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loop: don't hold lo_mutex during __loop_clr_fd()

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syzbot is reporting circular locking problem at __loop_clr_fd() [1], for
commit 87579e9b7d ("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.ne.jp
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Tetsuo Handa 2021-11-24 19:47:40 +09:00 committed by Jens Axboe
parent a30e344132
commit 6050fa4c84
1 changed files with 22 additions and 33 deletions
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55
drivers/block/loop.c
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@ -1082,13 +1082,10 @@ out_putf:
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 @@ out_unlock:
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 @@ out_unlock:
* 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