Jens, any questions?
I'm testing this patch in linux-next.git since next-20210714, and got
no regression reports. I think this patch is ready to be merged.
On 2021/07/06 23:40, Tetsuo Handa wrote:
> Commit 6cc8e7430801fa23 ("loop: scale loop device by introducing per
> device lock") re-opened a race window for NULL pointer dereference at
> loop_validate_file() where commit 310ca162d779efee ("block/loop: Use
> global lock for ioctl() operation.") has closed.
>
> Although we need to guarantee that other loop devices will not change
> during traversal, we can't take remote "struct loop_device"->lo_mutex
> inside loop_validate_file() in order to avoid AB-BA deadlock. Therefore,
> introduce a global lock dedicated for loop_validate_file() which is
> conditionally taken before local "struct loop_device"->lo_mutex is taken.
>
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Fixes: 6cc8e7430801fa23 ("loop: scale loop device by introducing per device lock")
> ---
> Changes in v3:
> Add memory barrier between loop_configure() and loop_validate_file().
> Flush at loop_change_fd() to avoid possible use-after-free.
>
> Changes in v2:
> Minimize lock duration in __loop_clr_fd().
> Add kerneldoc to lock/unlock helper functions.
> Reformat local variables declaration.
>
> drivers/block/loop.c | 128 ++++++++++++++++++++++++++++++++-----------
> 1 file changed, 97 insertions(+), 31 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index cc0e8c39a48b..2a5b3d365250 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -88,6 +88,47 @@
>
> static DEFINE_IDR(loop_index_idr);
> static DEFINE_MUTEX(loop_ctl_mutex);
> +static DEFINE_MUTEX(loop_validate_mutex);
> +
> +/**
> + * loop_global_lock_killable() - take locks for safe loop_validate_file() test
> + *
> + * @lo: struct loop_device
> + * @global: true if @lo is about to bind another "struct loop_device", false otherwise
> + *
> + * Returns 0 on success, -EINTR otherwise.
> + *
> + * Since loop_validate_file() traverses on other "struct loop_device" if
> + * is_loop_device() is true, we need a global lock for serializing concurrent
> + * loop_configure()/loop_change_fd()/__loop_clr_fd() calls.
> + */
> +static int loop_global_lock_killable(struct loop_device *lo, bool global)
> +{
> + int err;
> +
> + if (global) {
> + err = mutex_lock_killable(&loop_validate_mutex);
> + if (err)
> + return err;
> + }
> + err = mutex_lock_killable(&lo->lo_mutex);
> + if (err && global)
> + mutex_unlock(&loop_validate_mutex);
> + return err;
> +}
> +
> +/**
> + * loop_global_unlock() - release locks taken by loop_global_lock_killable()
> + *
> + * @lo: struct loop_device
> + * @global: true if @lo was about to bind another "struct loop_device", false otherwise
> + */
> +static void loop_global_unlock(struct loop_device *lo, bool global)
> +{
> + mutex_unlock(&lo->lo_mutex);
> + if (global)
> + mutex_unlock(&loop_validate_mutex);
> +}
>
> static int max_part;
> static int part_shift;
> @@ -672,13 +713,15 @@ static int loop_validate_file(struct file *file, struct block_device *bdev)
> while (is_loop_device(f)) {
> struct loop_device *l;
>
> + lockdep_assert_held(&loop_validate_mutex);
> if (f->f_mapping->host->i_rdev == bdev->bd_dev)
> return -EBADF;
>
> l = I_BDEV(f->f_mapping->host)->bd_disk->private_data;
> - if (l->lo_state != Lo_bound) {
> + if (l->lo_state != Lo_bound)
> return -EINVAL;
> - }
> + /* Order wrt setting lo->lo_backing_file in loop_configure(). */
> + rmb();
> f = l->lo_backing_file;
> }
> if (!S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode))
> @@ -697,13 +740,18 @@ static int loop_validate_file(struct file *file, struct block_device *bdev)
> static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
> unsigned int arg)
> {
> - struct file *file = NULL, *old_file;
> - int error;
> - bool partscan;
> + struct file *file = fget(arg);
> + struct file *old_file;
> + int error;
> + bool partscan;
> + bool is_loop;
>
> - error = mutex_lock_killable(&lo->lo_mutex);
> + if (!file)
> + return -EBADF;
> + is_loop = is_loop_device(file);
> + error = loop_global_lock_killable(lo, is_loop);
> if (error)
> - return error;
> + goto out_putf;
> error = -ENXIO;
> if (lo->lo_state != Lo_bound)
> goto out_err;
> @@ -713,11 +761,6 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
> if (!(lo->lo_flags & LO_FLAGS_READ_ONLY))
> goto out_err;
>
> - error = -EBADF;
> - file = fget(arg);
> - if (!file)
> - goto out_err;
> -
> error = loop_validate_file(file, bdev);
> if (error)
> goto out_err;
> @@ -740,7 +783,16 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
> loop_update_dio(lo);
> blk_mq_unfreeze_queue(lo->lo_queue);
> partscan = lo->lo_flags & LO_FLAGS_PARTSCAN;
> - mutex_unlock(&lo->lo_mutex);
> + loop_global_unlock(lo, is_loop);
> +
> + /*
> + * Flush loop_validate_file() before fput(), for l->lo_backing_file
> + * might be pointing at old_file which might be the last reference.
