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 <[email protected]>
> Reported-by: syzbot
> <syzbot+4684a000d5abdade83fac55b1e7d1f935ef19...@syzkaller.appspotmail.com>
> ---
> 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 <[email protected]>
SUSE Labs, CR
