• Ming Lei's avatar
    block: don't release queue's sysfs lock during switching elevator · b89f625e
    Ming Lei authored
    cecf5d87 ("block: split .sysfs_lock into two locks") starts to
    release & acquire sysfs_lock before registering/un-registering elevator
    queue during switching elevator for avoiding potential deadlock from
    showing & storing 'queue/iosched' attributes and removing elevator's
    kobject.
    
    Turns out there isn't such deadlock because 'q->sysfs_lock' isn't
    required in .show & .store of queue/iosched's attributes, and just
    elevator's sysfs lock is acquired in elv_iosched_store() and
    elv_iosched_show(). So it is safe to hold queue's sysfs lock when
    registering/un-registering elevator queue.
    
    The biggest issue is that commit cecf5d87 assumes that concurrent
    write on 'queue/scheduler' can't happen. However, this assumption isn't
    true, because kernfs_fop_write() only guarantees that concurrent write
    aren't called on the same open file, but the write could be from
    different open on the file. So we can't release & re-acquire queue's
    sysfs lock during switching elevator, otherwise use-after-free on
    elevator could be triggered.
    
    Fixes the issue by not releasing queue's sysfs lock during switching
    elevator.
    
    Fixes: cecf5d87 ("block: split .sysfs_lock into two locks")
    Cc: Christoph Hellwig <hch@infradead.org>
    Cc: Hannes Reinecke <hare@suse.com>
    Cc: Greg KH <gregkh@linuxfoundation.org>
    Cc: Mike Snitzer <snitzer@redhat.com>
    Reviewed-by: default avatarBart Van Assche <bvanassche@acm.org>
    Signed-off-by: default avatarMing Lei <ming.lei@redhat.com>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    b89f625e
elevator.c 18.6 KB