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Date:   Fri, 13 Jan 2023 08:04:20 -0500
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Subject: Re: [PATCH v2] vfio: fix potential deadlock on vfio group lock
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From:   Matthew Rosato <mjrosato@linux.ibm.com>
To:     alex.williamson@redhat.com, pbonzini@redhat.com
Cc:     jgg@nvidia.com, cohuck@redhat.com, farman@linux.ibm.com,
        pmorel@linux.ibm.com, borntraeger@linux.ibm.com,
        frankja@linux.ibm.com, imbrenda@linux.ibm.com, david@redhat.com,
        akrowiak@linux.ibm.com, jjherne@linux.ibm.com, pasic@linux.ibm.com,
        zhenyuw@linux.intel.com, zhi.a.wang@intel.com, seanjc@google.com,
        linux-s390@vger.kernel.org, kvm@vger.kernel.org,
        intel-gvt-dev@lists.freedesktop.org,
        intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org
References: <20230112203844.41179-1-mjrosato@linux.ibm.com>
In-Reply-To: <20230112203844.41179-1-mjrosato@linux.ibm.com>
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On 1/12/23 3:38 PM, Matthew Rosato wrote:
> Currently it is possible that the final put of a KVM reference comes from
> vfio during its device close operation.  This occurs while the vfio group
> lock is held; however, if the vfio device is still in the kvm device list,
> then the following call chain could result in a deadlock:
> 
> kvm_put_kvm
>  -> kvm_destroy_vm
>   -> kvm_destroy_devices
>    -> kvm_vfio_destroy
>     -> kvm_vfio_file_set_kvm
>      -> vfio_file_set_kvm
>       -> group->group_lock/group_rwsem
> 
> Avoid this scenario by having vfio core code acquire a KVM reference
> the first time a device is opened and hold that reference until the
> device fd is closed, at a point after the group lock has been released.
> 
> Fixes: 421cfe6596f6 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM")
> Reported-by: Alex Williamson <alex.williamson@redhat.com>
> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> ---
> Changes from v1:
> * Re-write using symbol get logic to get kvm ref during first device
>   open, release the ref during device fd close after group lock is
>   released
> * Drop kvm get/put changes to drivers; now that vfio core holds a
>   kvm ref until sometime after the device_close op is called, it
>   should be fine for drivers to get and put their own references to it.
> ---
>  drivers/vfio/group.c     |  6 ++---
>  drivers/vfio/vfio_main.c | 48 +++++++++++++++++++++++++++++++++++++---
>  include/linux/vfio.h     |  1 -
>  3 files changed, 48 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> index bb24b2f0271e..2b0da82f82f4 100644
> --- a/drivers/vfio/group.c
> +++ b/drivers/vfio/group.c
> @@ -165,9 +165,9 @@ static int vfio_device_group_open(struct vfio_device *device)
>  	}
>  
>  	/*
> -	 * Here we pass the KVM pointer with the group under the lock.  If the
> -	 * device driver will use it, it must obtain a reference and release it
> -	 * during close_device.
> +	 * Here we pass the KVM pointer with the group under the lock.  A
> +	 * reference will be obtained the first time the device is opened and
> +	 * will be held until the device fd is closed.
>  	 */
>  	ret = vfio_device_open(device, device->group->iommufd,
>  			       device->group->kvm);
> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 5177bb061b17..c969e2a0ecd3 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -16,6 +16,7 @@
>  #include <linux/fs.h>
>  #include <linux/idr.h>
>  #include <linux/iommu.h>
> +#include <linux/kvm_host.h>

Ugh, looks like including linux/kvm_host.h here breaks architectures that don't have an arch/*/include/uapi/asm/kvm.h

AFAICT this should be implicit with the CONFIG_HAVE_KVM bool, so unless someone has a better idea, to avoid I think we can key off of CONFIG_HAVE_KVM like so...

#ifdef CONFIG_HAVE_KVM
#include <linux/kvm_host.h>
#endif

[...]

#ifdef CONFIG_HAVE_KVM
[...symbol_get implementation here...]
#else
static bool vfio_kvm_get_kvm_safe(struct kvm *kvm)
{
	return false;
}
static void vfio_kvm_put_kvm(struct kvm *kvm)
{

}
#endif