In the Linux kernel, the following vulnerability has been resolved:

perf/core: Order the PMU list to fix warning about unordered pmu_ctx_list

Syskaller triggers a warning due to prev_epc->pmu != next_epc->pmu in perf_event_swap_task_ctx_data(). vmcore shows that two lists have the same perf_event_pmu_context, but not in the same order.

The problem is that the order of pmu_ctx_list for the parent is impacted by the time when an event/PMU is added. While the order for a child is impacted by the event order in the pinned_groups and flexible_groups. So the order of pmu_ctx_list in the parent and child may be different.

To fix this problem, insert the perf_event_pmu_context to its proper place after iteration of the pmu_ctx_list.

The follow testcase can trigger above warning:

perf record -e cycles –call-graph lbr – taskset -c 3 ./a.out &

perf stat -e cpu-clock,cs -p xxx // xxx is the pid of a.out

test.c

void main() { int count = 0; pid_t pid;

    printf(%d runningn, getpid());
    sleep(30);
    printf(runningn);

    pid = fork();
    if (pid == -1) {
            printf(fork errorn);
            return;
    }
    if (pid == 0) {
            while (1) {
                    count++;
            }
    } else {
            while (1) {
                    count++;
            }
    }

}

The testcase first opens an LBR event, so it will allocate task_ctx_data, and then open tracepoint and software events, so the parent context will have 3 different perf_event_pmu_contexts. On inheritance, child ctx will insert the perf_event_pmu_context in another order and the warning will trigger.

[ mingo: Tidied up the changelog. ]

Weakness

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

Affected Software

Extended Description

A race condition occurs within concurrent environments, and it is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/2016066c66192a99d9e0ebf433789c490a6785a2
• https://git.kernel.org/stable/c/3e812a70732d84b7873cea61a7f6349b9a9dcbf5
• https://git.kernel.org/stable/c/7d582eb6e4e100959ba07083d7563453c8c2a343
• https://git.kernel.org/stable/c/f0c3971405cef6892844016aa710121a02da3a23