Re: INFO: task hung in __get_super

[Al Viro <viro@zeniv.linux.org.uk>]

On Sun, Apr 28, 2019 at 11:14:06AM -0700, syzbot wrote:
>  down_read+0x49/0x90 kernel/locking/rwsem.c:26
>  __get_super.part.0+0x203/0x2e0 fs/super.c:788
>  __get_super include/linux/spinlock.h:329 [inline]
>  get_super+0x2e/0x50 fs/super.c:817
>  fsync_bdev+0x19/0xd0 fs/block_dev.c:525
>  invalidate_partition+0x36/0x60 block/genhd.c:1581
>  drop_partitions block/partition-generic.c:443 [inline]
>  rescan_partitions+0xef/0xa20 block/partition-generic.c:516
>  __blkdev_reread_part+0x1a2/0x230 block/ioctl.c:173
>  blkdev_reread_part+0x27/0x40 block/ioctl.c:193
>  loop_reread_partitions+0x1c/0x40 drivers/block/loop.c:633
>  loop_set_status+0xe57/0x1380 drivers/block/loop.c:1296
>  loop_set_status64+0xc2/0x120 drivers/block/loop.c:1416
>  lo_ioctl+0x8fc/0x2150 drivers/block/loop.c:1559
>  __blkdev_driver_ioctl block/ioctl.c:303 [inline]
>  blkdev_ioctl+0x6f2/0x1d10 block/ioctl.c:605
>  block_ioctl+0xee/0x130 fs/block_dev.c:1933
>  vfs_ioctl fs/ioctl.c:46 [inline]
>  file_ioctl fs/ioctl.c:509 [inline]
>  do_vfs_ioctl+0xd6e/0x1390 fs/ioctl.c:696
>  ksys_ioctl+0xab/0xd0 fs/ioctl.c:713
>  __do_sys_ioctl fs/ioctl.c:720 [inline]
>  __se_sys_ioctl fs/ioctl.c:718 [inline]
>  __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:718
>  do_syscall_64+0x103/0x610 arch/x86/entry/common.c:290
>  entry_SYSCALL_64_after_hwframe+0x49/0xbe

ioctl(..., BLKRRPART) blocked on ->s_umount in __get_super().
The trouble is, the only things holding ->s_umount appears to be
these:

> 2 locks held by syz-executor274/11716:
>  #0: 00000000a19e2025 (&type->s_umount_key#38/1){+.+.}, at:
> alloc_super+0x158/0x890 fs/super.c:228
>  #1: 00000000bde6230e (loop_ctl_mutex){+.+.}, at: lo_simple_ioctl
> drivers/block/loop.c:1514 [inline]
>  #1: 00000000bde6230e (loop_ctl_mutex){+.+.}, at: lo_ioctl+0x266/0x2150
> drivers/block/loop.c:1572

> 2 locks held by syz-executor274/11717:
>  #0: 00000000e185c083 (&type->s_umount_key#38/1){+.+.}, at:
> alloc_super+0x158/0x890 fs/super.c:228
>  #1: 00000000bde6230e (loop_ctl_mutex){+.+.}, at: lo_simple_ioctl
> drivers/block/loop.c:1514 [inline]
>  #1: 00000000bde6230e (loop_ctl_mutex){+.+.}, at: lo_ioctl+0x266/0x2150
> drivers/block/loop.c:1572

... and that's bollocks.  ->s_umount held there is that on freshly allocated
superblock.  It *MUST* be in mount(2); no other syscall should be able to
call alloc_super() in the first place.  So what the hell is that doing
trying to call lo_ioctl() inside mount(2)?  Something like isofs attempting
cdrom ioctls on the underlying device?

Why do we have loop_func_table->ioctl(), BTW?  All in-tree instances are
either NULL or return -EINVAL unconditionally.  Considering that the
caller is
                err = lo->ioctl ? lo->ioctl(lo, cmd, arg) : -EINVAL;
we could bloody well just get rid of cryptoloop_ioctl() (the only
non-NULL instance) and get rid of calling lo_simple_ioctl() in
lo_ioctl() switch's default.

