tracing: Fix crash from reading trace_pipe with sendfile

commit a29054d9478d0435ab01b7544da4f674ab13f533 upstream.

If tracing contains data and the trace_pipe file is read with sendfile(),
then it can trigger a NULL pointer dereference and various BUG_ON within the
VM code.

There's a patch to fix this in the splice_to_pipe() code, but it's also a
good idea to not let that happen from trace_pipe either.

Link: http://lkml.kernel.org/r/1457641146-9068-1-git-send-email-rabin@rab.in
Reported-by: Rabin Vincent <rabin.vincent@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 0c348a6..f4b93a2 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3565,7 +3565,10 @@ static ssize_t tracing_splice_read_pipe(struct file *filp,
 
        spd.nr_pages = i;
 
-       ret = splice_to_pipe(pipe, &spd);
+       if (i)
+               ret = splice_to_pipe(pipe, &spd);
+       else
+               ret = 0;
 out:
        splice_shrink_spd(&spd);
        return ret;