linux-kernel - Re: [PATCH 2/2] vfio: Prevent open

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Message-ID: <b1c2e3a0-f030-40dc-9b6f-e195b9aa0041@intel.com>
Date: Wed, 28 May 2025 15:24:04 +0800
From: Yi Liu <yi.l.liu@...el.com>
To: Jacob Pan <jacob.pan@...ux.microsoft.com>, <linux-kernel@...r.kernel.org>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>, Alex Williamson
	<alex.williamson@...hat.com>, "jgg@...dia.com" <jgg@...dia.com>
CC: Zhang Yu <zhangyu1@...rosoft.com>, Easwar Hariharan
	<eahariha@...ux.microsoft.com>
Subject: Re: [PATCH 2/2] vfio: Prevent open_count decrement to negative

On 2025/5/17 00:45, Jacob Pan wrote:
> When vfio_df_close() is called with open_count=0, it triggers a warning in
> vfio_assert_device_open() but still decrements open_count to -1. This allows
> a subsequent open to incorrectly pass the open_count == 0 check, leading to
> unintended behavior, such as setting df->access_granted = true.
> 
> For example, running an IOMMUFD compat no-IOMMU device with VFIO tests
> (https://github.com/awilliam/tests/blob/master/vfio-noiommu-pci-device-open.c)
> results in a warning and a failed VFIO_GROUP_GET_DEVICE_FD ioctl on the first
> run, but the second run succeeds incorrectly.
> 
> Add checks to avoid decrementing open_count below zero
> 
> Signed-off-by: Jacob Pan <jacob.pan@...ux.microsoft.com>
> ---
>   drivers/vfio/vfio_main.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)

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

> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 1fd261efc582..5046cae05222 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -583,7 +583,8 @@ void vfio_df_close(struct vfio_device_file *df)
>   
>   	lockdep_assert_held(&device->dev_set->lock);
>   
> -	vfio_assert_device_open(device);
> +	if (!vfio_assert_device_open(device))
> +		return;
>   	if (device->open_count == 1)
>   		vfio_df_device_last_close(df);
>   	device->open_count--;

-- 
Regards,
Yi Liu