oss-security - CVE-2020-10708 kernel: race condition in kernel/audit.c may allow low privilege users trigger kernel panic

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Message-ID: <43b894ae-c437-4d49-bb57-6fa33535fb4e.splendidsky.cwc@alibaba-inc.com>
Date: Fri, 17 Apr 2020 12:40:10 +0800
From: "陈伟宸(田各)" <splendidsky.cwc@...baba-inc.com>
To: "oss-security" <oss-security@...ts.openwall.com>
Subject: CVE-2020-10708 kernel: race condition in kernel/audit.c may allow low privilege users trigger kernel panic


"A race condition was found in the Linux kernel audit subsystem. When the system is configured to panic on events being dropped, an attacker who is able to trigger an audit event that starts while auditd is in the process of starting may be able to cause the system to panic by exploiting a race condition in audit event handling. This creates a denial of service by causing a panic."

https://bugzilla.redhat.com/show_bug.cgi?id=1822593

Env:
    Red Hat Enterprise Linux Server release 7.7 (Maipo)
    3.10.0-1062.12.1.el7.x86_64

Details:
Function audit_log_end and audit_panic may have race conditions when auditd is restarting because audit_pid can be NULL in audit_log_end and then become not NULL in audit_panic, which may allow attackers to trigger kernel panic. Here is panic call stack:


void audit_log_end(struct audit_buffer *ab)
{
    if (!ab)
        return;
    if (!audit_rate_check()) {
        audit_log_lost("rate limit exceeded");
    } else {
        struct nlmsghdr *nlh = nlmsg_hdr(ab->skb);
        nlh->nlmsg_len = ab->skb->len - NLMSG_HDRLEN;

        if (audit_pid) {
            skb_queue_tail(&audit_skb_queue, ab->skb);
            wake_up_interruptible(&kauditd_wait);
        } else {
            audit_printk_skb(ab->skb); // <- audit_pid == NULL when auditd is killed
        }
        ab->skb = NULL;
    }
    audit_buffer_free(ab);
}
-> audit_printk_skb -> audit_log_lost ->
void audit_panic(const char *message)
{
    switch (audit_failure)
    {
    case AUDIT_FAIL_SILENT:
        break;
    case AUDIT_FAIL_PRINTK:
        if (printk_ratelimit())
            printk(KERN_ERR "audit: %s\n", message);
        break;
    case AUDIT_FAIL_PANIC:
        /* test audit_pid since printk is always losey, why bother? */
        if (audit_pid) // <- audit_pid not NULL because auditd is restarting
            panic("audit: %s\n", message);
        break;
    }
}

How to reproduce：
1. set audit-failure to AUDIT_FAIL_PANIC(2) and add a random audit rule like:
[root@...t ~]# cat /etc/audit/rules.d/audit.rules
-D
-b 8192
-f 2
-w /etc/hosts -p rwa -k hosts
2. keep killing auditd and then starting auditd, for example:
while true; do ps aux | grep "/sbin/auditd" | grep -v "grep" | awk '{print $2}' | xargs kill; service auditd start; systemctl reset-failed auditd.service; done
3. log in a low privilege user and keep reading /etc/hosts, for example:
while true; do cat /etc/hosts > /dev/null; done
4. kernel panic will happen within several minutes

Thanks.


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