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MacOS/iOS kernel double free due to incorrect API usage in flow divert socket option handling(CVE-2017-13867)

Description

SO_FLOW_DIVERT_TOKEN is a socket option on the `SOL_SOCKET`layer. It's implemented by ``` flow_divert_token_set(struct socket *so, struct sockopt *sopt) in flow_divert.c. The relevant code is: error = soopt_getm(sopt, &token); if (error) { goto done; } error = soopt_mcopyin(sopt, token); if (error) { goto done; } ... done: if (token != NULL) { mbuf_freem(token); } ``` `soopt_getm` allocates an mbuf. `soopt_mcopyin`, which should copyin the data for the mbuf from userspace, has the following code: ``` error = copyin(sopt->sopt_val, mtod(m, char *), m->m_len); if (error != 0) { m_freem(m0); return (error); } ``` This means that if the copyin fails, by for example providing an invalid userspace pointer, soopt_mcopyin will free the mbuf. `flow_divert_token_set` isn't aware of these semantics and if it sees that soopt_mcopyin returns an error it also calls mbuf_freem on that same mbuf which `soopy_mcopyin` already freed. mbufs are aggressivly cached but with sufficiently full caches m_freem will eventually fall through to freeing back to a zalloc zone, and that zone could potentially be garbage collected leading to the ability to actually exploit such an issue. This PoC will just hit a panic inside m_free when it detects a double-free but do note that this cannot detect all double frees and this issue is still exploitable with sufficient grooming/cache manipulation. Tested on MacOS 10.13 (17A365) on MacBookAir5,2


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