tracing: Move mutex to protect against resetting of seq data

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")
Cc: stable@vger.kernel.org # 2.6.30+
Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 8 insertions, 7 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 8a4bd6b68a0b..8fb4847b0450 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -4890,19 +4890,20 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
         struct trace_iterator *iter = filp->private_data;
         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);
-
         /*
          * Avoid more than one consumer on a single file descriptor
          * This is just a matter of traces coherency, the ring buffer itself
          * 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)