On 1/21/21 10:09 PM, Steven Rostedt wrote:
> On Thu, 21 Jan 2021 17:30:40 +0300
> Denis Efremov <efremov@xxxxxxxxx> wrote:
>
>> Hi,
>>
>> This patch (CVE-2020-27825) was tagged with
>> Fixes: b23d7a5f4a07a ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU")
>>
>> I'm not an expert here but it seems like b23d7a5f4a07a only refactored
>> ring_buffer_reset_cpu() by introducing reset_disabled_cpu_buffer() without
>> significant changes. Hence, mutex_lock(&buffer->mutex)/mutex_unlock(&buffer->mutex)
>> can be backported further than b23d7a5f4a07a~ and to all LTS kernels. Is
>> b23d7a5f4a07a the actual cause of the bug?
>>
>
> Ug, that looks to be a mistake. Looking back at the thread about this:
>
> https://lore.kernel.org/linux-arm-msm/20200915141304.41fa7c30@xxxxxxxxxxxxxxxxxx/
I see from the link that it was planned to backport the patch to LTS kernels:
> Actually we are seeing issue in older kernel like 4.19/4.14/5.4 and there below patch was not
> present in stable branches:
> Commit b23d7a5f4a07 ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU")
The point is that it's not backported yet. Maybe because of Fixes tag. I've discovered
this while trying to formalize CVE-2020-27825 bug in cvehound
https://github.com/evdenis/cvehound/blob/master/cvehound/cve/CVE-2020-27825.cocci
I think that the backport to the 4.4+ should be something like:
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 547a3a5ac57b..2171b377bbc1 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -4295,6 +4295,8 @@ void ring_buffer_reset_cpu(struct ring_buffer *buffer, int cpu)
if (!cpumask_test_cpu(cpu, buffer->cpumask))
return;
+ mutex_lock(&buffer->mutex);
+
atomic_inc(&buffer->resize_disabled);
atomic_inc(&cpu_buffer->record_disabled);
@@ -4317,6 +4319,8 @@ void ring_buffer_reset_cpu(struct ring_buffer *buffer, int cpu)
atomic_dec(&cpu_buffer->record_disabled);
atomic_dec(&buffer->resize_disabled);
+
+ mutex_unlock(&buffer->mutex);
}
EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu);
Thanks,
Denis
