linux-kernel - [PATCH 3.2 138/152] tracing: Move mutex to protect against resetting of seq data

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Message-ID: <lsq.1479082447.578788147@decadent.org.uk>
Date:   Mon, 14 Nov 2016 00:14:07 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Al Viro" <viro@...IV.linux.org.uk>,
        "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
Subject: [PATCH 3.2 138/152] tracing: Move mutex to protect against
 resetting of seq data

3.2.84-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>

commit 1245800c0f96eb6ebb368593e251d66c01e61022 upstream.

The iter->seq can be reset outside the protection of the mutex. So can
reading of user data. Move the mutex up to the beginning of the function.

Fixes: d7350c3f45694 ("tracing/core: make the read callbacks reentrants")
Reported-by: Al Viro <viro@...IV.linux.org.uk>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 kernel/trace/trace.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3331,13 +3331,6 @@ tracing_read_pipe(struct file *filp, cha
 	static struct tracer *old_tracer;
 	ssize_t sret;
 
-	/* return any leftover data */
-	sret = trace_seq_to_user(&iter->seq, ubuf, cnt);
-	if (sret != -EBUSY)
-		return sret;
-
-	trace_seq_init(&iter->seq);
-
 	/* copy the tracer to avoid using a global lock all around */
 	mutex_lock(&trace_types_lock);
 	if (unlikely(old_tracer != current_trace && current_trace)) {
@@ -3352,6 +3345,14 @@ tracing_read_pipe(struct file *filp, cha
 	 * is protected.
 	 */
 	mutex_lock(&iter->mutex);
+
+	/* return any leftover data */
+	sret = trace_seq_to_user(&iter->seq, ubuf, cnt);
+	if (sret != -EBUSY)
+		goto out;
+
+	trace_seq_init(&iter->seq);
+
 	if (iter->trace->read) {
 		sret = iter->trace->read(iter, filp, ubuf, cnt, ppos);
 		if (sret)