iomap: add a workaround for racy i_size updates on block devices

A szybot reproducer that does write I/O while truncating the size of a
block device can end up in clean_bdev_aliases, which tries to clean the
bdev aliases that it uses.  This is because iomap_to_bh automatically
sets the BH_New flag when outside of i_size.  For block devices updates
to i_size are racy and we can hit this case in a tiny race window,
leading to the eventual clean_bdev_aliases call.  Fix this by erroring
out of > i_size I/O on block devices.

Reported-by: syzbot+1fa947e7f09e136925b8@syzkaller.appspotmail.com
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: syzbot+1fa947e7f09e136925b8@syzkaller.appspotmail.com
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

Author: Christoph Hellwig

fs/buffer.c

@@ -2058,8 +2058,17 @@ iomap_to_bh(struct inode *inode, sector_t block, struct buffer_head *bh,
 		fallthrough;
 	case IOMAP_MAPPED:
 		if ((iomap->flags & IOMAP_F_NEW) ||
-		    offset >= i_size_read(inode))
+		    offset >= i_size_read(inode)) {
+			/*
+			 * This can happen if truncating the block device races
+			 * with the check in the caller as i_size updates on
+			 * block devices aren't synchronized by i_rwsem for
+			 * block devices.
+			 */
+			if (S_ISBLK(inode->i_mode))
+				return -EIO;
 			set_buffer_new(bh);
+		}
 		bh->b_blocknr = (iomap->addr + offset - iomap->offset) >>
 				inode->i_blkbits;
 		set_buffer_mapped(bh);