Something like this:

diff --git a/drivers/block/cryptoloop.c b/drivers/block/cryptoloop.c
index 254ee7d54e91..f16468a562f5 100644
--- a/drivers/block/cryptoloop.c
+++ b/drivers/block/cryptoloop.c
@@ -167,12 +167,6 @@ cryptoloop_transfer(struct loop_device *lo, int cmd,
 }
 
 static int
-cryptoloop_ioctl(struct loop_device *lo, int cmd, unsigned long arg)
-{
-	return -EINVAL;
-}
-
-static int
 cryptoloop_release(struct loop_device *lo)
 {
 	struct crypto_sync_skcipher *tfm = lo->key_data;
@@ -188,7 +182,6 @@ cryptoloop_release(struct loop_device *lo)
 static struct loop_func_table cryptoloop_funcs = {
 	.number = LO_CRYPT_CRYPTOAPI,
 	.init = cryptoloop_init,
-	.ioctl = cryptoloop_ioctl,
 	.transfer = cryptoloop_transfer,
 	.release = cryptoloop_release,
 	.owner = THIS_MODULE
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index bf1c61cab8eb..2ec162b80562 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -955,7 +955,6 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
 	lo->lo_flags = lo_flags;
 	lo->lo_backing_file = file;
 	lo->transfer = NULL;
-	lo->ioctl = NULL;
 	lo->lo_sizelimit = 0;
 	lo->old_gfp_mask = mapping_gfp_mask(mapping);
 	mapping_set_gfp_mask(mapping, lo->old_gfp_mask & ~(__GFP_IO|__GFP_FS));
@@ -1064,7 +1063,6 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
 
 	loop_release_xfer(lo);
 	lo->transfer = NULL;
-	lo->ioctl = NULL;
 	lo->lo_device = NULL;
 	lo->lo_encryption = NULL;
 	lo->lo_offset = 0;
@@ -1262,7 +1260,6 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
 	if (!xfer)
 		xfer = &none_funcs;
 	lo->transfer = xfer->transfer;
-	lo->ioctl = xfer->ioctl;
 
 	if ((lo->lo_flags & LO_FLAGS_AUTOCLEAR) !=
 	     (info->lo_flags & LO_FLAGS_AUTOCLEAR))
@@ -1525,7 +1522,7 @@ static int lo_simple_ioctl(struct loop_device *lo, unsigned int cmd,
 		err = loop_set_block_size(lo, arg);
 		break;
 	default:
-		err = lo->ioctl ? lo->ioctl(lo, cmd, arg) : -EINVAL;
+		err = -EINVAL;
 	}
 	mutex_unlock(&loop_ctl_mutex);
 	return err;
@@ -1567,10 +1564,9 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,
 	case LOOP_SET_BLOCK_SIZE:
 		if (!(mode & FMODE_WRITE) && !capable(CAP_SYS_ADMIN))
 			return -EPERM;
-		/* Fall through */
+		return lo_simple_ioctl(lo, cmd, arg);
 	default:
-		err = lo_simple_ioctl(lo, cmd, arg);
-		break;
+		return -EINVAL;
 	}
 
 	return err;
diff --git a/drivers/block/loop.h b/drivers/block/loop.h
index af75a5ee4094..56a9a0c161d7 100644
--- a/drivers/block/loop.h
+++ b/drivers/block/loop.h
@@ -84,7 +84,6 @@ struct loop_func_table {
 	int (*init)(struct loop_device *, const struct loop_info64 *); 
 	/* release is called from loop_unregister_transfer or clr_fd */
 	int (*release)(struct loop_device *); 
-	int (*ioctl)(struct loop_device *, int cmd, unsigned long arg);
 	struct module *owner;
 };