> + */
> + if (!is_loop) {
> + mutex_lock(&loop_validate_mutex);
> + mutex_unlock(&loop_validate_mutex);
> + }
> /*
> * We must drop file reference outside of lo_mutex as dropping
> * the file ref can take open_mutex which creates circular locking
> @@ -752,9 +804,9 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
> return 0;
>
> out_err:
> - mutex_unlock(&lo->lo_mutex);
> - if (file)
> - fput(file);
> + loop_global_unlock(lo, is_loop);
> +out_putf:
> + fput(file);
> return error;
> }
>
> @@ -1136,22 +1188,22 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
> struct block_device *bdev,
> const struct loop_config *config)
> {
> - struct file *file;
> - struct inode *inode;
> + struct file *file = fget(config->fd);
> + struct inode *inode;
> struct address_space *mapping;
> - int error;
> - loff_t size;
> - bool partscan;
> - unsigned short bsize;
> + int error;
> + loff_t size;
> + bool partscan;
> + unsigned short bsize;
> + bool is_loop;
> +
> + if (!file)
> + return -EBADF;
> + is_loop = is_loop_device(file);
>
> /* This is safe, since we have a reference from open(). */
> __module_get(THIS_MODULE);
>
> - error = -EBADF;
> - file = fget(config->fd);
> - if (!file)
> - goto out;
> -
> /*
> * If we don't hold exclusive handle for the device, upgrade to it
> * here to avoid changing device under exclusive owner.
> @@ -1162,7 +1214,7 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
> goto out_putf;
> }
>
> - error = mutex_lock_killable(&lo->lo_mutex);
> + error = loop_global_lock_killable(lo, is_loop);
> if (error)
> goto out_bdev;
>
> @@ -1242,6 +1294,9 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
> size = get_loop_size(lo, file);
> loop_set_size(lo, size);
>
> + /* Order wrt reading lo_state in loop_validate_file(). */
> + wmb();
> +
> lo->lo_state = Lo_bound;
> if (part_shift)
> lo->lo_flags |= LO_FLAGS_PARTSCAN;
> @@ -1253,7 +1308,7 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
> * put /dev/loopXX inode. Later in __loop_clr_fd() we bdput(bdev).
> */
> bdgrab(bdev);
> - mutex_unlock(&lo->lo_mutex);
> + loop_global_unlock(lo, is_loop);
> if (partscan)
> loop_reread_partitions(lo);
> if (!(mode & FMODE_EXCL))
> @@ -1261,13 +1316,12 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
> return 0;
>
> out_unlock:
> - mutex_unlock(&lo->lo_mutex);
> + loop_global_unlock(lo, is_loop);
> out_bdev:
> if (!(mode & FMODE_EXCL))
> bd_abort_claiming(bdev, loop_configure);
> out_putf:
> fput(file);
> -out:
> /* This is safe: open() is still holding a reference. */
> module_put(THIS_MODULE);
> return error;
> @@ -1283,6 +1337,18 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
> int lo_number;
> struct loop_worker *pos, *worker;
>
> + /*
> + * Flush loop_configure() and loop_change_fd(). It is acceptable for
> + * loop_validate_file() to succeed, for actual clear operation has not
> + * started yet.
> + */
> + mutex_lock(&loop_validate_mutex);
> + mutex_unlock(&loop_validate_mutex);
> + /*
> + * loop_validate_file() now fails because l->lo_state != Lo_bound
> + * became visible.
> + */
> +
> mutex_lock(&lo->lo_mutex);
> if (WARN_ON_ONCE(lo->lo_state != Lo_rundown)) {
> err = -ENXIO;
>
