Re: [PATCH net] net/mlx5: HV VHCA: Fix the agent registration race

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- [Medium] The patch's second hunk does not achieve its stated purpose
  and leaves the lock coverage of…

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net/mlx5: HV VHCA: Fix the agent registration race

This keeps agents_lock held across the duplicate-type check, the agent
allocation and the publish into hv_vhca->agents[] in
mlx5_hv_vhca_agent_create(), so two concurrent create calls for the same
type can no longer both pass the check.  It also wraps the read of
hv_vhca->agents[MLX5_HV_VHCA_AGENT_CONTROL] in mlx5_hv_vhca_cleanup()
with agents_lock.

> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/lib/hv_vhca.c b/drivers/net/ethernet/mellanox/mlx5/core/lib/hv_vhca.c
> index 4c4cf6da519fa..eac8afdfc67a3 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/lib/hv_vhca.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/lib/hv_vhca.c
> @@ -228,7 +228,10 @@ void mlx5_hv_vhca_cleanup(struct mlx5_hv_vhca *hv_vhca)
>  	if (!hv_vhca)
>  		return;
>  
> +	mutex_lock(&hv_vhca->agents_lock);
>  	agent = hv_vhca->agents[MLX5_HV_VHCA_AGENT_CONTROL];
> +	mutex_unlock(&hv_vhca->agents_lock);
> +
>  	if (agent)
>  		mlx5_hv_vhca_control_agent_destroy(agent);

[Severity: Medium]
What does this new critical section in mlx5_hv_vhca_cleanup() actually
protect?  The lock is dropped immediately after the load, and the pointer
is consumed afterwards:

	mutex_lock(&hv_vhca->agents_lock);
	agent = hv_vhca->agents[MLX5_HV_VHCA_AGENT_CONTROL];
	mutex_unlock(&hv_vhca->agents_lock);

	if (agent)
		mlx5_hv_vhca_control_agent_destroy(agent);

So the only guarantee gained over the previous code is a non-torn load of
an aligned pointer, which a plain load already provides.  Nothing is
serialized against the destroy itself.  Extending the lock over the call
is not an option either, since mlx5_hv_vhca_agent_destroy() takes
agents_lock itself and that would self-deadlock.

The changelog says:

    While at it, read hv_vhca->agents[MLX5_HV_VHCA_AGENT_CONTROL] under
    the lock in mlx5_hv_vhca_cleanup() as well, since that slot is
    written under the lock.

Is that accurate?  mlx5_hv_vhca_init() still stores to the same slot with
no lock held:

	agent = mlx5_hv_vhca_control_agent_create(hv_vhca);
	if (IS_ERR_OR_NULL(agent)) {
		mlx5_hv_unregister_invalidate(hv_vhca->dev);
		return;
	}

	hv_vhca->agents[MLX5_HV_VHCA_AGENT_CONTROL] = agent;

That store is now fully redundant with the locked publish that
mlx5_hv_vhca_agent_create() already performs:

	hv_vhca->agents[type] = agent;
	mutex_unlock(&hv_vhca->agents_lock);

and it contradicts the field's own contract:

	struct mutex                agents_lock; /* Protect agents array */

Can it also race with the locked reader?  mlx5_hv_register_invalidate()
has already run by that point, and mlx5_hv_vhca_agent_create() queues
work before returning to init:

mlx5_hv_vhca_agent_create()
  mlx5_hv_vhca_agents_update()
    mlx5_hv_vhca_invalidate()
      queue_work(hv_vhca->work_queue, &work->invalidate_work);

while the worker reads the array under the mutex:

	mutex_lock(&hv_vhca->agents_lock);
	for (i = 0; i < MLX5_HV_VHCA_AGENT_MAX; i++) {
		struct mlx5_hv_vhca_agent *agent = hv_vhca->agents[i];

Since the value stored by init() is identical to the one already
published under the lock, the practical effect today looks like a benign
data race that KCSAN would flag rather than a functional failure.  Would
it be cleaner to drop the unlocked assignment in mlx5_hv_vhca_init()
instead, so agents[] has exactly one locked writer per slot, drop the
no-op lock/unlock in mlx5_hv_vhca_cleanup(), and adjust the changelog
wording accordingly?

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Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260922074246.20411-1-ginger.jzllee%40gmail.com