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The sk pointer is initialized to tun->sk before checking if tun is a null pointer. Because null pointer dereferencing is undefined behavior, the compiler (GCC in this case) can optimize away the if (!tun) check because it is performed after tun->sk is accessed, implying that tun is non-null. As a result, this noncompliant code example is vulnerable to a null pointer dereference exploit, because null pointer dereferencing can be permitted on several platforms, for example, by using mmap(2) with the MAP_FIXED flag on Linux and Mac OS X or by using the shmat() POSIX function with the SHM_RND flag [Liu 2009].
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Dereferencing a null pointer is undefined behavior, typically abnormal program termination. In some situations, however, dereferencing a null pointer can lead to the execution of arbitrary code [Jack 2007], [van Sprundel 2006]. The indicated severity is for this more severe case; on platforms where it is not possible to exploit a null pointer dereference to execute arbitrary code, the actual severity is low.
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