[PATCH bpf-next v3 1/6] bpf: Allow passing BTF pointers as PTR_TO_SOCKET

[Lorenz Bauer <lmb@cloudflare.com>]

Tracing programs can derive struct sock pointers from a variety
of sources, e.g. a bpf_iter for sk_storage maps receives one as
part of the context. It's desirable to be able to pass these to
functions that expect PTR_TO_SOCKET. For example, it enables us
to insert such a socket into a sockmap via map_elem_update.

Teach the verifier that a PTR_TO_BTF_ID for a struct sock is
equivalent to PTR_TO_SOCKET. There is one hazard here:
bpf_sk_release also takes a PTR_TO_SOCKET, but expects it to be
refcounted. Since this isn't the case for pointers derived from
BTF we must prevent them from being passed to the function.
Luckily, we can simply check that the ref_obj_id is not zero
in release_reference, and return an error otherwise.

Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
---
 kernel/bpf/verifier.c | 61 +++++++++++++++++++++++++------------------
 1 file changed, 36 insertions(+), 25 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index b4e9c56b8b32..509754c3aa7d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3908,6 +3908,9 @@ static int resolve_map_arg_type(struct bpf_verifier_env *env,
 	return 0;
 }
 
+BTF_ID_LIST(btf_fullsock_ids)
+BTF_ID(struct, sock)
+
 static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 			  struct bpf_call_arg_meta *meta,
 			  const struct bpf_func_proto *fn)
@@ -4000,37 +4003,15 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 		expected_type = PTR_TO_SOCKET;
 		if (!(register_is_null(reg) &&
 		      arg_type == ARG_PTR_TO_SOCKET_OR_NULL)) {
-			if (type != expected_type)
+			if (type != expected_type &&
+			    type != PTR_TO_BTF_ID)
 				goto err_type;
 		}
+		meta->btf_id = btf_fullsock_ids[0];
 	} else if (arg_type == ARG_PTR_TO_BTF_ID) {
-		bool ids_match = false;
-
 		expected_type = PTR_TO_BTF_ID;
 		if (type != expected_type)
 			goto err_type;
-		if (!fn->check_btf_id) {
-			if (reg->btf_id != meta->btf_id) {
-				ids_match = btf_struct_ids_match(&env->log, reg->off, reg->btf_id,
-								 meta->btf_id);
-				if (!ids_match) {
-					verbose(env, "Helper has type %s got %s in R%d\n",
-						kernel_type_name(meta->btf_id),
-						kernel_type_name(reg->btf_id), regno);
-					return -EACCES;
-				}
-			}
-		} else if (!fn->check_btf_id(reg->btf_id, arg)) {
-			verbose(env, "Helper does not support %s in R%d\n",
-				kernel_type_name(reg->btf_id), regno);
-
-			return -EACCES;
-		}
-		if ((reg->off && !ids_match) || !tnum_is_const(reg->var_off) || reg->var_off.value) {
-			verbose(env, "R%d is a pointer to in-kernel struct with non-zero offset\n",
-				regno);
-			return -EACCES;
-		}
 	} else if (arg_type == ARG_PTR_TO_SPIN_LOCK) {
 		if (meta->func_id == BPF_FUNC_spin_lock) {
 			if (process_spin_lock(env, regno, true))
@@ -4085,6 +4066,33 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 		return -EFAULT;
 	}
 
+	if (type == PTR_TO_BTF_ID) {
+		bool ids_match = false;
+
+		if (!fn->check_btf_id) {
+			if (reg->btf_id != meta->btf_id) {
+				ids_match = btf_struct_ids_match(&env->log, reg->off, reg->btf_id,
+								 meta->btf_id);
+				if (!ids_match) {
+					verbose(env, "Helper has type %s got %s in R%d\n",
+						kernel_type_name(meta->btf_id),
+						kernel_type_name(reg->btf_id), regno);
+					return -EACCES;
+				}
+			}
+		} else if (!fn->check_btf_id(reg->btf_id, arg)) {
+			verbose(env, "Helper does not support %s in R%d\n",
+				kernel_type_name(reg->btf_id), regno);
+
+			return -EACCES;
+		}
+		if ((reg->off && !ids_match) || !tnum_is_const(reg->var_off) || reg->var_off.value) {
+			verbose(env, "R%d is a pointer to in-kernel struct with non-zero offset\n",
+				regno);
+			return -EACCES;
+		}
+	}
+
 	if (arg_type == ARG_CONST_MAP_PTR) {
 		/* bpf_map_xxx(map_ptr) call: remember that map_ptr */
 		meta->map_ptr = reg->map_ptr;
@@ -4561,6 +4569,9 @@ static int release_reference(struct bpf_verifier_env *env,
 	int err;
 	int i;
 
+	if (!ref_obj_id)
+		return -EINVAL;
+
 	err = release_reference_state(cur_func(env), ref_obj_id);
 	if (err)
 		return err;
-- 
2.25.1