Hi, On Wed 26-09-18 00:26:49, Tetsuo Handa wrote: > syzbot is reporting circular locking dependency between bdev->bd_mutex > and lo->lo_ctl_mutex [1] which is caused by calling blkdev_reread_part() > with lock held. We need to drop lo->lo_ctl_mutex in order to fix it. > > This patch fixes it by combining loop_index_mutex and loop_ctl_mutex into > loop_mutex, and releasing loop_mutex before calling blkdev_reread_part() > or fput() or path_put() or leaving ioctl(). > > The rule is that current thread calls lock_loop() before accessing > "struct loop_device", and current thread no longer accesses "struct > loop_device" after unlock_loop() is called. > > Since syzbot is reporting various bugs [2] where a race in the loop module > is suspected, let's check whether this patch affects these bugs too. > > [1] https://syzkaller.appspot.com/bug?id=bf154052f0eea4bc7712499e4569505907d15889 > [2] https://syzkaller.appspot.com/bug?id=b3c7e1440aa8ece16bf557dbac427fdff1dad9d6 > > Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> > Reported-by: syzbot <syzbot+4684a000d5abdade83fac55b1e7d1f935ef1936e@xxxxxxxxxxxxxxxxxxxxxxxxx> > --- > drivers/block/loop.c | 187 ++++++++++++++++++++++++++++----------------------- > 1 file changed, 101 insertions(+), 86 deletions(-) I still don't like this patch. I'll post a patch series showing what I have in mind. Admittedly, it's a bit tedious but the locking is much saner afterwards... Honza > > diff --git a/drivers/block/loop.c b/drivers/block/loop.c > index 4b05a27..04389bb 100644 > --- a/drivers/block/loop.c > +++ b/drivers/block/loop.c > @@ -84,12 +84,50 @@ > #include <linux/uaccess.h> > > static DEFINE_IDR(loop_index_idr); > -static DEFINE_MUTEX(loop_index_mutex); > -static DEFINE_MUTEX(loop_ctl_mutex); > +static DEFINE_MUTEX(loop_mutex); > +static void *loop_mutex_owner; /* == __mutex_owner(&loop_mutex) */ > > static int max_part; > static int part_shift; > > +/* > + * lock_loop - Lock loop_mutex. > + */ > +static void lock_loop(void) > +{ > + mutex_lock(&loop_mutex); > + loop_mutex_owner = current; > +} > + > +/* > + * lock_loop_killable - Lock loop_mutex unless killed. > + */ > +static int lock_loop_killable(void) > +{ > + int err = mutex_lock_killable(&loop_mutex); > + > + if (err) > + return err; > + loop_mutex_owner = current; > + return 0; > +} > + > +/* > + * unlock_loop - Unlock loop_mutex as needed. > + * > + * Explicitly call this function before calling fput() or blkdev_reread_part() > + * in order to avoid circular lock dependency. After this function is called, > + * current thread is no longer allowed to access "struct loop_device" memory, > + * for another thread would access that memory as soon as loop_mutex is held. > + */ > +static void unlock_loop(void) > +{ > + if (loop_mutex_owner == current) { > + loop_mutex_owner = NULL; > + mutex_unlock(&loop_mutex); > + } > +} > + > static int transfer_xor(struct loop_device *lo, int cmd, > struct page *raw_page, unsigned raw_off, > struct page *loop_page, unsigned loop_off, > @@ -637,6 +675,7 @@ static void loop_reread_partitions(struct loop_device *lo, > const int count = atomic_read(&lo->lo_refcnt); > > memcpy(filename, lo->lo_file_name, sizeof(filename)); > + unlock_loop(); > > /* > * bd_mutex has been held already in release path, so don't > @@ -699,6 +738,7 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev, > struct file *file, *old_file; > int error; > > + lockdep_assert_held(&loop_mutex); > error = -ENXIO; > if (lo->lo_state != Lo_bound) > goto out; > @@ -737,10 +777,12 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev, > > if (lo->lo_flags & LO_FLAGS_PARTSCAN) > loop_reread_partitions(lo, bdev); > + unlock_loop(); > fput(old_file); > return 0; > > out_putf: > + unlock_loop(); > fput(file); > out: > return error; > @@ -918,6 +960,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode, > int error; > loff_t size; > > + lockdep_assert_held(&loop_mutex); > /* This is safe, since we have a reference from open(). */ > __module_get(THIS_MODULE); > > @@ -991,6 +1034,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode, > return 0; > > out_putf: > + unlock_loop(); > fput(file); > out: > /* This is safe: open() is still holding a reference. */ > @@ -1042,6 +1086,7 @@ static int loop_clr_fd(struct loop_device *lo) > struct block_device *bdev = lo->lo_device; > bool reread; > > + lockdep_assert_held(&loop_mutex); > if (lo->lo_state != Lo_bound) > return -ENXIO; > > @@ -1057,7 +1102,6 @@ static int loop_clr_fd(struct loop_device *lo) > */ > if (atomic_read(&lo->lo_refcnt) > 1) { > lo->lo_flags |= LO_FLAGS_AUTOCLEAR; > - mutex_unlock(&loop_ctl_mutex); > return 0; > } > > @@ -1111,13 +1155,7 @@ static int loop_clr_fd(struct loop_device *lo) > lo->lo_disk->flags |= GENHD_FL_NO_PART_SCAN; > if (reread) > loop_reread_partitions(lo, bdev); > - mutex_unlock(&loop_ctl_mutex); > - /* > - * Need not hold loop_ctl_mutex to fput backing file. > - * Calling fput holding loop_ctl_mutex triggers a circular > - * lock dependency possibility warning as fput can take > - * bd_mutex which is usually taken before loop_ctl_mutex. > - */ > + unlock_loop(); > fput(filp); > return 0; > } > @@ -1129,6 +1167,7 @@ static int loop_clr_fd(struct loop_device *lo) > struct loop_func_table *xfer; > kuid_t uid = current_uid(); > > + lockdep_assert_held(&loop_mutex); > if (lo->lo_encrypt_key_size && > !uid_eq(lo->lo_key_owner, uid) && > !capable(CAP_SYS_ADMIN)) > @@ -1220,10 +1259,9 @@ static int loop_clr_fd(struct loop_device *lo) > struct kstat stat; > int ret; > > - if (lo->lo_state != Lo_bound) { > - mutex_unlock(&loop_ctl_mutex); > + lockdep_assert_held(&loop_mutex); > + if (lo->lo_state != Lo_bound) > return -ENXIO; > - } > > memset(info, 0, sizeof(*info)); > info->lo_number = lo->lo_number; > @@ -1240,10 +1278,10 @@ static int loop_clr_fd(struct loop_device *lo) > lo->lo_encrypt_key_size); > } > > - /* Drop loop_ctl_mutex while we call into the filesystem. */ > + /* Drop loop_mutex while we call into the filesystem. */ > path = lo->lo_backing_file->f_path; > path_get(&path); > - mutex_unlock(&loop_ctl_mutex); > + unlock_loop(); > ret = vfs_getattr(&path, &stat, STATX_INO, AT_STATX_SYNC_AS_STAT); > if (!ret) { > info->lo_device = huge_encode_dev(stat.dev); > @@ -1334,10 +1372,8 @@ static int loop_clr_fd(struct loop_device *lo) > struct loop_info64 info64; > int err; > > - if (!arg) { > - mutex_unlock(&loop_ctl_mutex); > + if (!arg) > return -EINVAL; > - } > err = loop_get_status(lo, &info64); > if (!err) > err = loop_info64_to_old(&info64, &info); > @@ -1352,10 +1388,8 @@ static int loop_clr_fd(struct loop_device *lo) > struct loop_info64 info64; > int err; > > - if (!arg) { > - mutex_unlock(&loop_ctl_mutex); > + if (!arg) > return -EINVAL; > - } > err = loop_get_status(lo, &info64); > if (!err && copy_to_user(arg, &info64, sizeof(info64))) > err = -EFAULT; > @@ -1365,6 +1399,7 @@ static int loop_clr_fd(struct loop_device *lo) > > static int loop_set_capacity(struct loop_device *lo) > { > + lockdep_assert_held(&loop_mutex); > if (unlikely(lo->lo_state != Lo_bound)) > return -ENXIO; > > @@ -1374,6 +1409,8 @@ static int loop_set_capacity(struct loop_device *lo) > static int loop_set_dio(struct loop_device *lo, unsigned long arg) > { > int error = -ENXIO; > + > + lockdep_assert_held(&loop_mutex); > if (lo->lo_state != Lo_bound) > goto out; > > @@ -1387,6 +1424,7 @@ static int loop_set_dio(struct loop_device *lo, unsigned long arg) > > static int loop_set_block_size(struct loop_device *lo, unsigned