erofs: include the trailing NUL in FS_IOC_GETFSLABEL

erofs_ioctl_get_volume_label() passes strlen(sbi->volume_name) as
the length to copy_to_user(), which copies the label string without
the trailing NUL byte.  Since FS_IOC_GETFSLABEL callers expect a
NUL-terminated string in the FSLABEL_MAX-sized buffer and may not
pre-zero the buffer, this can cause userspace to read past the label
into uninitialised stack memory.

Fix this by using strlen() + 1 to include the NUL terminator,
consistent with how ext4 and xfs implement FS_IOC_GETFSLABEL.

Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Fixes: 1cf12c7177 ("erofs: Add support for FS_IOC_GETFSLABEL")
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chunhai Guo <guochunhai@vivo.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/inode.c

@@ -351,7 +351,7 @@ static int erofs_ioctl_get_volume_label(struct inode *inode, void __user *arg)
 		ret = clear_user(arg, 1);
 	else
 		ret = copy_to_user(arg, sbi->volume_name,
-				   strlen(sbi->volume_name));
+				   strlen(sbi->volume_name) + 1);
 	return ret ? -EFAULT : 0;
 }