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?
Thanks, Denis
On 10/6/20 12:33 PM, Gaurav Kohli wrote:
Below race can come, if trace_open and resize of cpu buffer is running parallely on different cpus CPUX CPUY ring_buffer_resize atomic_read(&buffer->resize_disabled) tracing_open tracing_reset_online_cpus ring_buffer_reset_cpu rb_reset_cpu rb_update_pages remove/insert pages resetting pointer
This race can cause data abort or some times infinte loop in rb_remove_pages and rb_insert_pages while checking pages for sanity.
Take buffer lock to fix this.
Signed-off-by: Gaurav Kohli gkohli@codeaurora.org Cc: stable@vger.kernel.org
Changes since v0: -Addressed Steven's review comments.
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 93ef0ab..15bf28b 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -4866,6 +4866,9 @@ void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu) if (!cpumask_test_cpu(cpu, buffer->cpumask)) return;
- /* prevent another thread from changing buffer sizes */
- mutex_lock(&buffer->mutex);
- atomic_inc(&cpu_buffer->resize_disabled); atomic_inc(&cpu_buffer->record_disabled);
@@ -4876,6 +4879,8 @@ void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu) atomic_dec(&cpu_buffer->record_disabled); atomic_dec(&cpu_buffer->resize_disabled);
- mutex_unlock(&buffer->mutex);
} EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu); @@ -4889,6 +4894,9 @@ void ring_buffer_reset_online_cpus(struct trace_buffer *buffer) struct ring_buffer_per_cpu *cpu_buffer; int cpu;
- /* prevent another thread from changing buffer sizes */
- mutex_lock(&buffer->mutex);
- for_each_online_buffer_cpu(buffer, cpu) { cpu_buffer = buffer->buffers[cpu];
@@ -4907,6 +4915,8 @@ void ring_buffer_reset_online_cpus(struct trace_buffer *buffer) atomic_dec(&cpu_buffer->record_disabled); atomic_dec(&cpu_buffer->resize_disabled); }
- mutex_unlock(&buffer->mutex);
} /**