long arg) > { > + lockdep_assert_held(&loop_mutex); > if (lo->lo_state != Lo_bound) > return -ENXIO; > > @@ -1409,11 +1447,10 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode, > unsigned int cmd, unsigned long arg) > { > struct loop_device *lo = bdev->bd_disk->private_data; > - int err; > + int err = lock_loop_killable(); > > - err = mutex_lock_killable_nested(&loop_ctl_mutex, 1); > if (err) > - goto out_unlocked; > + return err; > > switch (cmd) { > case LOOP_SET_FD: > @@ -1423,10 +1460,7 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode, > err = loop_change_fd(lo, bdev, arg); > break; > case LOOP_CLR_FD: > - /* loop_clr_fd would have unlocked loop_ctl_mutex on success */ > err = loop_clr_fd(lo); > - if (!err) > - goto out_unlocked; > break; > case LOOP_SET_STATUS: > err = -EPERM; > @@ -1436,8 +1470,7 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode, > break; > case LOOP_GET_STATUS: > err = loop_get_status_old(lo, (struct loop_info __user *) arg); > - /* loop_get_status() unlocks loop_ctl_mutex */ > - goto out_unlocked; > + break; > case LOOP_SET_STATUS64: > err = -EPERM; > if ((mode & FMODE_WRITE) || capable(CAP_SYS_ADMIN)) > @@ -1446,8 +1479,7 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode, > break; > case LOOP_GET_STATUS64: > err = loop_get_status64(lo, (struct loop_info64 __user *) arg); > - /* loop_get_status() unlocks loop_ctl_mutex */ > - goto out_unlocked; > + break; > case LOOP_SET_CAPACITY: > err = -EPERM; > if ((mode & FMODE_WRITE) || capable(CAP_SYS_ADMIN)) > @@ -1466,9 +1498,7 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode, > default: > err = lo->ioctl ? lo->ioctl(lo, cmd, arg) : -EINVAL; > } > - mutex_unlock(&loop_ctl_mutex); > - > -out_unlocked: > + unlock_loop(); > return err; > } > > @@ -1582,10 +1612,8 @@ struct compat_loop_info { > struct loop_info64 info64; > int err; > > - if (!arg) { > - mutex_unlock(&loop_ctl_mutex); > + if (!arg) > return -EINVAL; > - } > err = loop_get_status(lo, &info64); > if (!err) > err = loop_info64_to_compat(&info64, arg); > @@ -1600,20 +1628,16 @@ static int lo_compat_ioctl(struct block_device *bdev, fmode_t mode, > > switch(cmd) { > case LOOP_SET_STATUS: > - err = mutex_lock_killable(&loop_ctl_mutex); > - if (!err) { > + err = lock_loop_killable(); > + if (!err) > err = loop_set_status_compat(lo, > (const struct compat_loop_info __user *)arg); > - mutex_unlock(&loop_ctl_mutex); > - } > break; > case LOOP_GET_STATUS: > - err = mutex_lock_killable(&loop_ctl_mutex); > - if (!err) { > + err = lock_loop_killable(); > + if (!err) > err = loop_get_status_compat(lo, > (struct compat_loop_info __user *)arg); > - /* loop_get_status() unlocks loop_ctl_mutex */ > - } > break; > case LOOP_SET_CAPACITY: > case LOOP_CLR_FD: > @@ -1630,6 +1654,7 @@ static int lo_compat_ioctl(struct block_device *bdev, fmode_t mode, > err = -ENOIOCTLCMD; > break; > } > + unlock_loop(); > return err; > } > #endif > @@ -1637,37 +1662,31 @@ static int lo_compat_ioctl(struct block_device *bdev, fmode_t mode, > static int lo_open(struct block_device *bdev, fmode_t mode) > { > struct loop_device *lo; > - int err = 0; > + int err = lock_loop_killable(); > > - mutex_lock(&loop_index_mutex); > + if (err) > + return err; > lo = bdev->bd_disk->private_data; > - if (!lo) { > + if (!lo) > err = -ENXIO; > - goto out; > - } > - > - atomic_inc(&lo->lo_refcnt); > -out: > - mutex_unlock(&loop_index_mutex); > + else > + atomic_inc(&lo->lo_refcnt); > + unlock_loop(); > return err; > } > > static void __lo_release(struct loop_device *lo) > { > - int err; > - > if (atomic_dec_return(&lo->lo_refcnt)) > return; > > - mutex_lock(&loop_ctl_mutex); > + lockdep_assert_held(&loop_mutex); > if (lo->lo_flags & LO_FLAGS_AUTOCLEAR) { > /* > * In autoclear