tracing: Fix out of bounds write in get_trace_buf

commit c1acb4ac1a892cf08d27efcb964ad281728b0545 upstream.

The nesting count of trace_printk allows for 4 levels of nesting. The
nesting counter starts at zero and is incremented before being used to
retrieve the current context's buffer. But the index to the buffer uses the
nesting counter after it was incremented, and not its original number,
which in needs to do.

Link: https://lkml.kernel.org/r/20201029161905.4269-1-hqjagain@gmail.com
Cc: stable@vger.kernel.org
Fixes: 3d9622c12c887 ("tracing: Add barrier to trace_printk() buffer nesting modification")
Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index d3e5de717df2fd82062ab7bba5fe2fc7260912ed..6e2fb7dc41bf3184dc1f019e53289e3bd318b281 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3114,7 +3114,7 @@ static char *get_trace_buf(void)
 
        /* Interrupts must see nesting incremented before we use the buffer */
        barrier();
-       return &buffer->buffer[buffer->nesting][0];
+       return &buffer->buffer[buffer->nesting - 1][0];
 }
 
 static void put_trace_buf(void)