Re: [PATCH v4 1/9] ring-buffer: Free cpu_buffer->free_page with subbuf_order

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [Critical] Concurrent execution of sub-buffer order resizing (`ring_buffer_subbuf_order_set`) and read operations (`ring_buffer_alloc_read_page`, `ring_buffer_read_page`) can lead to out-of-bounds writes, use-after-free, and page allocator corruption due to mismatched synchronization.
--

--- Patch [1]: [PATCH v4 1/9] ring-buffer: Free cpu_buffer->free_page with subbuf_order ---

commit 4100674474d6bd293fb872fecb17fc781ca58502
Author: Vincent Donnefort [off-list ref]

ring-buffer: Free cpu_buffer->free_page with subbuf_order
    
When sub-buffers use an order greater than 0, cpu_buffer->free_page is
allocated with subbuf_order. Use the correct order for
cpu_buffer->free_page.

quoted

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 2667992f0aa29..a3d28b2e2c94a 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c

@@ -2631,7 +2631,7 @@ static void rb_free_cpu_buffer(struct ring_buffer_per_cpu *cpu_buffer)
 		free_buffer_page(bpage);
 	}
 
-	free_page((unsigned long)cpu_buffer->free_page);
+	free_pages((unsigned long)cpu_buffer->free_page, cpu_buffer->buffer->subbuf_order);

[Severity: Critical]
This isn't a bug introduced by this patch, but is it possible for a race
condition between resizing and reading to lead to memory corruption?

If thread A changes the sub-buffer size via
/sys/kernel/tracing/buffer_subbuf_size_kb, ring_buffer_subbuf_order_set()
updates buffer->subbuf_order locklessly globally before tearing down the CPU
buffers and clearing their free_page.

If thread B concurrently reads /sys/kernel/tracing/per_cpu/cpu0/trace_pipe_raw
via tracing_buffers_read(), it observes the new order and calls
ring_buffer_alloc_read_page().

Because ring_buffer_subbuf_order_set() hasn't yet cleared
cpu_buffer->free_page for this CPU (they use distinct locks), thread B pops
the old page from free_page but tags it with the new order:

kernel/trace/ring_buffer.c:ring_buffer_alloc_read_page() {
    ...
    bpage->order = buffer->subbuf_order;
    ...
    if (cpu_buffer->free_page) {
        bpage->data = cpu_buffer->free_page;
        cpu_buffer->free_page = NULL;
    }
    ...
}

Thread B then calls ring_buffer_read_page(). The lockless check passes
because both match the new order:

kernel/trace/ring_buffer.c:ring_buffer_read_page() {
    ...
    if (data_page->order != buffer->subbuf_order)
        return -1;
    ...
    reader->page = data_page->data;
}

Could this allow the undersized old page to be swapped into the ring buffer
as the reader_page, leading to an out-of-bounds write when a writer attempts
to write large events? 

It seems it could also cause page allocator corruption when a smaller page
is later passed to free_pages() with a larger order.

quoted

 	kfree(cpu_buffer);
 }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260812153311.2328812-1-vdonnefort@google.com?part=1