I ran clang's static analyzer over the kernel sources. It identified (apparently, correctly) two places in the ICMP6 code where we could dereference a NULL pointer in the icmp6_input() function.
When processing an ICMP6_ECHO_REQUEST, if IP6_EXTHDR_GET fails, it will set nicmp6 and n to NULL. Therefore, we should condition our modification to nicmp6 on n being not NULL.
And, when processing an ICMP6_WRUREQUEST in the (mode != FQDN) case, if m_dup_pkthdr() fails, the code will set n to NULL. However, the very next line dereferences n. Therefore, when m_dup_pkthdr() fails, we should discontinue further processing and follow the same path as when m_gethdr() fails.
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