[PATCH bpf-next v1 10/13] bpf: lsm: Handle attachment of the same program

[KP Singh]

From: KP Singh <kpsingh at google.com>

Allow userspace to attach a newer version of a program without having
duplicates of the same program.

If BPF_F_ALLOW_OVERRIDE is passed, the attachment logic compares the
name of the new program to the names of existing attached programs. The
names are only compared till a "__" (or '\0', if there is no "__"). If
a successful match is found, the existing program is replaced with the
newer attachment.

./loader Attaches "env_dumper__v1" followed by "env_dumper__v2"
to the bprm_check_security hook..

./loader
./loader

Before:

  cat /sys/kernel/security/bpf/process_execution
  env_dumper__v1
  env_dumper__v2

After:

  cat /sys/kernel/security/bpf/process_execution
  env_dumper__v2

Signed-off-by: KP Singh <kpsingh at google.com>
---
 security/bpf/ops.c | 57 +++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 56 insertions(+), 1 deletion(-)

diff --git a/security/bpf/ops.c b/security/bpf/ops.c
index e9aae2ce718c..481e6ee75f27 100644
--- a/security/bpf/ops.c
+++ b/security/bpf/ops.c
@@ -64,11 +64,52 @@ static struct bpf_lsm_hook *get_hook_from_fd(int fd)
 	return ERR_PTR(ret);
 }
 
+/*
+ * match_prog_name matches the name of the program till "__"
+ * or the end of the string is encountered. This allows
+ * the matched program to be replaced by a newer version.
+ *
+ * For example:
+ *
+ *	env_dumper__v1 is matched with env_dumper__v2
+ *
+ */
+static bool match_prog_name(const char *a, const char *b)
+{
+	size_t m, n;
+	char *end;
+
+	end = strstr(a, "__");
+	n = end ? end - a : strlen(a);
+
+	end = strstr(b, "__");
+	m = end ? end - b : strlen(b);
+
+	if (m != n)
+		return false;
+
+	return strncmp(a, b, n) == 0;
+}
+
+static struct bpf_prog *find_attached_prog(struct bpf_prog_array *array,
+					   struct bpf_prog *prog)
+{
+	struct bpf_prog_array_item *item = array->items;
+
+	for (; item->prog; item++) {
+		if (match_prog_name(item->prog->aux->name, prog->aux->name))
+			return item->prog;
+	}
+
+	return NULL;
+}
+
 int bpf_lsm_attach(const union bpf_attr *attr, struct bpf_prog *prog)
 {
 	struct bpf_prog_array *old_array;
 	struct bpf_prog_array *new_array;
 	struct bpf_lsm_hook *h;
+	struct bpf_prog *old_prog = NULL;
 	int ret = 0;
 
 	h = get_hook_from_fd(attr->target_fd);
@@ -78,13 +119,27 @@ int bpf_lsm_attach(const union bpf_attr *attr, struct bpf_prog *prog)
 	mutex_lock(&h->mutex);
 	old_array = rcu_dereference_protected(h->progs,
 					      lockdep_is_held(&h->mutex));
+	/*
+	 * Check if a matching program already exists and replace
+	 * the existing program if BPF_F_ALLOW_OVERRIDE is specified in
+	 * the attach flags.
+	 */
+	if (old_array) {
+		old_prog = find_attached_prog(old_array, prog);
+		if (old_prog && !(attr->attach_flags & BPF_F_ALLOW_OVERRIDE)) {
+			ret = -EEXIST;
+			goto unlock;
+		}
+	}
 
-	ret = bpf_prog_array_copy(old_array, NULL, prog, &new_array);
+	ret = bpf_prog_array_copy(old_array, old_prog, prog, &new_array);
 	if (ret < 0)
 		goto unlock;
 
 	rcu_assign_pointer(h->progs, new_array);
 	bpf_prog_array_free(old_array);
+	if (old_prog)
+		bpf_prog_put(old_prog);
 
 unlock:
 	mutex_unlock(&h->mutex);
-- 
2.20.1