On 1/22/2021 4:29 PM, Greg KH wrote:
On Thu, Jan 21, 2021 at 03:37:32PM -0500, Steven Rostedt wrote:
On Thu, 21 Jan 2021 23:15:22 +0300
Denis Efremov <efremov@xxxxxxxxx> wrote:
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);
That could possibly work.
Yes, this will work, As i have tested similar patch for internal testing
for kernel branches like 5.4/4.19.
Ok, so what can I do here? Can someone resend this as a backport to the
other stable kernels in this way so that I can queue it up?
thanks,
greg k-h
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