Re: [PATCH] vfio: Get/put KVM only for the first/last vfio_df_open/close in cdev path

[Yi Liu <yi.l.liu@intel.com>]

On 2024/6/28 23:18, Yan Zhao wrote:
> In the device cdev path, adjust the handling of the KVM reference count to
> only increment with the first vfio_df_open() and decrement after the final
> vfio_df_close(). This change addresses a KVM reference leak that occurs
> when a device cdev file is opened multiple times and attempts to bind to
> iommufd repeatedly.
> 
> Currently, vfio_df_get_kvm_safe() is invoked prior to each vfio_df_open()
> in the cdev path during iommufd binding. The corresponding
> vfio_device_put_kvm() is executed either when iommufd is unbound or if an
> error occurs during the binding process.
> 
> However, issues arise when a device binds to iommufd more than once. The
> second vfio_df_open() will fail during iommufd binding, and
> vfio_device_put_kvm() will be triggered, setting device->kvm to NULL.
> Consequently, when iommufd is unbound from the first successfully bound
> device, vfio_device_put_kvm() becomes ineffective, leading to a leak in the
> KVM reference count.

Good catch!!!

> Below is the calltrace that will be produced in this scenario when the KVM
> module is unloaded afterwards, reporting "BUG kvm_vcpu (Tainted: G S):
> Objects remaining in kvm_vcpu on __kmem_cache_shutdown()".
> 
> Call Trace:
>   <TASK>
>   dump_stack_lvl+0x80/0xc0
>   slab_err+0xb0/0xf0
>   ? __kmem_cache_shutdown+0xc1/0x4e0
>   ? rcu_is_watching+0x11/0x50
>   ? lock_acquired+0x144/0x3c0
>   __kmem_cache_shutdown+0x1b7/0x4e0
>   kmem_cache_destroy+0xa6/0x260
>   kvm_exit+0x80/0xc0 [kvm]
>   vmx_exit+0xe/0x20 [kvm_intel]
>   __x64_sys_delete_module+0x143/0x250
>   ? ktime_get_coarse_real_ts64+0xd3/0xe0
>   ? syscall_trace_enter+0x143/0x210
>   do_syscall_64+0x6f/0x140
>   entry_SYSCALL_64_after_hwframe+0x76/0x7e
> 
> Fixes: 5fcc26969a16 ("vfio: Add VFIO_DEVICE_BIND_IOMMUFD")
> Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
> ---
>   drivers/vfio/device_cdev.c | 13 +++++++++----
>   1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/vfio/device_cdev.c b/drivers/vfio/device_cdev.c
> index bb1817bd4ff3..3b85d01d1b27 100644
> --- a/drivers/vfio/device_cdev.c
> +++ b/drivers/vfio/device_cdev.c
> @@ -65,6 +65,7 @@ long vfio_df_ioctl_bind_iommufd(struct vfio_device_file *df,
>   {
>   	struct vfio_device *device = df->device;
>   	struct vfio_device_bind_iommufd bind;
> +	bool put_kvm = false;
>   	unsigned long minsz;
>   	int ret;
>   
> @@ -101,12 +102,15 @@ long vfio_df_ioctl_bind_iommufd(struct vfio_device_file *df,
>   	}
>   
>   	/*
> -	 * Before the device open, get the KVM pointer currently
> +	 * Before the device's first open, get the KVM pointer currently
>   	 * associated with the device file (if there is) and obtain
> -	 * a reference.  This reference is held until device closed.
> +	 * a reference.  This reference is held until device's last closed.
>   	 * Save the pointer in the device for use by drivers.
>   	 */
> -	vfio_df_get_kvm_safe(df);
> +	if (device->open_count == 0) {
> +		vfio_df_get_kvm_safe(df);
> +		put_kvm = true;
> +	}
>   
>   	ret = vfio_df_open(df);
>   	if (ret)
> @@ -129,7 +133,8 @@ long vfio_df_ioctl_bind_iommufd(struct vfio_device_file *df,
>   out_close_device:
>   	vfio_df_close(df);
>   out_put_kvm:
> -	vfio_device_put_kvm(device);
> +	if (put_kvm)

you may use if (device->open_count == 0) as well here to save a bool. 
Otherwise looks good to me.

Reviewed-by: Yi Liu <yi.l.liu@intel.com>

> +		vfio_device_put_kvm(device);
>   	iommufd_ctx_put(df->iommufd);
>   	df->iommufd = NULL;
>   out_unlock:
> 
> base-commit: 6ba59ff4227927d3a8530fc2973b80e94b54d58f

BTW. The vfio_device_get_kvm_safe() is not supposed to be invoked multiple
times by design as the second call would override the device->put_kvm and
device->kvm. This does not change the put_kvm nor the kvm though. But not a
good thing anyghow. maybe worth a warn like below.

diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index ee72c0b61795..a4bead0e5820 100644
--- a/drivers/vfio/vfio_main.c
+++ b/drivers/vfio/vfio_main.c
@@ -408,6 +408,8 @@ void vfio_device_get_kvm_safe(struct vfio_device 
*device, struct kvm *kvm)
  	if (!kvm)
  		return;

+	WARN_ON(device->put_kvm || device->kvm);
+
  	pfn = symbol_get(kvm_put_kvm);
  	if (WARN_ON(!pfn))
  		return;

-- 
Regards,
Yi Liu