ring buffer was actually a bit too big:

  size = sizeof(*entry) + cnt;

But cnt includes the 'id' and the buffer data, so adding cnt to the size
of *entry actually allocates too much on the ring buffer.

Change the allocation to:

  size = struct_size(entry, buf, cnt - sizeof(entry->id));

and the memcpy() to unsafe_memcpy() with an added justification.

Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/20251011112032.77be18e4@gandalf.local.home
Fixes: 64cf7d058a00 ("tracing: Have trace_marker use per-cpu data to read user space")
Reported-by: syzbot+9a2ede1643175f350105@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/68e973f5.050a0220.1186a4.0010.GAE@google.com/
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index bbb89206a891..eb256378e65b 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7441,7 +7441,8 @@ static ssize_t write_raw_marker_to_buffer(struct trace_array *tr,
 	ssize_t written;
 	size_t size;
 
-	size = sizeof(*entry) + cnt;
+	/* cnt includes both the entry->id and the data behind it. */
+	size = struct_size(entry, buf, cnt - sizeof(entry->id));
 
 	buffer = tr->array_buffer.buffer;
 
@@ -7455,7 +7456,10 @@ static ssize_t write_raw_marker_to_buffer(struct trace_array *tr,
 		return -EBADF;
 
 	entry = ring_buffer_event_data(event);
-	memcpy(&entry->id, buf, cnt);
+	unsafe_memcpy(&entry->id, buf, cnt,
+		      "id and content already reserved on ring buffer"
+		      "'buf' includes the 'id' and the data."
+		      "'entry' was allocated with cnt from 'id'.");
 	written = cnt;
 
 	__buffer_unlock_commit(buffer, event);FAILED: patch "[PATCH] tracing: Stop fortify-string from warning in" failed to apply to 5.15-stable tree<gregkh@linuxfoundation.org> undefinedrostedt@goodmis.org,akpm@linux-foundation.org,mathieu.desnoyers@efficios.com,mhiramat@kernel.org undefined undefined undefined undefined undefined