bridge: check brport attr show in brport_show
Now br_sysfs_if file flush doesn't have attr show. To read it will cause kernel panic after users chmod u+r this file. Xiong found this issue when running the commands: ip link add br0 type bridge ip link add type veth ip link set veth0 master br0 chmod u+r /sys/devices/virtual/net/veth0/brport/flush timeout 3 cat /sys/devices/virtual/net/veth0/brport/flush kernel crashed with NULL a pointer dereference call trace. This patch is to fix it by return -EINVAL when brport_attr->show is null, just the same as the check for brport_attr->store in brport_store(). Fixes: 9cf63747 ("bridge: add sysfs hook to flush forwarding table") Reported-by: NXiong Zhou <xzhou@redhat.com> Signed-off-by: NXin Long <lucien.xin@gmail.com> Signed-off-by: NDavid S. Miller <davem@davemloft.net>
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