning transfer
>   * @completion: completion of virtio I2C message
> + * @read: true if this is a read message (I2C_M_RD is set)
>   * @out_hdr: the OUT header of the virtio I2C message
> - * @buf: the buffer into which data is read, or from which it's written
>   * @in_hdr: the IN header of the virtio I2C message
>   */
>  struct virtio_i2c_req {
> +	struct virtio_i2c_xfer *xfer;
>  	struct completion completion;
> +	bool read;
>  	struct virtio_i2c_out_hdr out_hdr	____cacheline_aligned;
> -	uint8_t *buf				____cacheline_aligned;
>  	struct virtio_i2c_in_hdr in_hdr		____cacheline_aligned;
>  };
>  
> +/**
> + * struct virtio_i2c_xfer - a queued I2C transfer
> + * @ref: one ref for the caller, plus one per in-flight virtqueue request
> + * @bounce_buf_base: start of bounce buffer region
> + * @reqs: the virtio I2C requests
> + *
> + * Allocation layout:
> + * - struct virtio_i2c_xfer xfer
> + * - struct virtio_i2c_req reqs[num]
> + * - u8 bounce_buf[msgs[0].len]
> + *   ...
> + * - u8 bounce_buf[msgs[num-1].len]

You seems to be ignoring DMA alignment requirements here.
Did you try running with DMA debugging enabled?


> + */
> +struct virtio_i2c_xfer {
> +	struct kref ref;
> +	u8 *bounce_buf_base;
> +	struct virtio_i2c_req reqs[];
> +};
> +
> +static void virtio_i2c_xfer_release(struct kref *ref)
> +{
> +	struct virtio_i2c_xfer *xfer = container_of(ref, struct virtio_i2c_xfer, ref);
> +	kfree(xfer);
> +}
> +
>  static void virtio_i2c_msg_done(struct virtqueue *vq)
>  {
>  	struct virtio_i2c_req *req;
>  	unsigned int len;
>  
> -	while ((req = virtqueue_get_buf(vq, &len)))
> +	while ((req = virtqueue_get_buf(vq, &len))) {
>  		complete(&req->completion);
> +		kref_put(&req->xfer->ref, virtio_i2c_xfer_release);
> +	}
>  }
>  
> -static int virtio_i2c_prepare_reqs(struct virtqueue *vq,
> -				   struct virtio_i2c_req *reqs,
> +static int virtio_i2c_prepare_xfer(struct virtqueue *vq,
> +				   struct virtio_i2c_xfer *xfer,
>  				   struct i2c_msg *msgs, int num)
>  {
>  	struct scatterlist *sgs[3], out_hdr, msg_buf, in_hdr;
> +	struct virtio_i2c_req *reqs = xfer->reqs;
> +	u8 *bounce_buf = xfer->bounce_buf_base;
>  	int i;
>  
>  	for (i = 0; i < num; i++) {
>  		int outcnt = 0, incnt = 0;
>  
> +		reqs[i].xfer = xfer;
> +		reqs[i].read = !!(msgs[i].flags & I2C_M_RD);
>  		init_completion(&reqs[i].completion);
>  
>  		/*
> @@ -82,23 +119,31 @@ static int virtio_i2c_prepare_reqs(struct virtqueue *vq,
>  		sgs[outcnt++] = &out_hdr;
>  
>  		if (msgs[i].len) {
> -			reqs[i].buf = i2c_get_dma_safe_msg_buf(&msgs[i], 1);
> -			if (!reqs[i].buf)
> -				break;
> +			/*
> +			 * Even if msg->flags has I2C_M_DMA_SAFE set, a bounce
> +			 * buffer is required because the transfer may be
> +			 * interrupted, after which msg->buf is no longer valid.
