linux-kernel - [for-next][PATCH 2/2] ring-buffer: Make reading page consistent with the code logic

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Message-ID: <20250116020313.739097483@goodmis.org>
Date: Wed, 15 Jan 2025 21:02:57 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: linux-kernel@...r.kernel.org
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Jeongjun Park <aha310510@...il.com>
Subject: [for-next][PATCH 2/2] ring-buffer: Make reading page consistent with the code logic

From: Jeongjun Park <aha310510@...il.com>

In the loop of __rb_map_vma(), the 's' variable is calculated from the
same logic that nr_pages is and they both come from nr_subbufs. But the
relationship is not obvious and there's a WARN_ON_ONCE() around the 's'
variable to make sure it never becomes equal to nr_subbufs within the
loop. If that happens, then the code is buggy and needs to be fixed.

The 'page' variable is calculated from cpu_buffer->subbuf_ids[s] which is
an array of 'nr_subbufs' entries. If the code becomes buggy and 's'
becomes equal to or greater than 'nr_subbufs' then this will be an out of
bounds hit before the WARN_ON() is triggered and the code exiting safely.

Make the 'page' initialization consistent with the code logic and assign
it after the out of bounds check.

Link: https://lore.kernel.org/20250110162612.13983-1-aha310510@gmail.com
Signed-off-by: Jeongjun Park <aha310510@...il.com>
[ sdr: rewrote change log ]
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 kernel/trace/ring_buffer.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 479bbbcbf61b..6d61ff78926b 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7028,7 +7028,7 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
 	}

 	while (p < nr_pages) {
-		struct page *page = virt_to_page((void *)cpu_buffer->subbuf_ids[s]);
+		struct page *page;
 		int off = 0;

 		if (WARN_ON_ONCE(s >= nr_subbufs)) {
@@ -7036,6 +7036,8 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
 			goto out;
 		}

+		page = virt_to_page((void *)cpu_buffer->subbuf_ids[s]);
+
 		for (; off < (1 << (subbuf_order)); off++, page++) {
 			if (p >= nr_pages)
 				break;
-- 
2.45.2