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

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Message-Id: <20250516164522.51905-2-jacob.pan@linux.microsoft.com>
Date: Fri, 16 May 2025 09:45:22 -0700
From: Jacob Pan <jacob.pan@...ux.microsoft.com>
To: linux-kernel@...r.kernel.org,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	Alex Williamson <alex.williamson@...hat.com>,
	"Liu, Yi L" <yi.l.liu@...el.com>,
	"jgg@...dia.com" <jgg@...dia.com>,
	Jacob Pan <jacob.pan@...ux.microsoft.com>
Cc: Zhang Yu <zhangyu1@...rosoft.com>,
	Easwar Hariharan <eahariha@...ux.microsoft.com>
Subject: [PATCH 2/2] vfio: Prevent open_count decrement to negative

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(-)

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--;
-- 
2.34.1