Re: [PATCH v4 1/9] ring-buffer: Free cpu_buffer->free_page with subbuf_order
From: sashiko-bot@kernel.org
Date: 2026-08-12 15:50:38
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 hunk ↗ jump to hunk
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.
kfree(cpu_buffer); }
-- Sashiko AI review · https://sashiko.dev/#/patchset/20260812153311.2328812-1-vdonnefort@google.com?part=1