tracing: Add barrier to trace_printk() buffer nesting modification

BugLink: http://bugs.launchpad.net/bugs/1720154
commit 3d9622c12c8873911f4cc0ccdabd0362c2fca06b upstream.

trace_printk() uses 4 buffers, one for each context (normal, softirq, irq
and NMI), such that it does not need to worry about one context preempting
the other. There's a nesting counter that gets incremented to figure out
which buffer to use. If the context gets preempted by another context which
calls trace_printk() it will increment the counter and use the next buffer,
and restore the counter when it is finished.

The problem is that gcc may optimize the modification of the buffer nesting
counter and it may not be incremented in memory before the buffer is used.
If this happens, and the context gets interrupted by another context, it
could pick the same buffer and corrupt the one that is being used.

Compiler barriers need to be added after the nesting variable is incremented
and before it is decremented to prevent usage of the context buffers by more
than one context at the same time.

Cc: Andy Lutomirski <luto@kernel.org>
Fixes: e2ace00117 ("tracing: Choose static tp_printk buffer by explicit nesting count")
Hat-tip-to: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 44004d8aa3b33f259ed90dd13ce4c102034da23a..4373d71bb4fd0b851c2a838aef31f63a2f779044 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2799,11 +2799,17 @@ static char *get_trace_buf(void)
        if (!buffer || buffer->nesting >= 4)
                return NULL;
 
-       return &buffer->buffer[buffer->nesting++][0];
+       buffer->nesting++;
+
+       /* Interrupts must see nesting incremented before we use the buffer */
+       barrier();
+       return &buffer->buffer[buffer->nesting][0];
 }
 
 static void put_trace_buf(void)
 {
+       /* Don't let the decrement of nesting leak before this */
+       barrier();
        this_cpu_dec(trace_percpu_buffer->nesting);
 }