Code: static void audit_log_execve_info(struct audit_context *context,
				  struct audit_buffer **ab)
{
	int i, len;
	size_t len_sent = 0;
	const char __user *p;
	char *buf;
 
	p = (const char __user *)current->mm->arg_start;
 
	audit_log_format(*ab, "argc=%d", context->execve.argc);
 
	/*
	 * we need some kernel buffer to hold the userspace args.  Just
	 * allocate one big one rather than allocating one of the right size
	 * for every single argument inside audit_log_single_execve_arg()
	 * should be <8k allocation so should be pretty safe.
	 */
	buf = kmalloc(MAX_EXECVE_AUDIT_LEN + 1, GFP_KERNEL);
	if (!buf) {
		audit_panic("out of memory for argv string");
		return;
	}
 
	for (i = 0; i < context->execve.argc; i++) {
		len = audit_log_single_execve_arg(context, ab, i,
						  &len_sent, p, buf);
		if (len <= 0)
			break;
		p += len;
	}
	kfree(buf);
 }

Vulnerability Type: Bypass 
CWE ID: CWE-362
Summary: Race condition in the audit_log_single_execve_arg function in kernel/auditsc.c in the Linux kernel through 4.7 allows local users to bypass intended character-set restrictions or disrupt system-call auditing by changing a certain string, aka a *double fetch* vulnerability.
Commit Message: audit: fix a double fetch in audit_log_single_execve_arg()

There is a double fetch problem in audit_log_single_execve_arg()
where we first check the execve(2) argumnets for any "bad" characters
which would require hex encoding and then re-fetch the arguments for
logging in the audit record[1].  Of course this leaves a window of
opportunity for an unsavory application to munge with the data.

This patch reworks things by only fetching the argument data once[2]
into a buffer where it is scanned and logged into the audit
records(s).  In addition to fixing the double fetch, this patch
improves on the original code in a few other ways: better handling
of large arguments which require encoding, stricter record length
checking, and some performance improvements (completely unverified,
but we got rid of some strlen() calls, that's got to be a good
thing).

As part of the development of this patch, I've also created a basic
regression test for the audit-testsuite, the test can be tracked on
GitHub at the following link:

 * https://github.com/linux-audit/audit-testsuite/issues/25

[1] If you pay careful attention, there is actually a triple fetch
problem due to a strnlen_user() call at the top of the function.

[2] This is a tiny white lie, we do make a call to strnlen_user()
prior to fetching the argument data.  I don't like it, but due to the
way the audit record is structured we really have no choice unless we
copy the entire argument at once (which would require a rather
wasteful allocation).  The good news is that with this patch the
kernel no longer relies on this strnlen_user() value for anything
beyond recording it in the log, we also update it with a trustworthy
value whenever possible.

Reported-by: Pengfei Wang <wpengfeinudt@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>