The patch below does not apply to the 5.16-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 6050fa4c84cc93ae509f5105f585a429dffc5633 Mon Sep 17 00:00:00 2001 From: Tetsuo Handa penguin-kernel@i-love.sakura.ne.jp Date: Wed, 24 Nov 2021 19:47:40 +0900 Subject: [PATCH] loop: don't hold lo_mutex during __loop_clr_fd()

syzbot is reporting circular locking problem at __loop_clr_fd() [1], for commit 87579e9b7d8dc36e ("loop: use worker per cgroup instead of kworker") is calling destroy_workqueue() with lo->lo_mutex held.

Since all functions where lo->lo_state matters are already checking lo->lo_state with lo->lo_mutex held (in order to avoid racing with e.g. ioctl(LOOP_CTL_REMOVE)), and __loop_clr_fd() can be called from either ioctl(LOOP_CLR_FD) xor close(), lo->lo_state == Lo_rundown is considered as an exclusive lock for __loop_clr_fd(). Therefore, hold lo->lo_mutex inside __loop_clr_fd() only when asserting/updating lo->lo_state.

Since ioctl(LOOP_CLR_FD) depends on lo->lo_state == Lo_bound, a valid lo->lo_backing_file must have been assigned by ioctl(LOOP_SET_FD) or ioctl(LOOP_CONFIGURE). Thus, we can remove lo->lo_backing_file test, and convert __loop_clr_fd() into a void function.

Link: https://syzkaller.appspot.com/bug?extid=63614029dfb79abd4383 [1] Reported-by: syzbot syzbot+63614029dfb79abd4383@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/8ebe3b2e-8975-7f26-0620-7144a3b8b8cd@i-love.sakura... Signed-off-by: Jens Axboe axboe@kernel.dk

diff --git a/drivers/block/loop.c b/drivers/block/loop.c index 0954ea8cf9e3..ba76319b5544 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -1082,13 +1082,10 @@ static int loop_configure(struct loop_device *lo, fmode_t mode, return error; }

-static int __loop_clr_fd(struct loop_device *lo, bool release) +static void __loop_clr_fd(struct loop_device *lo, bool release) { - struct file *filp = NULL; + struct file *filp; gfp_t gfp = lo->old_gfp_mask; - int err = 0; - bool partscan = false; - int lo_number; struct loop_worker *pos, *worker;

/* @@ -1103,17 +1100,14 @@ static int __loop_clr_fd(struct loop_device *lo, bool release) * became visible. */

+ /* + * Since this function is called upon "ioctl(LOOP_CLR_FD)" xor "close() + * after ioctl(LOOP_CLR_FD)", it is a sign of something going wrong if + * lo->lo_state has changed while waiting for lo->lo_mutex. + */ mutex_lock(&lo->lo_mutex); - if (WARN_ON_ONCE(lo->lo_state != Lo_rundown)) { - err = -ENXIO; - goto out_unlock; - } - - filp = lo->lo_backing_file; - if (filp == NULL) { - err = -EINVAL; - goto out_unlock; - } + BUG_ON(lo->lo_state != Lo_rundown); + mutex_unlock(&lo->lo_mutex);

if (test_bit(QUEUE_FLAG_WC, &lo->lo_queue->queue_flags)) blk_queue_write_cache(lo->lo_queue, false, false); @@ -1134,6 +1128,7 @@ static int __loop_clr_fd(struct loop_device *lo, bool release) del_timer_sync(&lo->timer);

spin_lock_irq(&lo->lo_lock); + filp = lo->lo_backing_file; lo->lo_backing_file = NULL; spin_unlock_irq(&lo->lo_lock);

@@ -1153,12 +1148,11 @@ static int __loop_clr_fd(struct loop_device *lo, bool release) module_put(THIS_MODULE); blk_mq_unfreeze_queue(lo->lo_queue);

- partscan = lo->lo_flags & LO_FLAGS_PARTSCAN; - lo_number = lo->lo_number; disk_force_media_change(lo->lo_disk, DISK_EVENT_MEDIA_CHANGE); -out_unlock: - mutex_unlock(&lo->lo_mutex); - if (partscan) { + + if (lo->lo_flags & LO_FLAGS_PARTSCAN) { + int err; + /* * open_mutex has been held already in release path, so don't * acquire it if this function is called in such case. @@ -1174,24 +1168,20 @@ static int __loop_clr_fd(struct loop_device *lo, bool release) mutex_unlock(&lo->lo_disk->open_mutex); if (err) pr_warn("%s: partition scan of loop%d failed (rc=%d)\n", - __func__, lo_number, err); + __func__, lo->lo_number, err); /* Device is gone, no point in returning error */ - err = 0; }

/* * lo->lo_state is set to Lo_unbound here after above partscan has - * finished. - * - * There cannot be anybody else entering __loop_clr_fd() as - * lo->lo_backing_file is already cleared and Lo_rundown state - * protects us from all the other places trying to change the 'lo' - * device. + * finished. There cannot be anybody else entering __loop_clr_fd() as + * Lo_rundown state protects us from all the other places trying to + * change the 'lo' device. */ - mutex_lock(&lo->lo_mutex); lo->lo_flags = 0; if (!part_shift) lo->lo_disk->flags |= GENHD_FL_NO_PART; + mutex_lock(&lo->lo_mutex); lo->lo_state = Lo_unbound; mutex_unlock(&lo->lo_mutex);

@@ -1200,9 +1190,7 @@ static int __loop_clr_fd(struct loop_device *lo, bool release) * lo_mutex triggers a circular lock dependency possibility warning as * fput can take open_mutex which is usually taken before lo_mutex. */ - if (filp) - fput(filp); - return err; + fput(filp); }

static int loop_clr_fd(struct loop_device *lo) @@ -1234,7 +1222,8 @@ static int loop_clr_fd(struct loop_device *lo) lo->lo_state = Lo_rundown; mutex_unlock(&lo->lo_mutex);

- return __loop_clr_fd(lo, false); + __loop_clr_fd(lo, false); + return 0; }

static int