bpf: sanitize bpf tracepoint access

during bpf program loading remember the last byte of ctx access
and at the time of attaching the program to tracepoint check that
the program doesn't access bytes beyond defined in tracepoint fields

This also disallows access to __dynamic_array fields, but can be
relaxed in the future.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 31 insertions, 1 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2e08f8e..58792fe 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -652,8 +652,12 @@ static int check_ctx_access(struct verifier_env *env, int off, int size,
 			    enum bpf_access_type t)
 {
 	if (env->prog->aux->ops->is_valid_access &&
-	    env->prog->aux->ops->is_valid_access(off, size, t))
+	    env->prog->aux->ops->is_valid_access(off, size, t)) {
+		/* remember the offset of last byte accessed in ctx */
+		if (env->prog->aux->max_ctx_offset < off + size)
+			env->prog->aux->max_ctx_offset = off + size;
 		return 0;
+	}
 
 	verbose("invalid bpf_context access off=%d size=%d\n", off, size);
 	return -EACCES;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index e5ffe97..9a01019 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7133,6 +7133,14 @@ static int perf_event_set_bpf_prog(struct perf_event *event, u32 prog_fd)
 		return -EINVAL;
 	}
 
+	if (is_tracepoint) {
+		int off = trace_event_get_offsets(event->tp_event);
+
+		if (prog->aux->max_ctx_offset > off) {
+			bpf_prog_put(prog);
+			return -EACCES;
+		}
+	}
 	event->tp_event->prog = prog;
 
 	return 0;
diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 05ddc08..ced9630 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -204,6 +204,24 @@ static void trace_destroy_fields(struct trace_event_call *call)
 	}
 }
 
+/*
+ * run-time version of trace_event_get_offsets_<call>() that returns the last
+ * accessible offset of trace fields excluding __dynamic_array bytes
+ */
+int trace_event_get_offsets(struct trace_event_call *call)
+{
+	struct ftrace_event_field *tail;
+	struct list_head *head;
+
+	head = trace_get_fields(call);
+	/*
+	 * head->next points to the last field with the largest offset,
+	 * since it was added last by trace_define_field()
+	 */
+	tail = list_first_entry(head, struct ftrace_event_field, link);
+	return tail->offset + tail->size;
+}
+
 int trace_event_raw_init(struct trace_event_call *call)
 {
 	int id;