selftests/bpf: Add dynptr helper tests

First test that we allow overwriting dynptr slots and reinitializing
them in unreferenced case, and disallow overwriting for referenced case.
Include tests to ensure slices obtained from destroyed dynptrs are being
invalidated on their destruction. The destruction needs to be scoped, as
in slices of dynptr A should not be invalidated when dynptr B is
destroyed. Next, test that MEM_UNINIT doesn't allow writing dynptr stack
slots.

Acked-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20230121002241.2113993-13-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/progs/dynptr_fail.c

@@ -900,3 +900,195 @@ int dynptr_partial_slot_invalidate(struct __sk_buff *ctx)
 	);
 	return 0;
 }
+
+/* Test that it is allowed to overwrite unreferenced dynptr. */
+SEC("?raw_tp")
+__success
+int dynptr_overwrite_unref(void *ctx)
+{
+	struct bpf_dynptr ptr;
+
+	if (get_map_val_dynptr(&ptr))
+		return 0;
+	if (get_map_val_dynptr(&ptr))
+		return 0;
+	if (get_map_val_dynptr(&ptr))
+		return 0;
+
+	return 0;
+}
+
+/* Test that slices are invalidated on reinitializing a dynptr. */
+SEC("?raw_tp")
+__failure __msg("invalid mem access 'scalar'")
+int dynptr_invalidate_slice_reinit(void *ctx)
+{
+	struct bpf_dynptr ptr;
+	__u8 *p;
+
+	if (get_map_val_dynptr(&ptr))
+		return 0;
+	p = bpf_dynptr_data(&ptr, 0, 1);
+	if (!p)
+		return 0;
+	if (get_map_val_dynptr(&ptr))
+		return 0;
+	/* this should fail */
+	return *p;
+}
+
+/* Invalidation of dynptr slices on destruction of dynptr should not miss
+ * mem_or_null pointers.
+ */
+SEC("?raw_tp")
+__failure __msg("R1 type=scalar expected=percpu_ptr_")
+int dynptr_invalidate_slice_or_null(void *ctx)
+{
+	struct bpf_dynptr ptr;
+	__u8 *p;
+
+	if (get_map_val_dynptr(&ptr))
+		return 0;
+
+	p = bpf_dynptr_data(&ptr, 0, 1);
+	*(__u8 *)&ptr = 0;
+	/* this should fail */
+	bpf_this_cpu_ptr(p);
+	return 0;
+}
+
+/* Destruction of dynptr should also any slices obtained from it */
+SEC("?raw_tp")
+__failure __msg("R7 invalid mem access 'scalar'")
+int dynptr_invalidate_slice_failure(void *ctx)
+{
+	struct bpf_dynptr ptr1;
+	struct bpf_dynptr ptr2;
+	__u8 *p1, *p2;
+
+	if (get_map_val_dynptr(&ptr1))
+		return 0;
+	if (get_map_val_dynptr(&ptr2))
+		return 0;
+
+	p1 = bpf_dynptr_data(&ptr1, 0, 1);
+	if (!p1)
+		return 0;
+	p2 = bpf_dynptr_data(&ptr2, 0, 1);
+	if (!p2)
+		return 0;
+
+	*(__u8 *)&ptr1 = 0;
+	/* this should fail */
+	return *p1;
+}
+
+/* Invalidation of slices should be scoped and should not prevent dereferencing
+ * slices of another dynptr after destroying unrelated dynptr
+ */
+SEC("?raw_tp")
+__success
+int dynptr_invalidate_slice_success(void *ctx)
+{
+	struct bpf_dynptr ptr1;
+	struct bpf_dynptr ptr2;
+	__u8 *p1, *p2;
+
+	if (get_map_val_dynptr(&ptr1))
+		return 1;
+	if (get_map_val_dynptr(&ptr2))
+		return 1;
+
+	p1 = bpf_dynptr_data(&ptr1, 0, 1);
+	if (!p1)
+		return 1;
+	p2 = bpf_dynptr_data(&ptr2, 0, 1);
+	if (!p2)
+		return 1;
+
+	*(__u8 *)&ptr1 = 0;
+	return *p2;
+}
+
+/* Overwriting referenced dynptr should be rejected */
+SEC("?raw_tp")
+__failure __msg("cannot overwrite referenced dynptr")
+int dynptr_overwrite_ref(void *ctx)
+{
+	struct bpf_dynptr ptr;
+
+	bpf_ringbuf_reserve_dynptr(&ringbuf, 64, 0, &ptr);
+	/* this should fail */
+	if (get_map_val_dynptr(&ptr))
+		bpf_ringbuf_discard_dynptr(&ptr, 0);
+	return 0;
+}
+
+/* Reject writes to dynptr slot from bpf_dynptr_read */
+SEC("?raw_tp")
+__failure __msg("potential write to dynptr at off=-16")
+int dynptr_read_into_slot(void *ctx)
+{
+	union {
+		struct {
+			char _pad[48];
+			struct bpf_dynptr ptr;
+		};
+		char buf[64];
+	} data;
+
+	bpf_ringbuf_reserve_dynptr(&ringbuf, 64, 0, &data.ptr);
+	/* this should fail */
+	bpf_dynptr_read(data.buf, sizeof(data.buf), &data.ptr, 0, 0);
+
+	return 0;
+}
+
+/* Reject writes to dynptr slot for uninit arg */
+SEC("?raw_tp")
+__failure __msg("potential write to dynptr at off=-16")
+int uninit_write_into_slot(void *ctx)
+{
+	struct {
+		char buf[64];
+		struct bpf_dynptr ptr;
+	} data;
+
+	bpf_ringbuf_reserve_dynptr(&ringbuf, 80, 0, &data.ptr);
+	/* this should fail */
+	bpf_get_current_comm(data.buf, 80);
+
+	return 0;
+}
+
+static int callback(__u32 index, void *data)
+{
+        *(__u32 *)data = 123;
+
+        return 0;
+}
+
+/* If the dynptr is written into in a callback function, its data
+ * slices should be invalidated as well.
+ */
+SEC("?raw_tp")
+__failure __msg("invalid mem access 'scalar'")
+int invalid_data_slices(void *ctx)
+{
+	struct bpf_dynptr ptr;
+	__u32 *slice;
+
+	if (get_map_val_dynptr(&ptr))
+		return 0;
+
+	slice = bpf_dynptr_data(&ptr, 0, sizeof(__u32));
+	if (!slice)
+		return 0;
+
+	bpf_loop(10, callback, &ptr, 0);
+
+	/* this should fail */
+	*slice = 1;
+
+	return 0;
+}