Re: [PATCH v3 14/15] iommufd: vfio container FD ioctl compatibility

From: Jason Gunthorpe <jgg@nvidia.com>
To: Lu Baolu <baolu.lu@linux.intel.com>,
	bpf@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
	David Woodhouse <dwmw2@infradead.org>,
	iommu@lists.linux.dev, Joerg Roedel <joro@8bytes.org>,
	Kevin Tian <kevin.tian@intel.com>,
	linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org,
	llvm@lists.linux.dev, Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Miguel Ojeda <ojeda@kernel.org>,
	Robin Murphy <robin.murphy@arm.com>,
	Shuah Khan <shuah@kernel.org>,
	Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
	Tom Rix <trix@redhat.com>, Will Deacon <will@kernel.org>
Cc: Alex Williamson <alex.williamson@redhat.com>,
	Chaitanya Kulkarni <chaitanyak@nvidia.com>,
	Cornelia Huck <cohuck@redhat.com>,
	Daniel Jordan <daniel.m.jordan@oracle.com>,
	David Gibson <david@gibson.dropbear.id.au>,
	Eric Auger <eric.auger@redhat.com>,
	Eric Farman <farman@linux.ibm.com>,
	Jason Wang <jasowang@redhat.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.org>,
	Joao Martins <joao.m.martins@oracle.com>,
	kvm@vger.kernel.org, Matthew Rosato <mjrosato@linux.ibm.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Nicolin Chen <nicolinc@nvidia.com>,
	Niklas Schnelle <schnelle@linux.ibm.com>,
	Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>,
	Yi Liu <yi.l.liu@intel.com>, Keqian Zhu <zhukeqian1@huawei.com>
Subject: Re: [PATCH v3 14/15] iommufd: vfio container FD ioctl compatibility
Date: Thu, 27 Oct 2022 11:12:57 -0300	[thread overview]
Message-ID: <Y1qR6Zxdmuk+ME5z@nvidia.com> (raw)
In-Reply-To: <14-v3-402a7d6459de+24b-iommufd_jgg@nvidia.com>

> +	down_read(&ioas->iopt.iova_rwsem);
> +	info.flags = VFIO_IOMMU_INFO_PGSIZES;
> +	info.iova_pgsizes = iommufd_get_pagesizes(ioas);
> +	info.cap_offset = 0;

The iommufd_get_pagesizes() obtains the domains_rwsem and cannot be
called under the iova_rwsem due to lock odering.

The test suite already covers this, but it turns out my test
environment had lockdep disabled since it hits a Intel iommu lockdep
splat on boot starting in v6.1-rc1 :\ Syzkaller found it because it
runs the VM with different options and avoids the boot splat.

@@ -371,11 +371,11 @@ static int iommufd_vfio_iommu_get_info(struct iommufd_ctx *ictx,
        if (IS_ERR(ioas))
                return PTR_ERR(ioas);
 
-       down_read(&ioas->iopt.iova_rwsem);
        info.flags = VFIO_IOMMU_INFO_PGSIZES;
        info.iova_pgsizes = iommufd_get_pagesizes(ioas);
        info.cap_offset = 0;
 
+       down_read(&ioas->iopt.iova_rwsem);
        total_cap_size = sizeof(info);
        for (i = 0; i != ARRAY_SIZE(fill_fns); i++) {
                int cap_size;

Jason
