linux-kernel - [PATCH] tracing: Have process

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Message-ID: <20241231000646.324fb5f7@gandalf.local.home>
Date: Tue, 31 Dec 2024 00:06:46 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: LKML <linux-kernel@...r.kernel.org>, Linux Trace Kernel
 <linux-trace-kernel@...r.kernel.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Genes Lists <lists@...ience.com>, Gene C
 <arch@...ience.com>
Subject: [PATCH] tracing: Have process_string() also allow arrays

From: Steven Rostedt <rostedt@...dmis.org>

In order to catch a common bug where a TRACE_EVENT() TP_fast_assign()
assigns an address of an allocated string to the ring buffer and then
references it in TP_printk(), which can be executed hours later when the
string is free, the function test_event_printk() runs on all events as
they are registered to make sure there's no unwanted dereferencing.

It calls process_string() to handle cases in TP_printk() format that has
"%s". It returns whether or not the string is safe. But it can have some
false positives.

For instance, xe_bo_move() has:

 TP_printk("move_lacks_source:%s, migrate object %p [size %zu] from %s to %s device_id:%s",
            __entry->move_lacks_source ? "yes" : "no", __entry->bo, __entry->size,
            xe_mem_type_to_name[__entry->old_placement],
            xe_mem_type_to_name[__entry->new_placement], __get_str(device_id))

Where the "%s" references into xe_mem_type_to_name[]. This is an array of
pointers that should be safe for the event to access. Instead of flagging
this as a bad reference, if a reference points to an array, where the
record field is the index, consider it safe.

Link: https://lore.kernel.org/all/9dee19b6185d325d0e6fa5f7cbba81d007d99166.camel@sapience.com/

Cc: stable@...r.kernel.org
Fixes: 65a25d9f7ac02 ("tracing: Add "%s" check in test_event_printk()")
Reported-by: Genes Lists <lists@...ience.com>
Tested-by: Gene C <arch@...ience.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 kernel/trace/trace_events.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 1545cc8b49d0..770e7ed91716 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -364,6 +364,18 @@ static bool process_string(const char *fmt, int len, struct trace_event_call *ca
 		s = r + 1;
 	} while (s < e);
 
+	/*
+	 * Check for arrays. If the argument has: foo[REC->val]
+	 * then it is very likely that foo is an array of strings
+	 * that are safe to use.
+	 */
+	r = strstr(s, "[");
+	if (r && r < e) {
+		r = strstr(r, "REC->");
+		if (r && r < e)
+			return true;
+	}
+
 	/*
 	 * If there's any strings in the argument consider this arg OK as it
 	 * could be: REC->field ? "foo" : "bar" and we don't want to get into
-- 
2.45.2