linux-kernel - [PATCH] tracing: Fix WARN_ON in tracing_buffers_mmap

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Message-ID: <20251119064019.25904-1-kartikey406@gmail.com>
Date: Wed, 19 Nov 2025 12:10:19 +0530
From: Deepanshu Kartikey <kartikey406@...il.com>
To: rostedt@...dmis.org,
	mhiramat@...nel.org,
	mathieu.desnoyers@...icios.com
Cc: linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org,
	Deepanshu Kartikey <kartikey406@...il.com>,
	syzbot+a72c325b042aae6403c7@...kaller.appspotmail.com
Subject: [PATCH] tracing: Fix WARN_ON in tracing_buffers_mmap_close for split VMAs

When a VMA is split (e.g., by partial munmap or MAP_FIXED), the kernel
calls vm_ops->close on each portion. For trace buffer mappings, this
results in ring_buffer_unmap() being called multiple times while
ring_buffer_map() was only called once.

This causes ring_buffer_unmap() to return -ENODEV on subsequent calls
because user_mapped is already 0, triggering a WARN_ON.

Trace buffer mappings cannot support partial mappings because the ring
buffer structure requires the complete buffer including the meta page.

Fix this by adding a may_split callback that returns -EINVAL to prevent
VMA splits entirely.

Closes: https://syzkaller.appspot.com/bug?extid=a72c325b042aae6403c7
Tested-by: syzbot+a72c325b042aae6403c7@...kaller.appspotmail.com
Reported-by: syzbot+a72c325b042aae6403c7@...kaller.appspotmail.com
Signed-off-by: Deepanshu Kartikey <kartikey406@...il.com>
---
 kernel/trace/trace.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index d1e527cf2aae..304e93597126 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -8781,8 +8781,18 @@ static void tracing_buffers_mmap_close(struct vm_area_struct *vma)
 	put_snapshot_map(iter->tr);
 }
 
+static int tracing_buffers_may_split(struct vm_area_struct *vma, unsigned long addr)
+{
+	/*
+	 * Trace buffer mappings require the complete buffer including
+	 * the meta page. Partial mappings are not supported.
+	 */
+	return -EINVAL;
+}
+
 static const struct vm_operations_struct tracing_buffers_vmops = {
 	.close		= tracing_buffers_mmap_close,
+	.may_split      = tracing_buffers_may_split,
 };
 
 static int tracing_buffers_mmap(struct file *filp, struct vm_area_struct *vma)
-- 
2.43.0