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[35.203.150.192]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-39d7738ce41sm1598219a91.3.2026.09.09.15.27.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 09 Sep 2026 15:27:45 -0700 (PDT) Date: Wed, 9 Sep 2026 22:27:42 +0000 From: David Matlack To: Alex Williamson Cc: Alex Williamson , kvm , linux-kernel , Jason Gunthorpe , Kevin Tian , Yi Liu Subject: Re: [PATCH 1/4] vfio: Reject a second cdev open before mutating shared device state Message-ID: References: <20260901215358.2421359-1-alex.williamson@nvidia.com> <20260901215358.2421359-2-alex.williamson@nvidia.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260901215358.2421359-2-alex.williamson@nvidia.com> On 2026-09-01 03:53 PM, Alex Williamson wrote: > The cdev single-open check lives in vfio_df_open(), which runs at the > end of the bind ioctl, after vfio_df_ioctl_bind_iommufd() has already > updated state shared across all opens: vfio_df_check_token() can set > the PF vf_token and vfio_df_get_kvm_safe() records the caller's KVM > pointer in device->kvm and takes a reference. > > A second cdev bind of an already-open device runs both, only to be > rejected in vfio_df_open(). The error path clears device->kvm and > drops the reference, tearing down the current opener's KVM association > and potentially resulting in an unbalanced reference on close or > premature release, while the vf_token remains clobbered. > > Move the single-open check into vfio_df_ioctl_bind_iommufd() ahead of > both mutations, so a bind that cannot complete leaves the current > opener's state untouched. df->group is NULL on this path, so a > non-zero open_count is exactly what vfio_df_open() rejected. The test > in vfio_df_open() becomes redundant and is removed. > > Return -EBUSY rather than -EINVAL here. The arguments are not invalid, > the device is in use, which could be a transient condition due to a > delayed fput if the prior user is terminated. This provides > compatibility with the group path, where a group open returns -EBUSY, > and users may choose bounded polling to detect such a transient > condition. > > Fixes: 839e692fa4eb ("vfio: Make vfio_df_open() single open for device cdev path") > Fixes: 5fcc26969a16 ("vfio: Add VFIO_DEVICE_BIND_IOMMUFD") > Fixes: 86624ba3b522 ("vfio/pci: Do vf_token checks for VFIO_DEVICE_BIND_IOMMUFD") > Assisted-by: claude-opus-4-8 > Signed-off-by: Alex Williamson Can you add a regression test for this? The VF token clobbering can be reproduced in vfio_pci_sriov_uapi_test: diff --git a/tools/testing/selftests/vfio/vfio_pci_sriov_uapi_test.c b/tools/testing/selftests/vfio/vfio_pci_sriov_uapi_test.c index 19d657d00b75..de8408b90a25 100644 --- a/tools/testing/selftests/vfio/vfio_pci_sriov_uapi_test.c +++ b/tools/testing/selftests/vfio/vfio_pci_sriov_uapi_test.c @@ -157,6 +157,42 @@ TEST_F(vfio_pci_sriov_uapi_test, override_token) ASSERT_COND_VF_CREATION(ret); } +TEST(failed_second_open_does_not_clobber_token) +{ + struct vfio_pci_device *pf = NULL, *pf_second_fd = NULL, *vf = NULL; + struct iommu *iommu; + int ret; + + iommu = iommu_init("iommufd"); + if (!iommu) + SKIP(return, "iommufd mode not supported"); + + /* Create and bind PF using UUID_1 */ + ret = device_init(pf_bdf, iommu, UUID_1, &pf); + ASSERT_EQ(ret, 0); + + /* + * Attempt to open the same PF again and bind it with a *different* + * token (UUID_2). This must fail with EBUSY because it's a second open. + */ + ret = device_init(pf_bdf, iommu, UUID_2, &pf_second_fd); + ASSERT_EQ(ret, -EBUSY); + + /* + * Attempt to initialize a VF using the original PF token (UUID_1). + * If the failed open above clobbered the PF's token (i.e. updated it to + * UUID_2), this VF initialization will fail. + */ + ret = device_init(vf_bdf, iommu, UUID_1, &vf); + ASSERT_EQ(ret, 0); + + device_cleanup(vf); + if (pf_second_fd) + device_cleanup(pf_second_fd); + device_cleanup(pf); + iommu_cleanup(iommu); +} + static void vf_teardown(void) { /*