On 14/09/2016 21:07, Jann Horn wrote:

On Wed, Sep 14, 2016 at 09:24:00AM +0200, Mickaël Salaün wrote:

quoted

Add eBPF functions to compare file system access with a Landlock file
system handle:
* bpf_landlock_cmp_fs_prop_with_struct_file(prop, map, map_op, file)
  This function allows to compare the dentry, inode, device or mount
  point of the currently accessed file, with a reference handle.
* bpf_landlock_cmp_fs_beneath_with_struct_file(opt, map, map_op, file)
  This function allows an eBPF program to check if the current accessed
  file is the same or in the hierarchy of a reference handle.

[...]

quoted

diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
index 94256597eacd..edaab4c87292 100644
--- a/kernel/bpf/arraymap.c
+++ b/kernel/bpf/arraymap.c

@@ -603,6 +605,9 @@ static void landlock_put_handle(struct map_landlock_handle *handle)
 	enum bpf_map_handle_type handle_type = handle->type;
 
 	switch (handle_type) {
+	case BPF_MAP_HANDLE_TYPE_LANDLOCK_FS_FD:
+		path_put(&handle->path);
+		break;
 	case BPF_MAP_HANDLE_TYPE_UNSPEC:
 	default:
 		WARN_ON(1);

[...]

quoted

diff --git a/security/landlock/checker_fs.c b/security/landlock/checker_fs.c
new file mode 100644
index 000000000000..39eb85dc7d18
--- /dev/null
+++ b/security/landlock/checker_fs.c

[...]

quoted

+static inline u64 bpf_landlock_cmp_fs_prop_with_struct_file(u64 r1_property,
+		u64 r2_map, u64 r3_map_op, u64 r4_file, u64 r5)
+{
+	u8 property = (u8) r1_property;
+	struct bpf_map *map = (struct bpf_map *) (unsigned long) r2_map;
+	enum bpf_map_array_op map_op = r3_map_op;
+	struct file *file = (struct file *) (unsigned long) r4_file;
+	struct bpf_array *array = container_of(map, struct bpf_array, map);
+	struct path *p1, *p2;
+	struct map_landlock_handle *handle;
+	int i;

Please don't use int when iterating over an array, use size_t.

OK, I will use size_t.

quoted

+	/* for now, only handle OP_OR */

Is "OP_OR" an appropriate name for something that ANDs the success of
checks?


[...]

quoted

+	synchronize_rcu();

Can you put a comment here that explains what's going on?

Hum, this should not be here.

quoted

+	for (i = 0; i < array->n_entries; i++) {
+		bool result_dentry = !(property & LANDLOCK_FLAG_FS_DENTRY);
+		bool result_inode = !(property & LANDLOCK_FLAG_FS_INODE);
+		bool result_device = !(property & LANDLOCK_FLAG_FS_DEVICE);
+		bool result_mount = !(property & LANDLOCK_FLAG_FS_MOUNT);
+
+		handle = (struct map_landlock_handle *)
+				(array->value + array->elem_size * i);
+
+		if (handle->type != BPF_MAP_HANDLE_TYPE_LANDLOCK_FS_FD) {
+			WARN_ON(1);
+			return -EFAULT;
+		}
+		p1 = &handle->path;
+
+		if (!result_dentry && p1->dentry == p2->dentry)
+			result_dentry = true;

Why is this safe? As far as I can tell, this is not in an RCU read-side
critical section (synchronize_rcu() was just called), and no lock has been
taken. What prevents someone from removing the arraymap entry while we're
looking at it? Am I missing something?

I will try to properly deal with RCU.