mode, stop the loop thread > * and remove configuration after last close. > */ > - err = loop_clr_fd(lo); > - if (!err) > - return; > + loop_clr_fd(lo); > } else if (lo->lo_state == Lo_bound) { > /* > * Otherwise keep thread (if running) and config, > @@ -1676,15 +1695,13 @@ static void __lo_release(struct loop_device *lo) > blk_mq_freeze_queue(lo->lo_queue); > blk_mq_unfreeze_queue(lo->lo_queue); > } > - > - mutex_unlock(&loop_ctl_mutex); > } > > static void lo_release(struct gendisk *disk, fmode_t mode) > { > - mutex_lock(&loop_index_mutex); > + lock_loop(); > __lo_release(disk->private_data); > - mutex_unlock(&loop_index_mutex); > + unlock_loop(); > } > > static const struct block_device_operations lo_fops = { > @@ -1723,10 +1740,8 @@ static int unregister_transfer_cb(int id, void *ptr, void *data) > struct loop_device *lo = ptr; > struct loop_func_table *xfer = data; > > - mutex_lock(&loop_ctl_mutex); > if (lo->lo_encryption == xfer) > loop_release_xfer(lo); > - mutex_unlock(&loop_ctl_mutex); > return 0; > } > > @@ -1738,8 +1753,14 @@ int loop_unregister_transfer(int number) > if (n == 0 || n >= MAX_LO_CRYPT || (xfer = xfer_funcs[n]) == NULL) > return -EINVAL; > > + /* > + * cleanup_cryptoloop() cannot handle errors because it is called > + * from module_exit(). Thus, don't give up upon SIGKILL here. > + */ > + lock_loop(); > xfer_funcs[n] = NULL; > idr_for_each(&loop_index_idr, &unregister_transfer_cb, xfer); > + unlock_loop(); > return 0; > } > > @@ -1982,20 +2003,18 @@ static int loop_lookup(struct loop_device **l, int i) > static struct kobject *loop_probe(dev_t dev, int *part, void *data) > { > struct loop_device *lo; > - struct kobject *kobj; > - int err; > + struct kobject *kobj = NULL; > + int err = lock_loop_killable(); > > - mutex_lock(&loop_index_mutex); > + *part = 0; > + if (err) > + return NULL; > err = loop_lookup(&lo, MINOR(dev) >> part_shift); > if (err < 0) > err = loop_add(&lo, MINOR(dev) >> part_shift); > - if (err < 0) > - kobj = NULL; > - else > + if (err >= 0) > kobj = get_disk_and_module(lo->lo_disk); > - mutex_unlock(&loop_index_mutex); > - > - *part = 0; > + unlock_loop(); > return kobj; > } > > @@ -2003,9 +2022,11 @@ static long loop_control_ioctl(struct file *file, unsigned int cmd, > unsigned long parm) > { > struct loop_device *lo; > - int ret = -ENOSYS; > + int ret = lock_loop_killable(); > > - mutex_lock(&loop_index_mutex); > + if (ret) > + return ret; > + ret = -ENOSYS; > switch (cmd) { > case LOOP_CTL_ADD: > ret = loop_lookup(&lo, parm); > @@ -2019,21 +2040,15 @@ static long loop_control_ioctl(struct file *file, unsigned int cmd, > ret = loop_lookup(&lo, parm); > if (ret < 0) > break; > - ret = mutex_lock_killable(&loop_ctl_mutex); > - if (ret) > - break; > if (lo->lo_state != Lo_unbound) { > ret = -EBUSY; > - mutex_unlock(&loop_ctl_mutex); > break; > } > if (atomic_read(&lo->lo_refcnt) > 0) { > ret = -EBUSY; > - mutex_unlock(&loop_ctl_mutex); > break; > } > lo->lo_disk->private_data = NULL; > - mutex_unlock(&loop_ctl_mutex); > idr_remove(&loop_index_idr, lo->lo_number); > loop_remove(lo); > break; > @@ -2043,7 +2058,7 @@ static long loop_control_ioctl(struct file *file, unsigned int cmd, > break; > ret = loop_add(&lo, -1); > } > - mutex_unlock(&loop_index_mutex); > + unlock_loop(); > > return ret; > } > @@ -2127,10 +2142,10 @@ static int __init loop_init(void) > THIS_MODULE, loop_probe, NULL, NULL); > > /* pre-create number of devices given by config or max_loop */ > - mutex_lock(&loop_index_mutex); > + lock_loop(); > for (i = 0; i < nr; i++) > loop_add(&lo, i); > - mutex_unlock(&loop_index_mutex); > + unlock_loop(); > > printk(KERN_INFO "loop: module loaded\n"); > return 0; > -- > 1.8.3.1 > -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
