tracing: Prevent hist_field_var_ref() from accessing NULL tracing_map_elts [Linux 5.2]

tracing: Prevent hist_field_var_ref() from accessing NULL tracing_map_elts [Linux 5.2]

This Linux kernel change "tracing: Prevent hist_field_var_ref() from accessing NULL tracing_map_elts" is included in the Linux 5.2 release. This change is authored by Tom Zanussi <tom.zanussi [at] linux.intel.com> on Thu Apr 18 10:18:50 2019 -0500. The commit for this change in Linux stable tree is 55267c8 (patch).

tracing: Prevent hist_field_var_ref() from accessing NULL tracing_map_elts

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/64ec[email protected]linux.intel.com

Reported-by: Vincent Bernat <[email protected]>
Signed-off-by: Tom Zanussi <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>

There are 3 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 kernel/trace/trace_events_hist.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 7fca345..06e7b9f 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -1854,6 +1854,9 @@ 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];

Leave a Reply

Your email address will not be published. Required fields are marked *