• Parav Pandit's avatar
    vfio/mdev: Synchronize device create/remove with parent removal · 5715c4dd
    Parav Pandit authored
    In following sequences, child devices created while removing mdev parent
    device can be left out, or it may lead to race of removing half
    initialized child mdev devices.
    
    issue-1:
    --------
           cpu-0                         cpu-1
           -----                         -----
                                      mdev_unregister_device()
                                        device_for_each_child()
                                          mdev_device_remove_cb()
                                            mdev_device_remove()
    create_store()
      mdev_device_create()                   [...]
        device_add()
                                      parent_remove_sysfs_files()
    
    /* BUG: device added by cpu-0
     * whose parent is getting removed
     * and it won't process this mdev.
     */
    
    issue-2:
    --------
    Below crash is observed when user initiated remove is in progress
    and mdev_unregister_driver() completes parent unregistration.
    
           cpu-0                         cpu-1
           -----                         -----
    remove_store()
       mdev_device_remove()
       active = false;
                                      mdev_unregister_device()
                                      parent device removed.
       [...]
       parents->ops->remove()
     /*
      * BUG: Accessing invalid parent.
      */
    
    This is similar race like create() racing with mdev_unregister_device().
    
    BUG: unable to handle kernel paging request at ffffffffc0585668
    PGD e8f618067 P4D e8f618067 PUD e8f61a067 PMD 85adca067 PTE 0
    Oops: 0000 [#1] SMP PTI
    CPU: 41 PID: 37403 Comm: bash Kdump: loaded Not tainted 5.1.0-rc6-vdevbus+ #6
    Hardware name: Supermicro SYS-6028U-TR4+/X10DRU-i+, BIOS 2.0b 08/09/2016
    RIP: 0010:mdev_device_remove+0xfa/0x140 [mdev]
    Call Trace:
     remove_store+0x71/0x90 [mdev]
     kernfs_fop_write+0x113/0x1a0
     vfs_write+0xad/0x1b0
     ksys_write+0x5a/0xe0
     do_syscall_64+0x5a/0x210
     entry_SYSCALL_64_after_hwframe+0x49/0xbe
    
    Therefore, mdev core is improved as below to overcome above issues.
    
    Wait for any ongoing mdev create() and remove() to finish before
    unregistering parent device.
    This continues to allow multiple create and remove to progress in
    parallel for different mdev devices as most common case.
    At the same time guard parent removal while parent is being accessed by
    create() and remove() callbacks.
    create()/remove() and unregister_device() are synchronized by the rwsem.
    
    Refactor device removal code to mdev_device_remove_common() to avoid
    acquiring unreg_sem of the parent.
    
    Fixes: 7b96953b ("vfio: Mediated device Core driver")
    Signed-off-by: default avatarParav Pandit <parav@mellanox.com>
    Reviewed-by: default avatarCornelia Huck <cohuck@redhat.com>
    Signed-off-by: default avatarAlex Williamson <alex.williamson@redhat.com>
    5715c4dd
mdev_private.h 1.84 KB