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loop: reduce the loop_ctl_mutex scope
syzbot is reporting circular locking problem at __loop_clr_fd() [1], for commit a160c61 ("block: add an optional probe callback to major_names") is calling the module's probe function with major_names_lock held. Fortunately, since commit 990e781 ("block: loop: fix deadlock between open and remove") stopped holding loop_ctl_mutex in lo_open(), current role of loop_ctl_mutex is to serialize access to loop_index_idr and loop_add()/loop_remove(); in other words, management of id for IDR. To avoid holding loop_ctl_mutex during whole add/remove operation, use a bool flag to indicate whether the loop device is ready for use. loop_unregister_transfer() which is called from cleanup_cryptoloop() currently has possibility of use-after-free problem due to lack of serialization between kfree() from loop_remove() from loop_control_remove() and mutex_lock() from unregister_transfer_cb(). But since lo->lo_encryption should be already NULL when this function is called due to module unload, and commit 222013f ("cryptoloop: add a deprecation warning") indicates that we will remove this function shortly, this patch updates this function to emit warning instead of checking lo->lo_encryption. Holding loop_ctl_mutex in loop_exit() is pointless, for all users must close /dev/loop-control and /dev/loop$num (in order to drop module's refcount to 0) before loop_exit() starts, and nobody can open /dev/loop-control or /dev/loop$num afterwards. Link: https://syzkaller.appspot.com/bug?id=7bb10e8b62f83e4d445cdf4c13d69e407e629558 [1] Reported-by: syzbot <syzbot+f61766d5763f9e7a118f@syzkaller.appspotmail.com> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> Reviewed-by: Christoph Hellwig <hch@lst.de> Link: https://lore.kernel.org/r/adb1e792-fc0e-ee81-7ea0-0906fc36419d@i-love.sakura.ne.jp Signed-off-by: Jens Axboe <axboe@kernel.dk>
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