Re: [PATCH v3 1/3] dmaengine: Fix device kref underflow in dma_chan_put()

Thank you for your contribution! Sashiko AI review found 3 potential issue(s) to consider:

Pre-existing issues:
- [High] dma_chan_put() accesses chan->device->owner after potentially freeing chan and chan->device via dma_device_put().
- [High] dma_release_channel() accesses `chan` fields after calling `dma_chan_put()`, which may have freed the channel.
- [High] RCU read-side critical section in `dma_issue_pending_all()` can access a synchronously freed `struct dma_device`.
--

--- Patch [1]: [PATCH v3 1/3] dmaengine: Fix device kref underflow in dma_chan_put() ---
commit 0f010c1f7452238748341b884bda3d3bba2c74ac
Author: Shivank Garg <[email protected]>

dmaengine: Fix device kref underflow in dma_chan_put()

This commit fixes a device reference underflow bug by ensuring the device 
reference is only dropped on the last put of a channel, matching the behavior 
of the get path.

> diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
> index 6ffd8bd82154a..516d6d9332083 100644
> --- a/drivers/dma/dmaengine.c
> +++ b/drivers/dma/dmaengine.c
> @@ -515,7 +515,9 @@ static void dma_chan_put(struct dma_chan *chan)
>  		chan->route_data = NULL;
>  	}
>  
> -	dma_device_put(chan->device);
> +	/* This channel is not in use anymore, drop the device ref */
> +	if (!chan->client_count)
> +		dma_device_put(chan->device);
>  	module_put(dma_chan_to_owner(chan));

[Severity: High]
This is a pre-existing issue, but does accessing chan after dma_device_put()
risk a use-after-free?

If dma_device_put() drops the last reference to the DMA device, it can
trigger synchronous release of the device memory, which might also free the
embedded dma_chan.

Could dma_chan_to_owner(chan) then dereference freed memory by evaluating
chan->device->owner?

>  }

[Severity: High]
This isn't a bug introduced by this patch, but does dma_release_channel()
also access freed memory under the same conditions?

drivers/dma/dmaengine.c:dma_release_channel() {
	...
	dma_chan_put(chan);

	if (chan->slave) {
		sysfs_remove_link(&chan->dev->device.kobj, DMA_SLAVE_NAME);
	...
}

If dma_chan_put() frees the channel when the last device reference is dropped,
is it safe to access chan->slave and chan->dev afterwards to tear down
the sysfs links?

[Severity: High]
This is also a pre-existing issue, but does the RCU read side in
dma_issue_pending_all() race with synchronous device release?

dma_issue_pending_all() relies on an RCU read lock to traverse the device list:

drivers/dma/dmaengine.c:dma_issue_pending_all() {
	...
	rcu_read_lock();
	list_for_each_entry_rcu(device, &dma_device_list, global_node) {
		if (dma_has_cap(DMA_PRIVATE, device->cap_mask))
	...
}

However, dma_device_release() unlinks the device and synchronously frees
it without observing an RCU grace period:

drivers/dma/dmaengine.c:dma_device_release() {
	...
	list_del_rcu(&device->global_node);
	dma_channel_rebalance();

	if (device->device_release)
		device->device_release(device);
	...
}

Could the RCU reader dereference a struct dma_device that was just
freed by dma_device_release()?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=1