macOS/iOS - Kernel Double Free due to Incorrect API Usage in Flow Divert Socket Option Handling

EDB-ID: 43328
Author: Google Security Research
Published: 2017-12-12
CVE: CVE-2017-13867
Type: Dos
Platform: Multiple
Aliases: N/A
Advisory/Source: Link
Tags: N/A
Vulnerable App: N/A

 Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1373 

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
*/

// ianbeer

#if 0
MacOS/iOS kernel double free due to incorrect API usage in flow divert socket option handling

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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