Messages in this thread Patch in this message | | | From | Desmond Cheong Zhi Xi <> | Subject | [PATCH v2] mtd: break circular locks in register_mtd_blktrans | Date | Fri, 18 Jun 2021 00:09:04 +0800 |
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Syzbot reported a circular locking dependency: https://syzkaller.appspot.com/bug?id=7bd106c28e846d1023d4ca915718b1a0905444cb
This happens because of the following lock dependencies:
1. loop_ctl_mutex -> bdev->bd_mutex (when loop_control_ioctl calls loop_remove, which then calls del_gendisk; this also happens in loop_exit which eventually calls loop_remove)
2. bdev->bd_mutex -> mtd_table_mutex (when blkdev_get_by_dev calls __blkdev_get, which then calls blktrans_open)
3. mtd_table_mutex -> major_names_lock (when register_mtd_blktrans calls __register_blkdev)
4. major_names_lock -> loop_ctl_mutex (when blk_request_module calls loop_probe)
Hence there's an overall dependency of:
loop_ctl_mutex ----------> bdev->bd_mutex ^ | | | | v major_names_lock <--------- mtd_table_mutex
We can break this circular dependency by holding mtd_table_mutex only for the required critical section in register_mtd_blktrans. This avoids the mtd_table_mutex -> major_names_lock dependency.
Reported-and-tested-by: syzbot+6a8a0d93c91e8fbf2e80@syzkaller.appspotmail.com Co-developed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com> ---
Changes in v1 -> v2:
Break the circular dependency in register_mtd_blktrans instead of blk_request_module, as suggested by Christoph Hellwig.
drivers/mtd/mtd_blkdevs.c | 8 ++------ 1 file changed, 2 insertions(+), 6 deletions(-)
diff --git a/drivers/mtd/mtd_blkdevs.c b/drivers/mtd/mtd_blkdevs.c index fb8e12d590a1..7d26cfe24d05 100644 --- a/drivers/mtd/mtd_blkdevs.c +++ b/drivers/mtd/mtd_blkdevs.c @@ -528,14 +528,10 @@ int register_mtd_blktrans(struct mtd_blktrans_ops *tr) if (!blktrans_notifier.list.next) register_mtd_user(&blktrans_notifier); - - mutex_lock(&mtd_table_mutex); - ret = register_blkdev(tr->major, tr->name); if (ret < 0) { printk(KERN_WARNING "Unable to register %s block device on major %d: %d\n", tr->name, tr->major, ret); - mutex_unlock(&mtd_table_mutex); return ret; } @@ -545,12 +541,12 @@ int register_mtd_blktrans(struct mtd_blktrans_ops *tr) tr->blkshift = ffs(tr->blksize) - 1; INIT_LIST_HEAD(&tr->devs); - list_add(&tr->list, &blktrans_majors); + mutex_lock(&mtd_table_mutex); + list_add(&tr->list, &blktrans_majors); mtd_for_each_device(mtd) if (mtd->type != MTD_ABSENT) tr->add_mtd(tr, mtd); - mutex_unlock(&mtd_table_mutex); return 0; } -- 2.25.1
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