In the Linux kernel, the following vulnerability has been resolved:

s390/ctcm: Prevent XID null dereference

The mpc_validate_xid() function sets grp->saved_xid2->xid2_flag2 to 0x40
to signal XID validation error. If peer XID is NULL or r/w channel
pairing mismatch happens, grp->saved_xid2 is never initialized. An
attempt to set the flag in such case leads to NULL dereference.

Fix this by using the always available priv->xid->xid2_flag2 instead of
grp->saved_xid2->xid2_flag2 for validation errors.