删除yum源，重建官方
cd /etc/yum.repos.d/ #删除所有
rpm -Uvh --force http://mirror.centos.org/centos-7/7.7.1908/os/x86_64/Packages/centos-release-7-7.1908.0.el7.centos.x86_64.rpm
ll
yum clean all
yum makecache
cd /etc/yum.repos.d/ #删除所有
rpm -Uvh --force http://mirror.centos.org/centos-7/7.7.1908/os/x86_64/Packages/centos-release-7-7.1908.0.el7.centos.x86_64.rpm
ll
yum clean all
yum makecache