> +			 */
> +			if (!(msgs[i].flags & I2C_M_RD))
> +				memcpy(bounce_buf, msgs[i].buf, msgs[i].len);
>  
> -			sg_init_one(&msg_buf, reqs[i].buf, msgs[i].len);
> +			sg_init_one(&msg_buf, bounce_buf, msgs[i].len);
>  
>  			if (msgs[i].flags & I2C_M_RD)
>  				sgs[outcnt + incnt++] = &msg_buf;
>  			else
>  				sgs[outcnt++] = &msg_buf;
>  		}
> +		bounce_buf += msgs[i].len;
>  
>  		sg_init_one(&in_hdr, &reqs[i].in_hdr, sizeof(reqs[i].in_hdr));
>  		sgs[outcnt + incnt++] = &in_hdr;
>  
> +		/* This reference is released in virtio_i2c_msg_done(). */
> +		kref_get(&xfer->ref);
> +
>  		if (virtqueue_add_sgs(vq, sgs, outcnt, incnt, &reqs[i], GFP_KERNEL)) {
> -			i2c_put_dma_safe_msg_buf(reqs[i].buf, &msgs[i], false);
> +			kref_put(&xfer->ref, virtio_i2c_xfer_release);
>  			break;
>  		}
>  	}
> @@ -106,26 +151,38 @@ static int virtio_i2c_prepare_reqs(struct virtqueue *vq,
>  	return i;
>  }
>  
> -static int virtio_i2c_complete_reqs(struct virtqueue *vq,
> -				    struct virtio_i2c_req *reqs,
> -				    struct i2c_msg *msgs, int num)
> +static int virtio_i2c_complete_xfer(struct virtio_i2c_xfer *xfer,
> +				    struct i2c_msg *msgs,
> +				    int num)
>  {
> +	struct virtio_i2c_req *reqs = xfer->reqs;
> +	u8 *bounce_buf = xfer->bounce_buf_base;
>  	bool failed = false;
>  	int i, j = 0;
>  
>  	for (i = 0; i < num; i++) {
>  		struct virtio_i2c_req *req = &reqs[i];
> +		struct i2c_msg *msg = &msgs[i];
> +
> +		if (wait_for_completion_interruptible(&req->completion))
> +			return -EINTR;
> +
> +		if (req->in_hdr.status != VIRTIO_I2C_MSG_OK) {
> +			/*
> +			 * Don't break yet. Try to wait until all requests
> +			 * complete to ensure that the virtqueue has enough
> +			 * descriptor slots for the next transfer.
> +			 */
> +			failed = true;
> +		}
>  
>  		if (!failed) {
> -			if (wait_for_completion_interruptible(&req->completion))
> -				failed = true;
> -			else if (req->in_hdr.status != VIRTIO_I2C_MSG_OK)
> -				failed = true;
> -			else
> -				j++;
> +			if (req->read)
> +				memcpy(msg->buf, bounce_buf, msg->len);
> +			j++;
>  		}
>  
> -		i2c_put_dma_safe_msg_buf(reqs[i].buf, &msgs[i], !failed);
> +		bounce_buf += msg->len;
>  	}
>  
>  	return j;
> @@ -136,14 +193,24 @@ static int virtio_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg *msgs,
>  {
>  	struct virtio_i2c *vi = i2c_get_adapdata(adap);
>  	struct virtqueue *vq = vi->vq;
> -	struct virtio_i2c_req *reqs;
> -	int count;
> +	struct virtio_i2c_xfer *xfer;
> +	size_t alloc_size;
> +	int i, count;
>  
> -	reqs = kzalloc_objs(*reqs, num);
> -	if (!reqs)
> +	alloc_size = struct_size(xfer, reqs, num);
> +	for (i = 0; i < num; i++) {
> +		if (check_add_overflow(alloc_size, msgs[i].len, &alloc_size))
> +			return -EOVERFLOW;
> +	}
> +
> +	xfer = kzalloc(alloc_size, GFP_KERNEL);
> +	if (!xfer)
>  		return -ENOMEM;
>  
> -	count = virtio_i2c_prepare_reqs(vq, reqs, msgs, num);
> +	kref_init(&xfer->ref);
> +	xfer->bounce_buf_base = (u8 *)(xfer->reqs + num);
> +
> +	count = virtio_i2c_prepare_xfer(vq, xfer, msgs, num);
>  	if (!count)
>  		goto err_free;
>  
> @@ -157,10 +224,10 @@ static int virtio_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg *msgs,
>  	 */
>  	virtqueue_kick(vq);
>  
> -	count = virtio_i2c_complete_reqs(vq, reqs, msgs, count);
> +	count = virtio_i2c_complete_xfer(xfer, msgs, count);
>  
>  err_free:
> -	kfree(reqs);
> +	kref_put(&xfer->ref, virtio_i2c_xfer_release);
>  	return count;
>  }
>  
> -- 
> 2.54.0