tracing: Prevent hist_field_var_ref() from accessing NULL tracing_map_elts

[ Upstream commit 55267c88c003a3648567beae7c90512d3e2ab15e ]

hist_field_var_ref() is an implementation of hist_field_fn_t(), which
can be called with a null tracing_map_elt elt param when assembling a
key in event_hist_trigger().

In the case of hist_field_var_ref() this doesn't make sense, because a
variable can only be resolved by looking it up using an already
assembled key i.e. a variable can't be used to assemble a key since
the key is required in order to access the variable.

Upper layers should prevent the user from constructing a key using a
variable in the first place, but in case one slips through, it
shouldn't cause a NULL pointer dereference.  Also if one does slip
through, we want to know about it, so emit a one-time warning in that
case.

Link: http://lkml.kernel.org/r/64ec8dc15c14d305295b64cdfcc6b2b9dd14753f.1555597045.git.tom.zanussi@linux.intel.com
Reported-by: Vincent Bernat <vincent@bernat.ch>
Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 11853e90b64972e176bb92f47fcb86429505aa2e..3f34cfb66a85f2742d368854ab5a4bf666a766f3 100644 (file)
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -1632,6 +1632,9 @@ static u64 hist_field_var_ref(struct hist_field *hist_field,
        struct hist_elt_data *elt_data;
        u64 var_val = 0;
 
+       if (WARN_ON_ONCE(!elt))
+               return var_val;
+
        elt_data = elt->private_data;
        var_val = elt_data->var_ref_vals[hist_field->var_ref_idx];